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'[OT] PIC-based Digital Storage Oscilloscope'
1998\01\18@014843 by Sean Breheny

face picon face
Hello all,

I am continuing work on my PIC-based digital storage osc. and I am actually
in the building phase right now, although the front end is not completely
finished. This brings me to my question, what should I make the input
impedance? I can remember when I asked a few other questions about this
device on the list before, at least one person said something like "if you
do [blah blah blah, etc.] it will then have a nice 50 ohm input impedance"
While 50 ohms is good for many devices where I want the load matched to the
generator, lets say I want my scope to see the output of an oscillator with
Zout = 1K, I will see little or nothing if my Zin(scope) = 50 ohms!
Besides, what happens when my 50 ohm scope gets attached to a signal which
is riding on a 12 V DC level? My scope will have to dissipate 2.88 Watts!
It is true that the cable going to the probe will probably have a
characteristic impedance of around 50 ohms, but if I compensate the
probe(or the internal input nework), I should be able to compensate for any
added reactance due to the cable, right?

I expect the scope's front end to have abt a 50 MHz BW. My input network
must connect the BNC input connector to a MAX4107 300MHz BW op amp which
has an input which looks like a 1Meg resistor in parallel with 2pF of cap.
This input network will have to provide variable attenuation.

I would appreciate any suggestions that anyone has regarding what input
impedance is best. I would think the higher, the better.

Thanks,

Sean

+--------------------------------+
| Sean Breheny                   |
| Amateur Radio Callsign: KA3YXM |
| Electrical Engineering Student |
+--------------------------------+
http://www.people.cornell.edu/pages/shb7
spam_OUTshb7TakeThisOuTspamcornell.edu
Phone(USA): (607) 253-0315

1998\01\18@082319 by peter

flavicon
face
Sean Breheny wrote:
>
> Hello all,
>
> I am continuing work on my PIC-based digital storage osc. and I am actually
> in the building phase right now, although the front end is not completely
> finished. This brings me to my question, what should I make the input
> impedance?

My scope's specs say its " 1Meg approx., 30pF approx "
(Hitachi VC6020 DSO)

--

.....peterKILLspamspam@spam@cousens.her.forthnet.gr

1998\01\18@102259 by Nigel Goodwin

flavicon
picon face
In message <3.0.2.32.19980118014641.00930100spamKILLspampostoffice2.mail.cornell.ed> u>, Sean Breheny <.....shb7KILLspamspam.....CORNELL.EDU> writes
>Hello all,
>
>I am continuing work on my PIC-based digital storage osc. and I am actually
>in the building phase right now, although the front end is not completely
>finished. This brings me to my question, what should I make the input
>impedance? I can remember when I asked a few other questions about this
>device on the list before, at least one person said something like "if you
>do [blah blah blah, etc.] it will then have a nice 50 ohm input impedance"
>While 50 ohms is good for many devices where I want the load matched to the
>generator, lets say I want my scope to see the output of an oscillator with
>Zout = 1K, I will see little or nothing if my Zin(scope) = 50 ohms!
>Besides, what happens when my 50 ohm scope gets attached to a signal which
>is riding on a 12 V DC level? My scope will have to dissipate 2.88 Watts!
>It is true that the cable going to the probe will probably have a
>characteristic impedance of around 50 ohms, but if I compensate the
>probe(or the internal input nework), I should be able to compensate for any
>added reactance due to the cable, right?

The impedance of a scope probe cable isn't 50 ohms - it's far higher
(although I've no idea what the actual value is!). If I can remember
back that far, I seem to recall that the impedance is calculated from
the diameter of the outer screen and the diameter of the inner core,
where the smaller the inner, the higher the impedance If you look in a
scope cable the inner is a single, very thin, wire - often corrugated, I
presume to keep it in the middle of the air-spacing.

>I expect the scope's front end to have abt a 50 MHz BW. My input network
>must connect the BNC input connector to a MAX4107 300MHz BW op amp which
>has an input which looks like a 1Meg resistor in parallel with 2pF of cap.
>This input network will have to provide variable attenuation.
>
>I would appreciate any suggestions that anyone has regarding what input
>impedance is best. I would think the higher, the better.

The standard input impedance for scopes is 1Meg and about 30pF
capacitance, I can't thing of any real reason for wanting to change from
that!. If you keep to the standard specs, that means you can use all the
usual scope probes etc.

It sounds a very interesting project, I would be intersted to hear how
you progress.

--

Nigel.

