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PICList Thread
'Protecting ADC inputs'
1998\10\19@120953 by Steve Parker

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      Hi there folks!

          I am currently contemplating the risk of damage to my humble
      16C71JW. Basically I am powering it with 5V, but I have the o/p
      of a dual rail OP AMP going to one of the ADC inputs. The OP AMP
      is capable of swinging to +/- 9V and I am therefore worried. I
      have put a 1K series resistance prior to the pin and a 5V6 Zener
      from the pin to earth. However, this is a bit hit and miss and I
      would love to be told the proper way to protect the i/p by some
      experts.
            Yours hoping for a technical slapping!!

                        Steve Parker

1998\10\19@163205 by Harold Hallikainen

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On Mon, 19 Oct 1998 15:22:27 BST Steve Parker
<spam_OUTSteve.ParkerTakeThisOuTspamSRC.BAE.CO.UK> writes:
>       Hi there folks!
>
>           I am currently contemplating the risk of damage to my humble
>       16C71JW. Basically I am powering it with 5V, but I have the o/p
>       of a dual rail OP AMP going to one of the ADC inputs. The OP AMP
>       is capable of swinging to +/- 9V and I am therefore worried. I
>       have put a 1K series resistance prior to the pin and a 5V6 Zener
>       from the pin to earth. However, this is a bit hit and miss and I
>       would love to be told the proper way to protect the i/p by some
>       experts.


       Since the PICs have internal clamp diodes, I'd say the zener is
not required.  I'd also set the resistor to the maximum the PIC A/D will
allow without error (10K, I believe).  I've got a circuit where one end
of a 24VCT transformer (center tap grounded) is driving the INT input of
a PIC through a 10K resistor.  The PIC clamls it great!

Harold



Harold Hallikainen
.....haroldKILLspamspam@spam@hallikainen.com
Hallikainen & Friends, Inc.
See the FCC Rules at http://hallikainen.com/FccRules and comments filed
in LPFM proceeding at http://hallikainen.com/lpfm




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1998\10\19@170654 by Bob Blick

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> >           I am currently contemplating the risk of damage to my humble
> >       16C71JW. Basically I am powering it with 5V, but I have the o/p
> >       of a dual rail OP AMP going to one of the ADC inputs. The OP AMP
> >       is capable of swinging to +/- 9V and I am therefore worried. I
> >       have put a 1K series resistance prior to the pin and a 5V6 Zener
> >       from the pin to earth. However, this is a bit hit and miss and I

There's two or three things you need to consider. The PIC has clamp diodes
to the supply rails, so you could just use a series resistor and forget
the zener. However, If your PIC and whatever loads it drives consumes less
than 3 milliamps, you could raise the PIC's supply rail when your opamp
drives high. You could increase the value of the resistor to 10k, and no
longer have that problem, but 10k is about 9k higher than I like to have
the source impedance to an ADC. You'll get errors on the order of 2 bits
under some conditions(especially on the channel next to the crystal pin).
You could put a .01 cap from the ADC pin to ground to zero out the high
impedance, but then you've increased the settling time you'll get out of
the opamp.

As your attorney <grin> I commend you on your choice of parts and advise
you to stick with the 1k resistor and 5.6v zener.

Cheers,
Bob

1998\10\19@193852 by Chris Eddy

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One more comment on this item, I have found that when a negative voltage is
exposed to a channel, the other analog channels go ballistic.  If you use
more than one channel on the 7X family, I strongly suggest using single
supply front end circuitry only.

Chris Eddy

Steve Parker wrote:

{Quote hidden}

1998\10\19@195351 by Dwayne Reid

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>           I am currently contemplating the risk of damage to my humble
>       16C71JW. Basically I am powering it with 5V, but I have the o/p
>       of a dual rail OP AMP going to one of the ADC inputs. The OP AMP
>       is capable of swinging to +/- 9V and I am therefore worried. I
>       have put a 1K series resistance prior to the pin and a 5V6 Zener
>       from the pin to earth. However, this is a bit hit and miss and I
>       would love to be told the proper way to protect the i/p by some
>       experts.

