Help with 7081 AC buffer circuit
Guys 'n gals,
I'm looking for some assistance with this circuit. The OP16 op-amp has an offset trim pot which I can use to adjust the output DC offset with the input to the meter shorted - so far so good. The catch is that it's really very noisy, wandering by as much as 20-100 uV in less than half a second, and wandering around the zero set point by as much as +/- 800uV over a period of time (say an hour).
This is much better than it was before I cooked the PCB for 6 hours at 120 celsius, as before I did that there was a huge monotonic drift as the PCB warmed up with the result that after an hour or so the DC output of the buffer circuit was many millivolts off.
The schematic is sheet 70817506 Sheet 4. In the soft copy of the service manual which can be found here:
http://www.ko4bb.com/manuals/index.php?dir=09%29_Misc_Test_Equipment/Solartron
it is on pages 117 and 118.
I'm looking for any suggestions as to what component in this circuit is likely to causing this problem.
Thanks
David Partridge
David,
Try enclosing the op-amp and nearby components with a little Styrofoam
"hat". The part dissipates a fair amount of power and the bias current,
although small, does show significant change with temperature. This change
multiplied by the 1 megohm input resistor may be responsible for some drift.
Regards,
mitch
----- Original Message -----
From: "David C. Partridge" david.partridge@perdrix.co.uk
To: "'Discussion of precise voltage measurement'" volt-nuts@febo.com
Sent: Friday, June 08, 2012 12:46 PM
Subject: [volt-nuts] Help with 7081 AC buffer circuit
Guys 'n gals,
I'm looking for some assistance with this circuit. The OP16 op-amp has an
offset trim pot which I can use to adjust the output DC offset with the
input to the meter shorted - so far so good. The catch is that it's
really very noisy, wandering by as much as 20-100 uV in less than half a
second, and wandering around the zero set point by as much as +/- 800uV
over a period of time (say an hour).
This is much better than it was before I cooked the PCB for 6 hours at 120
celsius, as before I did that there was a huge monotonic drift as the PCB
warmed up with the result that after an hour or so the DC output of the
buffer circuit was many millivolts off.
The schematic is sheet 70817506 Sheet 4. In the soft copy of the service
manual which can be found here:
http://www.ko4bb.com/manuals/index.php?dir=09%29_Misc_Test_Equipment/Solartron
it is on pages 117 and 118.
I'm looking for any suggestions as to what component in this circuit is
likely to causing this problem.
Thanks
David Partridge
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To unsubscribe, go to
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Mitch, I don't think that's likely to be the problem, if it were, all the 7081s and 7071s would have a huge problem, but they don't, just this one ...
Regards,
David Partridge
-----Original Message-----
From: volt-nuts-bounces@febo.com [mailto:volt-nuts-bounces@febo.com] On Behalf Of Mitch Van Ochten
Sent: 08 June 2012 20:49
To: Discussion of precise voltage measurement
Subject: Re: [volt-nuts] Help with 7081 AC buffer circuit
David,
Try enclosing the op-amp and nearby components with a little Styrofoam "hat". The part dissipates a fair amount of power and the bias current, although small, does show significant change with temperature. This change multiplied by the 1 megohm input resistor may be responsible for some drift.
Regards,
mitch
David,
I would approach it from this perspective (and I suspect you have already
done this as well).
-
Is the problem on this PCB? Do you have another board to swap with? If
not, where else could the problem be? Obviously, all the connections and a
'soaked' PCB would be problematic. Time for another 'bake'? -
If it is on that PCB, can you replace or clean the pot? Can you replace
the Op Amp?
Hope this helps but suspect you have already done these.
Joe
-----Original Message-----
From: volt-nuts-bounces@febo.com [mailto:volt-nuts-bounces@febo.com] On
Behalf Of David C. Partridge
Sent: Friday, June 08, 2012 11:46 AM
To: 'Discussion of precise voltage measurement'
Subject: [volt-nuts] Help with 7081 AC buffer circuit
Guys 'n gals,
I'm looking for some assistance with this circuit. The OP16 op-amp has an
offset trim pot which I can use to adjust the output DC offset with the
input to the meter shorted - so far so good. The catch is that it's really
very noisy, wandering by as much as 20-100 uV in less than half a second,
and wandering around the zero set point by as much as +/- 800uV over a
period of time (say an hour).