       /--------------------------------------------------------------\
       | Nigel Goodwin   | Internet : EraseMEnigelgspam_OUTspamTakeThisOuTlpilsley.demon.co.uk     |
       | Lower Pilsley   | Web Page : http://www.lpilsley.demon.co.uk |
       | Chesterfield    |                                            |
       | England         |                                            |
       \--------------------------------------------------------------/

1998\01\18@143133 by Sean Breheny

face picon face
Hello again,

Thanks for all the responses. I gather from the responses that it would be
best to keep the input impedance as high as possible, so that's what I will
try to do. I did forget to mention that the input buffer op amp that I am
using has a significant input bias current, several uA, so as far as I can
see, this will severly limit how high I can make my input impedance.
It will probably not be able to be high than a few K, in parallel with the
abt. 20pF or so of cap. due to the op-amp and the board, etc. However, if
the input atten. network is set to a higher atten, this impedance will go
up, of course. If anyone can see any way around the problem of input bias
current, please let me know.

As for input protection, I had looked at various diodes and TVS devices,
and they all had a rather high capacitance, so I rejected using them. The
op-amp that handles the input is a bi-polar op-amp, so I hope it won't be
too sensitive to esd. Now that I am beginning to realize that my input
impedance must be lower than I expected, I guess that the few added pF of
diode cap. won't make that much difference so I might go back to using,
say, two back-to-back 6.2V 500mW zeners from op-amp input to ground. These
will come after the input atten. network so I will actually be able to give
the scope inputs higher than 6.8 V.

I had also planned on attempting to make my own probe. Maybe this is not
cost effective? Last time I checked, probes were rather expensive, >$30. I
was just going to take a standard meter probe tip, place the appropriate
value resistor with a trim cap. across it, in the tip in series with the
center conductor of some coax, attach an aligator clip to the sheild, and
then solder a BNC onto the other end. Right idea, or wrong idea??

I sure do appreciate everyone's advice, you are saving me a lot of time,
money, and hassle. I'll let you know how it works/publish schematics on the
web when I am finished.

Thanks,

Sean




+--------------------------------+
| Sean Breheny                   |
| Amateur Radio Callsign: KA3YXM |
| Electrical Engineering Student |
+--------------------------------+
http://www.people.cornell.edu/pages/shb7
shb7spamspam_OUTcornell.edu
Phone(USA): (607) 253-0315

1998\01\18@163849 by Bill (WL) Boulton

flavicon
face
Sean

You're making a DSO so I think you should make the input "look" like a CRO.
I think it would be more marketable in that form. Personally, I wouldn't
purchase anything that didn't.

Good luck with the project.
Bill

At 01:46 AM 18/01/98 -0500, you wrote:
{Quote hidden}

1998\01\18@180053 by Mitchell D. Miller

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face
part 0 2442 bytes
-- Mitch



-----Original Message-----
From:   Bill (WL) Boulton [SMTP:KILLspambill-794KILLspamspamWINSHOP.COM.AU]
Sent:   Sunday, January 18, 1998 3:39 PM
To:     RemoveMEPICLISTTakeThisOuTspamMITVMA.MIT.EDU
Subject:        Re: [OT] PIC-based Digital Storage Oscilloscope

Sean

You're making a DSO so I think you should make the input "look" like a CRO.
I think it would be more marketable in that form. Personally, I wouldn't
purchase anything that didn't.

Good luck with the project.
Bill

At 01:46 AM 18/01/98 -0500, you wrote:
{Quote hidden}

1998\01\19@120348 by Sean Breheny

face picon face
At 04:56 PM 1/18/98 -0600, you wrote:
>What's a CRO?
>
>-- Mitch
>
>

Hi Mitch,

CRO stands for Cathode Ray Oscilloscope. It is another term for Analog
Oscilloscope. It just means that the input voltage is just amplified and
placed on the deflection plates of a cathode ray tube, rather than being
digitized, stored, and then displayed at some later time.(as it would be in
a DSO or Digital Storage Oscilloscope)

Sean

P.S. As for whether or not I ought to make it look like a CRO, I'd love to,
but I have several problems with doing than (i.e. input bias current on the
input buffer is several uA, which would cause too large a DC error if
source impedance is >10k) Also, I am mainly doing this a project to learn
about high speed ADCs and RF amplifier/attenuator design, not really to
market it.

Sean





+--------------------------------+
| Sean Breheny                   |
| Amateur Radio Callsign: KA3YXM |
| Electrical Engineering Student |
+--------------------------------+
http://www.people.cornell.edu/pages/shb7
TakeThisOuTshb7EraseMEspamspam_OUTcornell.edu
Phone(USA): (607) 253-0315

1998\01\19@120353 by Harri Suomalainen

flavicon
face
>I am continuing work on my PIC-based digital storage osc. and I am
actually
>in the building phase right now, although the front end is not
completely
>finished. This brings me to my question, what should I make the
input
>impedance? I can remember when I asked a few other questions about
this

I'd go for a selectable one. For usual scoping purposes a 1Mohm
impedance
is pretty common (or some fairly high impedance anyway). I'd include
a switch
to select a 50ohm impedance as well when needed.