Personally, I use a 10K series resistor, sometines with the zener, mostly
without.  Be aware that voltages that exceed (VCC + 0.5V) or (GND - 0.5V)
can cause erroneous readings on other a/d inputs on the chip.  No damage -
just bad readings.

I don't like using zener clamps because of their leakage currents.

If I am multiplexing a large number of inputs into the 'c71 with external
muliplexors (4051 or 4052), I dedicate 1 mux input from each a/d channel
used on the '71 to a known voltage.  What I mean is: if I am using a pair of
muxes to 2 inputs on the '71, 1 input from each mux reads a known voltage
(1/3 VCC, 2/3 VCC).  If those voltages read wrong, it means that one or more
of the other MUX inputs is out of range and I can't trust any of the
readings.  I take action accordingly (show the error and go fail-safe).

Hope this helps.

dwayne


Dwayne Reid   <dwaynerspamKILLspamplanet.eon.net>
Trinity Electronics Systems Ltd    Edmonton, AB, CANADA
(403) 489-3199 voice     (403) 487-6397 fax

1998\10\19@230133 by Mike Keitz

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On Mon, 19 Oct 1998 19:53:51 -0400 Dwayne Reid <.....dwaynerKILLspamspam.....PLANET.EON.NET>
writes:

>Personally, I use a 10K series resistor, sometines with the zener,
>mostly
>without.  Be aware that voltages that exceed (VCC + 0.5V) or (GND -
>0.5V)
>can cause erroneous readings on other a/d inputs on the chip.  No
>damage -
>just bad readings.

>I don't like using zener clamps because of their leakage currents.

Put the zener on the other end of the resistor -- directly at the op-amp
output.  The PIC diodes will see only small currents through the resistor
since the voltage on the other side is only 5.6 V.  Nearly all op-amps
have current limited outputs (to 10 or 20 mA) that won't mind running
into a zener for a while.  The op amp's feedback will null out any
leakage through the zener.  Of course the supply current will increase
considerably while the output voltage is being clamped.

Better performance with the voltage at the negative limit may be possible
using a Schottky diode in parallel with the zener to clamp to a less
negative level.  The forward voltage drop of a zener is higher than even
a regular silicon diode.  Or place a pull-up resistor to 5V on the PIC
side so even when the op-amp output/diode voltage is negative, the PIC
input is positive.  With that circuit, the input voltage will be shifted
and rescaled some.

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1998\10\20@134405 by John Payson

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part 0 1494 bytes
I personally don't usually bother with zeners on PIC inputs, given the
existence of clamp diodes; a zener between VDD and ground may be a good
idea, though, since PICs generally don't like excessive VDD's.

The 1K resistor's probably a pretty good call; I had a 1K between the
tap on a pot (0-5V) and the PIC, and found it to be adequate even when
a ground loop put 120VAC on that pot; the 1K resistor blew open, but
the PIC was undamaged.  When people say PICs are indestructible, believe
'em.  The following is a rough catalog of non-OTP PIC's I've destroyed:

16C84 -- Accidentally connected RB6 to about 90VDC @unlimitted current.
 Remainder of chip worked fine, but couldn't reprogram without that
 pin.

16C622's (a few) -- Eventually wore out due to problem in programming
 software (the prog. pulses were sometimes too long).

12C508's -- Some OTP's of these failed for the same reason as the 622's
 above.

17C756 -- Mis-inserted into hand-built programming fixture (no current-
 limit on that one).  One of the bonding wires was sorta glowing (it
 was a window part--OUCH!)

16C925 -- Applied VPP without VDD and tried to program (was ICSP'ing and
 didn't have enough juice on my prog'ger to power VDD).

Plus probably a half dozen or so PICs of various types whose legs gave
out from too many insertion/removal cycles.  If you look through the
list, only one failure was due to something other than a programming or
handling failure, and that's going through a LOT of chips.