This is much better than it was before I cooked the PCB for 6 hours at 120
celsius, as before I did that there was a huge monotonic drift as the PCB
warmed up with the result that after an hour or so the DC output of the
buffer circuit was many millivolts off.
The schematic is sheet 70817506 Sheet 4. In the soft copy of the service
manual which can be found here:
<http://www.ko4bb.com/manuals/index.php?dir=09%29_Misc_Test_Equipment/Solart
ron>
it is on pages 117 and 118.
I'm looking for any suggestions as to what component in this circuit is
likely to causing this problem.
Thanks
David Partridge
volt-nuts mailing list -- volt-nuts@febo.com
To unsubscribe, go to
https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts
and follow the instructions there.
Does that matter on the AC buffer in this case? Maybe that's the
normal 1/f noise of the circuit down near DC.
If it is problematic, then you may want to try reconditioning the
low level signal relay contacts. You can rig up a circuit with a 9V
battery and a 1 k or so ohm resistor, temporarily hooked up across
each contact, then run the relay through a few dozen cycles. If you
do that with each contact in the signal path, it may clean it up.
You may have to disconnect some of them to avoid damage to the
circuits - especially at the OP-16 input - it depends on the
overload rating. You may be able to first see if it's worth doing
this laborious process by very gently tapping each relay body to see
if there's any response.
Also, if you are working on the circuit while it's out in the open,
you could have light generating currents in some of the devices, so
make sure it's protected.
Lastly, the zero adjust pot itself could be the culprit - try running
it to each extreme a few dozen times to clean the wiper and element.
Ed
----- Original Message ----- From: "David C. Partridge"
david.partridge@perdrix.co.uk
To: "'Discussion of precise voltage measurement'" volt-nuts@febo.com
Sent: Friday, June 08, 2012 12:46 PM
Subject: [volt-nuts] Help with 7081 AC buffer circuit
Guys 'n gals,
I'm looking for some assistance with this circuit. The OP16 op-amp
has an offset trim pot which I can use to adjust the output DC
offset with the input to the meter shorted - so far so good. The
catch is that it's really very noisy, wandering by as much as
20-100 uV in less than half a second, and wandering around the zero
set point by as much as +/- 800uV over a period of time (say an hour).
This is much better than it was before I cooked the PCB for 6 hours
at 120 celsius, as before I did that there was a huge monotonic
drift as the PCB warmed up with the result that after an hour or so
the DC output of the buffer circuit was many millivolts off.
The schematic is sheet 70817506 Sheet 4. In the soft copy of the
service manual which can be found here:
http://www.ko4bb.com/manuals/index.php?dir=09%29_Misc_Test_Equipment/Solartron
it is on pages 117 and 118.
I'm looking for any suggestions as to what component in this
circuit is likely to causing this problem.
Thanks
David Partridge
volt-nuts mailing list -- volt-nuts@febo.com
To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts
and follow the instructions there.
volt-nuts mailing list -- volt-nuts@febo.com
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and follow the instructions there.
Yes the level here does matter, any output (DC or AC) at the test point in question is DC coupled into the RMS converter which responds to both AC and DC.
The calibration specification says:
"Monitor the DC level at TP705 "Buffer O/P" on the 100mV AC range. Adjust RV701 for a minimum value between +/- 100uV."
Yes, suggesting the erratic behaviour could be the potentiometer makes good sense. I don't think I fancy running a 25 turn pot back and forth through its entire adjustment range a few dozen times though - I'll borrow one from another meter and report back.
I've swapped the OP16 with another meter already.
I'm working on it with the lid in place so light sensitivity shouldn't be a problem.
Regards,
David Partridge
-----Original Message-----
From: volt-nuts-bounces@febo.com [mailto:volt-nuts-bounces@febo.com] On Behalf Of ed breya
Sent: 09 June 2012 07:39
To: Discussion of precise voltage measurement
Subject: Re: [volt-nuts] Help with 7081 AC buffer circuit
Does that matter on the AC buffer in this case? Maybe that's the normal 1/f noise of the circuit down near DC.
If it is problematic, then you may want to try reconditioning the low
level signal relay contacts. You can rig up a circuit with a 9V battery
and a 1 k or so ohm resistor, temporarily hooked up across each
contact, then run the relay through a few dozen cycles. If you do that
with each contact in the signal path, it may clean it up.