>I expect the scope's front end to have abt a 50 MHz BW. My input
network
>must connect the BNC input connector to a MAX4107 300MHz BW op amp
which
>has an input which looks like a 1Meg resistor in parallel with 2pF
of cap.

Sounds just great as a input impedance. Usual values for scopes are
something like
1Meg in parallel with some capasitance (a few picos up to around 20pF
or so).
Scope probes compensate the capasitance in that region just fine. A
very wierd
input impedance (like highly capasitive one) would mean problems with
normal
scope probes.

BTW, any further details or a general description about the project
available? Sounds
like interesting one. Definately a project I've been thinking for a
while too!


--
Harri Suomalainen     RemoveMEhabaspamTakeThisOuTcc.hut.fi

We have phone numbers, why would we need IP-numbers? - a person in a
bus

1998\01\19@120355 by Andrew Mayo

flavicon
face
A scope will have a very high input impedance at DC - 1M or greater. At
high frequencies the impedance will be a function of the coaxial cable
and probe assembly, primarily. If you are using normal scope probes (and
I would), then these internally divide the voltage input by 10 (i.e they
are called X10 probes) and present a lower impedance to the scope than
would otherwise be possible. This minimises the effect of capacitance on
rise time. Probes contain a compensation trimmer across the divider
network to further reduce the effect of cable,scope and probe
capacitance. This is manually trimmed while the input is driven from a
good-quality square wave until the scope waveform looks most like the
input i.e negligible droop, sag, risetime impairment or ringing.

The design of wide-band scope amplifiers is a non-trivial task, and you
may wish to read the section on this topic in the book 'Analogue circuit
design - art, science and personalities' before continuing.

{Quote hidden}

>       {Original Message removed}

1998\01\19@120442 by alex_holden

picon face
Mitchell D. Miller wrote:
>
> What's a CRO?

Didn't you know that? Of course, it's a Cave Rescue Organisation.
Or was it Cathode Ray Oscilloscope?

--
-----------------------------------------------------------------------
: Alex Holden- Electronics student, Caver, and Land Rover enthusiast. :
:        http://www.geocities.com/CapeCanaveral/Lab/1532/             :
------------  Linux- The choice of a GNU generation.  -----------------

1998\01\19@121359 by TONY NIXON 54964

flavicon
picon face
>
> What's a CRO?
>
A big black thing that lives in a tree.

Try sitting under that tree and play dead for a while. After a bit of twig
dropping to see if you are really dead, it will fly down and try to PIC you
to bits.

Tony.

For the beginner....
PicNPoke Multimedia 16F84 Simulator Assembler, and Tutorial.
Now with PicNPlay circuit simulator.
Plus animated Address Mode Tutor.

http://www.dontronics.com/picnpoke.html

1998\01\19@134323 by Harold Hallikainen

picon face
       Yes, I'd think "the higher the better" on scope input impedance.
A fairly typical input Z is 1 meg with so many pF across it.  If you go
for this, you can use standard compensated scope probes.  Checking scopes
here, I found one with 47 pF, the other with 20 pF.
       You might include a front panel switch to throw a 50 ohm
termination across it (maybe even allow selection of 50 or 75 ohms?).


Harold

1998\01\19@150654 by Mikal Hodvik

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face
Tektronix published an excellent book titled *Oscilloscope Probe Circuits*
in 1969-- part of their Circuit Concepts Series. If you can find a copy, it
will provide a LOT of insight on the issues involved. Trust me; if you want
to transfer a signal with decent fidelity from arbitrary test points into
your measuring system, standard scope probes are almost always the best
solution.

To get the 1M input resistance that's needed for compatibility with standard
scope voltage probes, its convenient to use a fast (video) op-amp with FET
inputs. Not a cheap component, but it makes life a lot simpler at this
interface!

Mike Hardwick, for Decade Engineering
In Oregon, Land of Magnificent Stumps
RemoveMEdecadespam_OUTspamKILLspamworldnet.att.net ~ http://www.decadenet.com


>        Yes, I'd think "the higher the better" on scope input impedance.
>A fairly typical input Z is 1 meg with so many pF across it.  If you go
>for this, you can use standard compensated scope probes.  Checking scopes
>here, I found one with 47 pF, the other with 20 pF.
>        You might include a front panel switch to throw a 50 ohm
>termination across it (maybe even allow selection of 50 or 75 ohms?).

