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'[EE] Meter amplifier'
2010\03\30@213020 by ivp

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Hi all,

Help

I'm boosting a meter movement to 4V / 1000rpm for a dynamometer and
have struck a small problem. The attached circuit shows the initial buffer
to find the difference between A and B. Point A is static, Point B is
variable

With no input pulses to the tachometer the meter is showing a reading

Point A is at 10.04V and Point B is at 9.96V. A difference of 80mV, which
equates to 80/210 * 1000 = 380rpm, and that's what the needle indicates

Disconnecting the meter from the circuit, I measure no voltage coming from
the op-amp + and - terminals and the op-amp output is 0V

Removing the 100n makes no difference

With just Point A connected to +, Point A reads 10.04V, so does Point B,
so there is zero potential across the meter with no input pulses

Otherwise, the tachometer and op-amp together work linearly, producing
210mV/1000rpm. The problem is the meter reads 400rpm high because
of this 80mV baseline which is there only when the meter is connected.
The deflection of the meter is caused by Point B dropping from 10.04V
@ 210mV / 1000rpm ie as frequency goes up Point B goes down

Cure suggestions please ?

Separate buffer for each meter lead ?
Higher value resistors ?
A bias somewhere ?

TIA


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2010\03\30@225634 by Russell McMahon

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Oscillating?
Scope says ?

2010\03\30@230526 by ivp

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> Oscillating?
> Scope says ?

Well, it did look flat-line in the volts range but on closer inspection
I see there's 5mV at ~14MHz on each lead

2010\03\30@230622 by Marcel Duchamp

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On 3/30/2010 6:29 PM, ivp wrote:
> Hi all,
>
> Help
>
> Cure suggestions please ?
>
> TIA
>

It looks like your RPM sensor has an offset voltage.  If this is the
case, try this:

a) A pot (say 10K) from +5 to ground.
b) A 510K resistor from the wiper to the negative input of the amp.

This should give around +/-100mV of offset adjustment.

2010\03\30@234951 by ivp

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> a) A pot (say 10K) from +5 to ground.
> b) A 510K resistor from the wiper to the negative input of the amp.

Thanks. No change unfortunately. Whether that voltage is injected
at the -ve terminal or at Point B there's no discernible difference to
the potential across the meter, which still reads ~400rpm

2010\03\31@012755 by Dwayne Reid

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At 07:29 PM 3/30/2010, ivp wrote:

>Point A is at 10.04V and Point B is at 9.96V. A difference of 80mV, which
>equates to 80/210 * 1000 = 380rpm, and that's what the needle indicates

I would try buffering each input.  I suspect that the meter driver
has significant output impedance and the unequal input impedance of
your diff amp is loading the meter driver un-equally.

Note: input impedance on the (+) input is 20K, input impedance on the
(-) input is 10K.

Its quick to try and should solve your problem.

dwayne

--
Dwayne Reid   <spam_OUTdwaynerTakeThisOuTspamplanet.eon.net>
Trinity Electronics Systems Ltd    Edmonton, AB, CANADA
(780) 489-3199 voice          (780) 487-6397 fax
http://www.trinity-electronics.com
Custom Electronics Design and Manufacturing

2010\03\31@022047 by Bob Blick

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> On 3/30/2010 6:29 PM, ivp wrote:
> > Hi all,
> >
> > Help
>

You are exceeding the common-mode range of the opamp by having 10v/2
going into each input.

Cheerful regards,

Bob

--
http://www.fastmail.fm - A no graphics, no pop-ups email service

2010\03\31@034315 by ivp

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> You are exceeding the common-mode range of the opamp by having
> 10v/2 going into each input

I did wonder about a 10V signal into a +/- 5V (actually +/- 5.42V)
amp. At the + and - inputs I measured 5.01V and 5.00V and amp
output of 80mV

I've breadboarded a voltage follower (12V supply) for each lead and
now find 5.01V on both the + and - inputs, with 0V out. The 380rpm
has gone from the meter. The linearity appears to have been preserved,
with 0V at 0rpm, 619mV at 3000rpm and 1238mV at 6000rpm
(subject to patient confirmation)

Thanks to Dwayne for the buffer suggestion . It was something I'd
considered but for no reason other than 'try it and see'

2010\03\31@103746 by Marcel Duchamp

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On 3/30/2010 11:20 PM, Bob Blick wrote:
>
>> On 3/30/2010 6:29 PM, ivp wrote:
>>> Hi all,
>>>
>>> Help
>>
>
> You are exceeding the common-mode range of the opamp by having 10v/2
> going into each input.
>
> Cheerful regards,
>
> Bob
>

Doh!  I believe Bob put his finger on it.  The input device is
attempting to raise the inputs of the amp above the power supply rails,
and probably is causing current to flow into the input diodes.

To test this, try increasing the positive power supply enough that the
above case is not true.  Check the input common mode range on the data
sheet to see just how much you need to raise the supply.

2010\03\31@172039 by ivp

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> You are exceeding the common-mode range of the opamp by having
> 10v/2 going into each input.

Bob, do I have an issue with this circuit, bearing in mind that the meter
now zeroes and the output (from subsequent stages) is a linear 4V /
1000rpm, with no baseline offset ?