You may have to disconnect some of them to avoid damage to the circuits
- especially at the OP-16 input - it depends on the overload rating.
You may be able to first see if it's worth doing this laborious process
by very gently tapping each relay body to see if there's any response.
Also, if you are working on the circuit while it's out in the open, you could have light generating currents in some of the devices, so make sure it's protected.
Lastly, the zero adjust pot itself could be the culprit - try running it to each extreme a few dozen times to clean the wiper and element.
Ed
----- Original Message ----- From: "David C. Partridge"
david.partridge@perdrix.co.uk
To: "'Discussion of precise voltage measurement'" volt-nuts@febo.com
Sent: Friday, June 08, 2012 12:46 PM
Subject: [volt-nuts] Help with 7081 AC buffer circuit
Guys 'n gals,
I'm looking for some assistance with this circuit. The OP16 op-amp
has an offset trim pot which I can use to adjust the output DC offset
with the input to the meter shorted - so far so good. The catch is
that it's really very noisy, wandering by as much as 20-100 uV in
less than half a second, and wandering around the zero set point by as
much as +/- 800uV over a period of time (say an hour).
This is much better than it was before I cooked the PCB for 6 hours at
120 celsius, as before I did that there was a huge monotonic drift as
the PCB warmed up with the result that after an hour or so the DC
output of the buffer circuit was many millivolts off.
The schematic is sheet 70817506 Sheet 4. In the soft copy of the
service manual which can be found here:
<http://www.ko4bb.com/manuals/index.php?dir=09%29_Misc_Test_Equipment/
Solartron>
it is on pages 117 and 118.
I'm looking for any suggestions as to what component in this circuit
is likely to causing this problem.
Thanks
David Partridge
volt-nuts mailing list -- volt-nuts@febo.com To unsubscribe, go to
https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts
and follow the instructions there.
volt-nuts mailing list -- volt-nuts@febo.com To unsubscribe, go to
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and follow the instructions there.
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and follow the instructions there.
Changing the potentiometer was indeed the solution - thanks to Ed and to John (whose post never made it here, though I found it on the site).
The DC level at TP705 is now relatively stable and close to 0uV (jumping around a bit but typically well less than 100uV).
Now that I get a stable reading near 0uV here, I see another problem :(
If I then switch from the VAC to the VAC plus VDC setting, the DC output level at TP705 jumps positive by about 600uV, which results in a difference in reading of around 60uV which I don't believe should happen. Strangely, if I leave the input open circuit, I can switch between VAC and VAC+VDC modes without this shift occurring.
I re-wired the circuitry around the relays using PTFE feed-through insulators and direct point to point wiring to eliminate leakage problems between the relay power lines (-10V, -15V) and the signals before I baked the PCB.
Regards,
David Partridge
-----Original Message-----
From: volt-nuts-bounces@febo.com [mailto:volt-nuts-bounces@febo.com] On Behalf Of ed breya
Sent: 09 June 2012 07:39
To: Discussion of precise voltage measurement
Subject: Re: [volt-nuts] Help with 7081 AC buffer circuit
Does that matter on the AC buffer in this case? Maybe that's the normal 1/f noise of the circuit down near DC.
If it is problematic, then you may want to try reconditioning the low
level signal relay contacts. You can rig up a circuit with a 9V battery
and a 1 k or so ohm resistor, temporarily hooked up across each
contact, then run the relay through a few dozen cycles. If you do that
with each contact in the signal path, it may clean it up.
You may have to disconnect some of them to avoid damage to the circuits
- especially at the OP-16 input - it depends on the overload rating.
You may be able to first see if it's worth doing this laborious process
by very gently tapping each relay body to see if there's any response.
Also, if you are working on the circuit while it's out in the open, you could have light generating currents in some of the devices, so make sure it's protected.
Lastly, the zero adjust pot itself could be the culprit - try running it to each extreme a few dozen times to clean the wiper and element.