1998\01\19@164804 by Sean Breheny

face picon face
Hello all,

Thanks again to all who made suggestions about my oscilloscope design. To
those who asked if any further info or a general description, I will post
one on my website as soon as I am more certain about how the front end will
turn out. Just briefly, it consists of an input network with variable
gain/attenuation, feeding a 40MS/s semi-flash ADC. The output of the ADC is
grabbed by a single 15ns SRAM chip whose address pins are driven by a pair
of 8-bit 100MHz 74F579 counter chips. The board has an interface which
allows an external computer (PIC based) to stop/start conversion runs, and
read out the data from the ram. The board, when completed, should be
capable of 5 to 40 MS/s conversion rates, with a 50MHz input bandwidth
(50MHz just to be safe, although 20MHz is the max.
BW without aliasing) and it can store up to 64Ksamples at a time before the
data needs to be read out of the ram.


I want to clarify an earlier question that I asked:

Does anyone know of an amplifier (op-amp style) with something
approximating these specs?

* Less than 10nA input bias current
* OK to operate off +5 and gnd (not necessarily input range incl. GND, just
doesn't need more than 5v spread from Vcc to Vee)
* Less than 10mV input offset voltage
* Same pin-out as 741 (not that I designed this around the 741, but the
amp I currently am trying to use has same pin out)
* 1 dB gain flatness up to 50MHz at Av = +10V/V
* Voltage feedback
* Can drive up to 40pF cap load with moderate (< 100 ohm) series resistor

If not, does anyone know of a buffer that can at least provide a flat gain
of 1 and has the rest of these characteristics?

I know that some have suggested that I read various documents on scope
input design, and I thank you for you suggestions, and I hope to do this,
however, I already have a board prototyped for this circuit (I guess I was
a bit premature, I never saw an op amp with an input bias current greater
than about 100 nA, so I never thought to check the input bias current spec
on the op amp I was using, which, by the way, is the only thing wrong with
the op amp that I am using.) I have not finished soldering in the
components, though, so I could switch op-amps if there was one which was
pin-compatible. Also, does anyone know of a way to some how cancel out
input bias current without having to know the source impedance?

If none of these things can be accomplished, I will finish construction
with the max input impedance I can get (abt 2K) and then make an adaptor
that will go between the scope and the probe to buffer the input. This
wouldn't be so bad, after all, I am just doing this as a learning experience.

Thanks again to all, and I appreciate everyones patience that I am tying up
the list with something that is only remotely PIC related.

Sean

+--------------------------------+
| Sean Breheny                   |
| Amateur Radio Callsign: KA3YXM |
| Electrical Engineering Student |
+--------------------------------+
http://www.people.cornell.edu/pages/shb7
RemoveMEshb7TakeThisOuTspamspamcornell.edu
Phone(USA): (607) 253-0315

1998\01\19@175643 by Bill (WL) Boulton

flavicon
face
Sean
At 06:28 PM 18/01/98 -0500, you wrote:
>
>Hi Mitch,
>
>CRO stands for Cathode Ray Oscilloscope. It is another term for Analog
>Oscilloscope. It just means that the input voltage is just amplified and
>placed on the deflection plates of a cathode ray tube, rather than being
>digitized, stored, and then displayed at some later time.(as it would be in
>a DSO or Digital Storage Oscilloscope)
>
>Sean
>
-------------- snip ------------------
Technically, CRO describes the output section of an oscilloscope. A lot of
high end units are both DSO and CRO having a digital input end and an
analogue output end. The hand held scopes and Techtronics TDS 2x0 series
are all digital with LCD screens.

>P.S. As for whether or not I ought to make it look like a CRO, I'd love to,
>but I have several problems with doing than (i.e. input bias current on the
>input buffer is several uA, which would cause too large a DC error if
>source impedance is >10k) Also, I am mainly doing this a project to learn
>about high speed ADCs and RF amplifier/attenuator design, not really to
>market it.

It's your project. You wrote the design specs (I hope) so you do with it as
you like.

You may be interested in an op-amp I found for a similar project. It is an
LM660C from NS. It is a CMOS device with very interesting specs (ie, offset
& bias currents of 1-2 pA MAX, rail to rail output and input impedance >
1Tohm). I have the data sheet as a ".pdf" (270k) on CD or you may like to
go directly to NS web site at <http://www.national.com> and do a search for
it. It's a good data sheet with oodles of info (12 pages). The device was
not expensive when I purchased some years ago.

Hope this device fits your design criteria and is of some use.