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2010\03\31@173608 by Mark Rages

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On Wed, Mar 31, 2010 at 3:19 PM, ivp <.....joecolquittKILLspamspam@spam@clear.net.nz> wrote:
>> You are exceeding the common-mode range of the opamp by having
>> 10v/2 going into each input.
>
> Bob, do I have an issue with this circuit, bearing in mind that the meter
> now zeroes and the output (from subsequent stages) is a linear 4V / 1000rpm,
> with no baseline offset ?

No, you are still exceeding the common-mode voltage range of the
TLC271.   At 10V supply, the common-mode range is 0.2 to 9V minimum.
This is measured from the negative supply, so max CM voltage in
circuit is -5+9, or 4 V.

Regards,
Mark
markrages@gmail

--
Midwest Telecine LLC
markragesspamKILLspammidwesttelecine.com

2010\03\31@174336 by Dwayne Reid

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At 12:20 AM 3/31/2010, Bob Blick wrote:

>You are exceeding the common-mode range of the opamp by having 10v/2
>going into each input.

I had considered that but discounted that possibility - several
op-amps (J-FET op-amps, especially) work just fine with the inputs at
or somewhat above Vdd.  Take a close look at the spec sheets for a
TL07x part, for example.

I saw the part number TLC something and assumed that the original
poster had checked that the common-mode voltage was within the
operating range of the chip.  Maybe I shouldn't have made that assumption . . .


I've also used op-amps from Linear Technology that allow operation
with the inputs significantly above Vdd (a couple of hundred volts,
for one particular part).

dwayne

--
Dwayne Reid   <.....dwaynerKILLspamspam.....planet.eon.net>
Trinity Electronics Systems Ltd    Edmonton, AB, CANADA
(780) 489-3199 voice          (780) 487-6397 fax
http://www.trinity-electronics.com
Custom Electronics Design and Manufacturing

2010\03\31@174625 by Bob Blick

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On Thu, 01 Apr 2010 10:19:20 +1300, "ivp" said:
> > You are exceeding the common-mode range of the opamp by having
> > 10v/2 going into each input.
>
> Bob, do I have an issue with this circuit, bearing in mind that the meter
> now zeroes and the output (from subsequent stages) is a linear 4V /
> 1000rpm, with no baseline offset ?

Looking at the specs of the TLC271, I'd say there is still a problem.
The common-mode range on that part extends down to the negative rail but
only to 1.5 volts below the positive rail(throughout temperature range).
You might try a heat gun alternating with cool spray on the opamp and
see if it still works at those temperature extremes.

If you have 12 volts available, why not power the opamp from +12 and -5
volts? You couldn't use the TLC271 since it's limited to 16 volts max,
but you could use an LM358 or do the whole circuit in a quad LM324.

Another source of offset could be whatever's driving the meter movement
might not have equal impedance on both terminals, so the new circuit
with buffering fixes that problem where the single amplifier circuit
loaded the meter driver.

Cheerful regards,

Bob

--
http://www.fastmail.fm - Faster than the air-speed velocity of an
                         unladen european swallow

2010\03\31@174657 by Mark Rages
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On Wed, Mar 31, 2010 at 3:36 PM, Mark Rages <EraseMEmarkragesspam_OUTspamTakeThisOuTgmail.com> wrote:
>
> No, you are still exceeding the common-mode voltage range of the
> TLC271.   At 10V supply, the common-mode range is 0.2 to 9V minimum.
> This is measured from the negative supply, so max CM voltage in
> circuit is -5+9, or 4 V.
>

Actually, if you aren't concerned with offset voltage, just ditch the
TLC271 and use one LM358 in the original circuit, with supplies of -5
and +12V.

Regards,
Mark
markrages@gmail
--
Mark Rages, Engineer
Midwest Telecine LLC
markragesspamspam_OUTmidwesttelecine.com

2010\03\31@175151 by Michael Watterson

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Mark Rages wrote:
{Quote hidden}

ditch the LM385 and put a pair of potential dividers  to reduce the
voltage and increase the opamp gain to compensate.

2010\03\31@191557 by ivp

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To repliers -

> ditch the LM385 and put a pair of potential dividers  to reduce the
> voltage and increase the opamp gain to compensate.

Michael, I hadn't appreciated a problem with CM (must get more
familiar with op amps), and the breadboarded prototype appeared to
work OK. Not until I made a PCB did I notice the meter deflection.
At the time the innards were out of the case so there was no dial for
visual reference

As there is gain available in following stages I could reduce the front
end gain to < 1 and compensate later

> At 10V supply, the common-mode range is 0.2 to 9V minimum.
> This is measured from the negative supply, so max CM voltage in
> circuit is -5+9, or 4 V

Ah, got it

> I saw the part number TLC something and assumed that the original
> poster had checked that the common-mode voltage was within the
> operating range of the chip

Dwayne, ditto

> If you have 12 volts available, why not power the opamp from +12 and
> -5 volts? You couldn't use the TLC271 since it's limited to 16 volts max,
> but you could use an LM358 or do the whole circuit in a quad LM324

Bob and Mark, not keen on hacking the PCB to fit dual amps. Reducing
the input voltages to the differential amp is what I'll try first

Thanks

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