Ed
----- Original Message ----- From: "David C. Partridge"
david.partridge@perdrix.co.uk
To: "'Discussion of precise voltage measurement'" volt-nuts@febo.com
Sent: Friday, June 08, 2012 12:46 PM
Subject: [volt-nuts] Help with 7081 AC buffer circuit
Guys 'n gals,
I'm looking for some assistance with this circuit. The OP16 op-amp
has an offset trim pot which I can use to adjust the output DC offset
with the input to the meter shorted - so far so good. The catch is
that it's really very noisy, wandering by as much as 20-100 uV in
less than half a second, and wandering around the zero set point by as
much as +/- 800uV over a period of time (say an hour).
This is much better than it was before I cooked the PCB for 6 hours at
120 celsius, as before I did that there was a huge monotonic drift as
the PCB warmed up with the result that after an hour or so the DC
output of the buffer circuit was many millivolts off.
The schematic is sheet 70817506 Sheet 4. In the soft copy of the
service manual which can be found here:
<http://www.ko4bb.com/manuals/index.php?dir=09%29_Misc_Test_Equipment/
Solartron>
it is on pages 117 and 118.
I'm looking for any suggestions as to what component in this circuit
is likely to causing this problem.
Thanks
David Partridge
volt-nuts mailing list -- volt-nuts@febo.com To unsubscribe, go to
https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts
and follow the instructions there.
volt-nuts mailing list -- volt-nuts@febo.com To unsubscribe, go to
https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts
and follow the instructions there.
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and follow the instructions there.
Argghhhh! Changing the pot didn't fix it - once the meter had warmed up for > 1hr it was backto its old tricks.
Dave
-----Original Message-----
From: volt-nuts-bounces@febo.com [mailto:volt-nuts-bounces@febo.com] On Behalf Of David C. Partridge
Sent: 09 June 2012 19:02
To: 'Discussion of precise voltage measurement'
Subject: Re: [volt-nuts] Help with 7081 AC buffer circuit
Changing the potentiometer was indeed the solution - thanks to Ed and to John (whose post never made it here, though I found it on the site).
The DC level at TP705 is now relatively stable and close to 0uV (jumping around a bit but typically well less than 100uV).
Now that I get a stable reading near 0uV here, I see another problem :(
If I then switch from the VAC to the VAC plus VDC setting, the DC output level at TP705 jumps positive by about 600uV, which results in a difference in reading of around 60uV which I don't believe should happen. Strangely, if I leave the input open circuit, I can switch between VAC and VAC+VDC modes without this shift occurring.
I re-wired the circuitry around the relays using PTFE feed-through insulators and direct point to point wiring to eliminate leakage problems between the relay power lines (-10V, -15V) and the signals before I baked the PCB.
Regards,
David Partridge
Well that is a clue within itself. Use a can of circuit freeze very lightly on a few components, especially resistors and capacitors, and see if one in particular component quietens the noise. Sounds like you have a case of Johnson's noise aggravated with warm up.
Don
----- Original Message -----
From: David C. Partridge
To: 'Discussion of precise voltage measurement'
Sent: Saturday, June 09, 2012 8:43 PM
Subject: Re: [volt-nuts] Help with 7081 AC buffer circuit
Argghhhh! Changing the pot didn't fix it - once the meter had warmed up for > 1hr it was backto its old tricks.
Dave
-----Original Message-----
From: volt-nuts-bounces@febo.com [mailto:volt-nuts-bounces@febo.com] On Behalf Of David C. Partridge
Sent: 09 June 2012 19:02
To: 'Discussion of precise voltage measurement'
Subject: Re: [volt-nuts] Help with 7081 AC buffer circuit
Changing the potentiometer was indeed the solution - thanks to Ed and to John (whose post never made it here, though I found it on the site).
The DC level at TP705 is now relatively stable and close to 0uV (jumping around a bit but typically well less than 100uV).
Now that I get a stable reading near 0uV here, I see another problem :(
If I then switch from the VAC to the VAC plus VDC setting, the DC output level at TP705 jumps positive by about 600uV, which results in a difference in reading of around 60uV which I don't believe should happen. Strangely, if I leave the input open circuit, I can switch between VAC and VAC+VDC modes without this shift occurring.
I re-wired the circuitry around the relays using PTFE feed-through insulators and direct point to point wiring to eliminate leakage problems between the relay power lines (-10V, -15V) and the signals before I baked the PCB.
Regards,
David Partridge
volt-nuts mailing list -- volt-nuts@febo.com
To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts
and follow the instructions there.