Good luck
Bill

1998\01\19@175647 by Bill (WL) Boulton

flavicon
face
Tony
I like that. Best possible reply.
Bill

At 11:01 AM 19/01/98 +1000, you wrote:
{Quote hidden}

1998\01\19@213643 by Mike Keitz

picon face
On Mon, 19 Jan 1998 16:46:04 -0500 Sean Breheny <EraseMEshb7spamspamspamBeGoneCORNELL.EDU>
writes:
>Hello all,
[...]
>I want to clarify an earlier question that I asked:
>
>Does anyone know of an amplifier (op-amp style) with something
>approximating these specs?
>
>* Less than 10nA input bias current

You can get around the bias current limitation by providing another path
for the bias current to flow rather than through the input terminating
resistor.  Put another large resistor at the input connected to an
adjustable voltage and adjust the voltage until the output voltage is
zero both with the input open and with it shorted.  The input resistance
is then approximately the two resistors in parallel (one to supply, one
to ground).  This is a good idea even if the amplifier you use has a
rather low bias current, as it prevents current from flowing out of the
scope and into the circuit under test.

>* OK to operate off +5 and gnd (not necessarily input range incl. GND,
>just
>doesn't need more than 5v spread from Vcc to Vee)

You may need a negative voltage to supply the bias current, but this is a
good idea anyway since using a dual-supply op amp opens up a lot more
choices.  Without a negative supply to the front-end, I'm not sure how
you intend to be able to measure negative input voltages without lifting
the input ground from the rest of the circuit ground (a rather bad idea).

A real simple way to get a few mA of bipolar supplies would be too use
the voltae converters in a MAX232 style chip.  Linear regulators after
the converter would be a good idea for consistent performance and to help
remove switching noise.

>* Less than 10mV input offset voltage

Don't get too hung up on maintaining a zero DC offset through the
amplifier, it makes it rather complicated.  Many, many useful scopes
aren't designed to measure absolute DC input voltage (i.e. the VERTICAL
POSITION dial isn't calibrated).  Digital scopes are lousy as voltmeters
anyway due to the small number of bits in the converter.

>* Voltage feedback

If you're configuring the amplifier as a noninverting amp with the signal
applied to the + input and feedback at the - input, a current feedback
amp will work fine.  They just have a low impedance at the - input
terminal, requiring the use of smaller resistors in the feedback network.
With proper feedback maintained, the + input terminal is voltage
sensitive and high impedance.

>If not, does anyone know of a buffer that can at least provide a flat
>gain
>of 1 and has the rest of these characteristics?

I'd be tempted to try JFET source followers between the input and the
voltage gain stage.  DC offset may go out the window, but my opinion is
that that isn't too critical in a scope.


>the op amp that I am using.) I have not finished soldering in the
>components, though, so I could switch op-amps if there was one which
>was
>pin-compatible.

I may be suggesting a total re-design, but you should explore your
present design some more to find out its limitations.  The first unit
built shouldn't be expected to work really well.

>Also, does anyone know of a way to some how cancel out
>input bias current without having to know the source impedance?

I touched on this above.  If the amplifier only has to handle AC signals,
then the bias curent can be automatically cancelled.  Use another
low-performance op amp to supply slow feedback to supply the proper bias
current to force the main amplifier's average output to zero.  I think
this is called a "DC Servo".  If you need to amplify signals near DC
(which most useful scopes do), then better hope the bias current is
constant and use a fixed adjustment.

An off-the wall idea would be to use a dual amp, one side connected just
to measure and amplify bias current.   This would be applied to the other
one (which is amplifying the signal) to cancel its bias current.  The two
amplifiers in a dual chip should track each other fairly close.

1998\01\19@221008 by Sean Breheny

face picon face
At 09:38 PM 1/19/98 -0500, you wrote:
>>* Less than 10nA input bias current
>
>You can get around the bias current limitation by providing another path
>for the bias current to flow rather than through the input terminating
>resistor.  Put another large resistor at the input connected to an
>adjustable voltage and adjust the voltage until the output voltage is
>zero both with the input open and with it shorted.  The input resistance
>is then approximately the two resistors in parallel (one to supply, one
>to ground).  This is a good idea even if the amplifier you use has a
>rather low bias current, as it prevents current from flowing out of the
>scope and into the circuit under test.
>

Mike, thanks for the idea. I had thought of something similar but I didn't
think that it would be possible to compensate like this without causing a
big DC offset. I'll try it.

{Quote hidden}

I did lift the input ground from the circuit ground, but not just with a
resistive divider, I have a slow speed op-amp with a resistive divider as a
reference, the op-amp then feeds a complimentary pair of bipolar
transistors and the negative input of the slow op-amp goes to the output of
the pair, and the output of the pair is also bypassed to ground through .1
uF and 22uF caps. Not the best solution , I realize, but I wanted the whole
thing to operate off a single supply for simplicity.
For scope inputs which do not pass DC, I have a resistor (value not yet
selected) across the input.