1998\10\20@154049 by Harold Hallikainen

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On Tue, 20 Oct 1998 12:42:38 -0500 John Payson <EraseMEsupercatspam_OUTspamTakeThisOuTCIRCAD.COM>
writes:

>'em.  The following is a rough catalog of non-OTP PIC's I've
>destroyed:
>
>16C84 -- Accidentally connected RB6 to about 90VDC @unlimitted
>current.
>  Remainder of chip worked fine, but couldn't reprogram without that
>  pin.
[deletia]
>17C756 -- Mis-inserted into hand-built programming fixture (no
>current-
>  limit on that one).  One of the bonding wires was sorta glowing (it
>  was a window part--OUCH!)
>


       Sure glad that wasn't a PicMaster Emulator!  Gets kinda
expensive!  In general, all my PIC stuff has only +5V on the board, so
damage possiblity is limited.  Recently I had a 16c74a driving a serial
DAC driving a 4051 demux, driving a 324 op amp.  Accidentally shorted the
+15V on the 324 to an input, blowing the 4051 and the DAC.  Luckily it
did not get back to the PicMaster!

Harold


Harold Hallikainen
haroldspamspam_OUThallikainen.com
Hallikainen & Friends, Inc.
See the FCC Rules at http://hallikainen.com/FccRules and comments filed
in LPFM proceeding at http://hallikainen.com/lpfm

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1998\10\20@174625 by John Payson
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part 0 1044 bytes
In all fairness to makers of emulators, how often is an emulator
useful for anything other than the code download ability (which
exists semi-natively in the 16C84/16F84 as well as many of Atmel's
parts)?  I have used emulators myself, and have on some occasions
made use of the live debugging facilities they offered, but often
I find the extra bulk of the emulator (having to balance everything
on the desk, etc.) to be a major nuisance.

I think the philosophy in the micro industry is that anyone who's
anyone will spend $10K-$20K on emulation equipment for any micro
they're going to use, and at the very bottom of the scale that
philosophy sorta makes sense (some 4-bit micros don't exist AT ALL
in anything other than mask-ROM versions).  In the PIC range, though,
I think in-circuit-erasable-programmable micros are in many cases as
useful as emulators, if not moreso (no giant ribbons hanging around);
I'm pained by the fact that we **STILL** don't have anything bigger
than the 16x84 in such a configuration...

[soap box mode off]



1998\10\20@215854 by Harold M Hallikainen

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On Tue, 20 Oct 1998 16:46:27 -0500 John Payson <@spam@supercatKILLspamspamCIRCAD.COM>
writes:
>In all fairness to makers of emulators, how often is an emulator
>useful for anything other than the code download ability (which
>exists semi-natively in the 16C84/16F84 as well as many of Atmel's
>parts)?  I have used emulators myself, and have on some occasions
>made use of the live debugging facilities they offered, but often
>I find the extra bulk of the emulator (having to balance everything
>on the desk, etc.) to be a major nuisance.
>


       I find emulators VERY useful.  I'm always using the WATCH, breakpoint,
and single step to figure out what my code is doing.  The only problem is
that the emulator for a 16c74 only runs at 4 MHz and my product requires
16 MHz (to receive data at 250 Kbps).  So I modified some of our DMX
generating and receiving equipment to run at 1/4 speed so I can see
what's going on.  Once it runs on the emulator, I move on to "crash and
burn" debugging, rotating my dozen erasable chips through the eraser and
the programmer 'til it works...
       I really like to see what's inside the chip using the emulator instead
of just staring at a dead product and trying to figure out where the code
went wrong.
       But... emulators ARE expensive!


Harold


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1998\10\21@015045 by paulb

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Mike Keitz wrote:

> Better performance with the voltage at the negative limit may be
> possible using a Schottky diode in parallel with the zener to clamp to
> a less negative level.  The forward voltage drop of a zener is higher
> than even a regular silicon diode.

 Seems to me the real answer if you don't mind buying the parts, is to
put a Schottky from the input to each rail, then put the Zener (5.6V or
perhaps 5.1V) across the supply.

 Minimal leakage, avoids problems of parasitics with the internal
"catch" or clamp diodes, and prevents the PIC supply from being jacked
up by an unreasonable amount.  Zener may not actually be necessary if
the regulator chip performs the function without complaint (up to a
reasonable number of mA).  But is that parameter in the spec sheets?
--
 Cheers,
       Paul B.

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