{Quote hidden}

Well, I'm not bothering to compensate for offset voltage, so I wanted it to
be low initially. a 10 mV difference times the 10x gain would place a .1 V
error (or 5 percent DC error on the whole 2V scale of my ADC) on the ADC. I
was considering this to be about the upper limit for the contribution from
offset voltage.

>>* Voltage feedback
>
>If you're configuring the amplifier as a noninverting amp with the signal
>applied to the + input and feedback at the - input, a current feedback
>amp will work fine.  They just have a low impedance at the - input
>terminal, requiring the use of smaller resistors in the feedback network.
> With proper feedback maintained, the + input terminal is voltage
>sensitive and high impedance.
>

I must admit, I was opting for voltage feedback mainly out of ignorance of
current feedback op amps, however, I have explored a few current feedback
ones and didn't find them much superior for my application.

>>If not, does anyone know of a buffer that can at least provide a flat
>>gain
>>of 1 and has the rest of these characteristics?
>
>I'd be tempted to try JFET source followers between the input and the
>voltage gain stage.  DC offset may go out the window, but my opinion is
>that that isn't too critical in a scope.
>
Well, I guess I may have to sacrifice the DC accuracy anyway so ultimately,
this may end up being what I do.


{Quote hidden}

Yeah, I realize that, but being a college student, my budget is quite small
and I can't order a new prototype board very often. I was hoping to get
decent, not perfect, performance out of this first prototype. I felt that a
professionally made board (AP circuits in this case) would give me the best
RF performance (double sided w/large ground plane, plated thru hole, using
surface mount for all analog ICs)

{Quote hidden}

Both of these ideas deserve serious consideration if I do make a second
prototype, but as I said, I don't think I am going to try to implement them
now, being that the board is already etched.

Thanks for all your suggestions and I will try the bias current
cancellation technique which you suggested.

Thanks again (I sure do get lots of great ideas on this list),

Sean

+--------------------------------+
| Sean Breheny                   |
| Amateur Radio Callsign: KA3YXM |
| Electrical Engineering Student |
+--------------------------------+
http://www.people.cornell.edu/pages/shb7
RemoveMEshb7KILLspamspamcornell.edu
Phone(USA): (607) 253-0315

1998\01\20@174306 by johnb

picon face
Mike Keitz wrote:
> You can get around the bias current limitation by providing another path
> for the bias current to flow rather than through the input terminating
> resistor.

Nobody EVER does this kind of thing at the input stage of
oscilloscope-type instruments. It's important to devise a circuit that
doesn't send a voltage or current back into the source; you risk
upsetting the circuit being tested or even stopping it working.

Even worse is a 'scope that lets digital switching signals leak back
into the circuit being scoped, which can upset the circuit very
seriously and produce an entirely misleading display on the 'scope
screen.

> I'd be tempted to try JFET source followers between the input and the
> voltage gain stage.
This is commonly done. Such a stage attenuates signals travelling in the
reverse direction - that is, back into the source. Very beneficial.

>  DC offset may go out the window, etc...
Not if it's done properly. There is a circuit that uses a matched pair
of high-transconductance junction FETs (both in the same package) that
has low voltage offset, low tempco, unity gain, wide bandwidth, high
input impedance and low output impedance. In my opinion God sent this
circuit for jobs like this. Or was it Siliconix? Anyway, the FETs are
used in series; the signal goes into the gate of the top FET, and the
bottom FET (powered from a -5V rail) acts as a matched current source
for the top FET.
The upper FET has a 1 Megohm (1% tolerance) resistor from gate to
ground, so the input resistance of the stage is 1 Megohm exactly. A X10
scope probe can then be used if the capacitance of the FET input circuit
is adjusted to be within the range of adjustment of the probe.
The remaining problem is then overload protection, which isn't hard.
And, of course, stopping it oscillating - this circuit has a very wide
bandwidth; I've used it up to 100MHz.

Siliconix used to make these FETs with numbers like E300 and E301. I
think it's called TEMIC or something now... Anyway, it's a great
circuit.

John Blackburn,
South London UK.

1998\01\20@181843 by peter

flavicon
face
Sean and anyone else who's interested

This thread has prompted me to dig out my scope operation manual
which has the full circuit diagrams
(don't you just love companys like that, thanks Hitachi)
it's ten years old only goes to 20Mhz
but if anyone wants I'll scan the input section and send it to you

peter

peterSTOPspamspamspam_OUTcousens.her.forthnet.gr

1998\01\20@234049 by Mike Keitz

picon face
On Tue, 20 Jan 1998 22:42:02 -0800 john blackburn <spamBeGonejohnbSTOPspamspamEraseMEdircon.co.uk>
writes:
>Mike Keitz wrote:
>> You can get around the bias current limitation by providing another
>path
>> for the bias current to flow rather than through the input
>terminating
>> resistor.
>
>Nobody EVER does this kind of thing at the input stage of
>oscilloscope-type instruments. It's important to devise a circuit that
>doesn't send a voltage or current back into the source; you risk
>upsetting the circuit being tested or even stopping it working.

That is the purpose of providing another bias current path:  to prevent
voltage or current from coming out of the input terminal.  The input
terminal should appear to be a resistor to ground.  Even this can of
course upset the operation of circuits being tested.

Here's what I'm talking about.  A conventional setup has
        |\
  in-*--|A> - out
     Rt |/
     |
    GND
If the amplifier's input has a bias current (for the sake of argument,
let's assume it's into the amplifier), then with the input open a
negative voltage appears across Rt and thus at the input terminal.  This
voltage is amplified and causes a DC offset in the output.  With the
input shorted to ground, the bias current flows through the short, and
the output voltage changes.  This amplifier arrangement is unsatisfactory
unless the bias current or Rt are small enough to reduce the voltage
developed to a negligible amount.

Now consider adding another resistor from the input to a constant voltage
source.

     Vb
     |
     Rb |\
  in-*--|A> - out
     Rt |/
     |
    GND

With Vb properly adjusted, the current through Rb is the amplifier's bias
current, so it all flows into the amplifier and none through Rt.  The
open-circuit voltage at the input is zero.  To small input signals, the
input appears to be a resistor to ground of the parallel combination of
Rt and Rb.  Rb should be large enough that Vb is considerably more than
the largest input signal.  Then the current through Rb is approximately
constant regardless of the input voltage.  If the amplifer's bias current
is also constant, the input appears reasonably linear and resistive,
without stray voltage.

The major problem is to adjust Vb so that the bias current is exactly
cancelled.  If the amplifer only handles AC signals, automatic adjustment
via a "DC servo" feedback loop can be established.  If it has to handle
DC signals, then it is not possible to tell a changing input signal from
a change in bias current.  A fixed adjustment may or may not work
depending on the characteristics of the amplifier and the level of
performance required.

This circuit is a compromise.  Sean's oscilloscope, at least at this
stage, is still a hacker project rather than a hardcore high-performance,
general-purpose instrument.  There is a reason why good scopes cost
considerable amounts of money, and it's not just greed.  Using a single
op-amp as the front end is not optimal, but it is small and inexpensive.
It should have enough usefullness to explore other aspects of scope
design, then come back and revisit the front-end.

[JFET buffer]
>Not if it's done properly. There is a circuit that uses a matched pair
>of high-transconductance junction FETs (both in the same package)

I'm interested in how this works.  I have no doubt that it does, but
can't quite grasp it from your description.  I assume the second FET is
used to develop a voltage equal and opposite to the gate-source voltage
of the first one, so the output voltage is again zero with zero input.
Or does it just maintain a constant gate-source voltage by using a
precision current source, which is then removed by a level shift later
on?

1998\01\20@235937 by John Griessen

flavicon
face
>[JFET buffer]
>>Not if it's done properly. There is a circuit that uses a matched pair
>>of high-transconductance junction FETs (both in the same package)
>
>I'm interested in how this works.  I have no doubt that it does, but
>can't quite grasp it from your description.

A JFET pair can be useful in dealing with the biasing of the two input
terminals of the op amp, one for each.  The FET's are connected so their
gates are all the probes touch, and no feedback is used so they need to be
matched to get a low CMRR--their drain to source resistance becomes part of
the bias network you were talking about before--and it creates the input
signal in the same place.  To adjust the bias network to zero out the
opamp's DC offsets you could use a programmable resistor (like we've heard
versions of on the list) controlled by your PIC while in CAL mode with a
request to the outside world (or with a relay) to short the inputs.

John Griessen  CIBOLO Metal Works
512 451-8207 FAX 451-0753


http://www.aus-etc.com/~cibolo/cibolo/

1998\01\21@022548 by Tom Handley

picon face
  Sean, I've been working on a PIC-based logic analyzer that will also
double as a DSO so I've been following this thread closely and have been
evaluating a variety of wideband amps. I've seen a lot of excellent advice
here. I'm looking for a usable bandwidth of 20Mhz since my logic analyzer
will sample around 40Mhz. Like you, I'm building this for myself and I'd
like to share it. This is a spare time project but a lot of low-level work
is comming together.

  I've looked at the MAX4107 that you mentioned. You might also want to
look at the 4100 and 4102 which have less bandwidth, 3ua bias, and are
pin-compatible. In any case, I strongly recommend a +/- 5 to 15V supply and
an FET source follower. Also, a 1M/~20pf input to accommodate common scope
probes.

  There's an old but classic, FET probe circuit with a 10M/8pf input and a
50ohm output in the "Circuit Design Idea Handbook" (1972) from EDN magazine.
It was designed by a (then) Sony engineer intended as an impedance converter
for use with 50ohm inputs. The original bandwidth was 20Mhz but you can
increase that by using an updated FET. The probe required +/- 6V (+/- 5V
would be fine) and was built in a metal `cigar' tube with a 6-32 threaded
stud to accept common probe accessories that are still available.

  Linear Technology has an excellent application note in their 1993 "Linear
Applications Handbook Volume II". This is AN47; "High Speed Amplifier
Techniques. A designer's Companion for Wideband Circuitry". This covers a
wealth of information including the "ABC's of Probes" contributed by
Tektronix. One application is a 100Mhz servo controlled FET input amp with
a 10M/3pf input and 100pa bias current. The example has a gain of 10. It's
based on the LT1223 and is intended for probe and ATE pin amplifier
applications. The LT1223 works fine with flash A/D's. Checking Digi-Key, the
components cost around $15. It requires a +/- 15V supply and Burr-Brown has
an nice self-contained, isolated, +5V in +/-15V out, converter in a 14 pin
DIP for $11 from Digi-Key.

  I've been running SPICE models for several amps and configurations. So
far I've been using Linear and Maxim's models. I recently received the v5.1
update to Electronics Workbench and the results have been fairly close to
vendor data. The Bode plotter comes in real handy here. `Gawd', I use to do
that by hand on graph paper with a slide rule... I'm getting ready to test
the FET probe and Linear's servo FET amp. I'll probably go with a modified
version of the Linear example. I still have'nt decided on an A/D. Which one
are you using? If you want, I can run some simulations based on your
front-end and the A/D input resistance/capacitance.

  I'm sorry you have already committed to a pc-board but I strongly suggest
you keep the option open to modify it even if you have to cut a trace or two
and solder short wires point-to-point. One good option is to use a standard,
small (ie: 1 x 1.5"), protoboard with a pad-per-hole and a ground plane on
one side. These are cheap and you could use gold-plated pc pins/receptacles
(See Digi-Key, pg 107) from the pads where your amp would go to the daughter
board without much problem. This would also allow you to try out other
circuits by plugging-in different daughter boards.

  One important engineering lesson to be learned here is never pass a design
to production until it's been prototyped and thoroughly tested or production
will take you out back and string you up from the highest tree ;-)

  - Tom

At 04:46 PM 1/19/98 -0500, you wrote:
{Quote hidden}

1998\01\21@123459 by Wayne Foletta

flavicon
face
Bias current cancellation is a common technique used in both discrete
and IC amplifier designs. Simple cancellation can be as suggested -
inject a current into the summing node from an adjustable source. In IC
advantage is taken from the matching of components so that collector or
drain currents is monitored and mirrored in a parallel path. For
example:

      --------+-----+
      |e    e |     |
       \|   |/      |
        |-+-|       |
       /| | |\    |/
      |   +---+---|
      |    ->~Ib  |\
      |            e|
      |             /
      |~Ib          \Rload
      |             /
      |             \
      |             |
  Vin |   ->Ib    |/
     -+-----------|
                  |\
                   e|


- Wayne Foletta
       BMI - Saratoga, CA

1998\01\22@101306 by Martin Nilsson

picon face
The activity and level of the discussion on building a PIC-based
Digital Storage Oscilloscope has been quite high. Wouldn't it be a
good idea to create a mailing list for just this purpose?  I think
there is broad interest for a project of this kind. It would be a pity
if the discussion dies away just because it is off topic on the PIC-list.

-- Martin

Martin Nilsson                           http://www.sics.se/~mn/
Swedish Institute of Computer Science    E-mail: KILLspammnspamBeGonespamsics.se
Box 1263, SE-164 29 Kista                Fax: +46-8-751-7230
Sweden                                   Tel: +46-8-752-1574

1998\01\28@204755 by Bob Blick

face
flavicon
face
> The activity and level of the discussion on building a PIC-based
> Digital Storage Oscilloscope has been quite high. Wouldn't it be a
> good idea to create a mailing list for just this purpose?  I think
> there is broad interest for a project of this kind. It would be a pity
> if the discussion dies away just because it is off topic on the PIC-list.

If the discussion moved, then I wouldn't see it anymore. Is does not seem
like a topic that is offensive to the piclist.

The down side to the freedom here is the volume of messages, but I've
resigned myself to getting 100 or more, and have a mail account just
for the piclist. Lucky I never get a vacation!

I like the community of the piclist and the variety of off-topic topics.

Cheerful regards,

Bob

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