opamp circuit offsets

Started by April 15, 2017
```I have a fairly bizarre 4-opamp circuit with differential output, and
I need to calculate the worst-case DC offset. The math is a nuisance,
and I'll have to repeat it if I change anything. So, Spice.

https://dl.dropboxusercontent.com/u/53724080/Circuits/Amps/E2_Offsets.jpg

The issue is now how to program the four opamp DC offset sources. One
way is to use four asynchronous sine waves, and run it and see how
things eventually spike. Sort of a cheap Monte Carlo.

There must be some number-theory choice of frequencies that's best,
but a few guesses seem to make peak DIFF offsets just below +-8 mV, so
that seems to be the answer. Eyeballing the graph sort of suggests the
probability that any given offset could ever really happen. I doubt
that we'd ever have all four amps at 2 mV in the worst-case
directions; the typ offset is 0.5 mV. So I guess I'll never see more
than, say, +-5 out.

--

John Larkin         Highland Technology, Inc

lunatic fringe electronics

```
```Just put 1 mV in series with each noninverting input in turn. That'll give you the partial derivatives, so it's simple to figure out what the worst case is (plus or minus the datasheet maximum, signs chosen so they all add up.

Cheers

Phil Hobbs
```
```On Sat, 15 Apr 2017 07:03:16 -0700, John Larkin
<jjlarkin@highlandtechnology.com> wrote:

>
>I have a fairly bizarre 4-opamp circuit with differential output, and
>I need to calculate the worst-case DC offset. The math is a nuisance,
>and I'll have to repeat it if I change anything. So, Spice.
>
>https://dl.dropboxusercontent.com/u/53724080/Circuits/Amps/E2_Offsets.jpg
>
>The issue is now how to program the four opamp DC offset sources. One
>way is to use four asynchronous sine waves, and run it and see how
>things eventually spike. Sort of a cheap Monte Carlo.
>
>There must be some number-theory choice of frequencies that's best,
>but a few guesses seem to make peak DIFF offsets just below +-8 mV, so
>that seems to be the answer. Eyeballing the graph sort of suggests the
>probability that any given offset could ever really happen. I doubt
>that we'd ever have all four amps at 2 mV in the worst-case
>directions; the typ offset is 0.5 mV. So I guess I'll never see more
>than, say, +-5 out.

Binary.  Assign each offset to a bit position, then count.

...Jim Thompson
--
| James E.Thompson                                 |    mens     |
| Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    |
| STV, Queen Creek, AZ 85142    Skype: skypeanalog |             |
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  |
| E-mail Icon at http://www.analog-innovations.com |    1962     |

Thinking outside the box... producing elegant solutions.
```
```On Sat, 15 Apr 2017 07:34:08 -0700 (PDT), pcdhobbs@gmail.com wrote:

>Just put 1 mV in series with each noninverting input in turn. That'll give you the partial derivatives, so it's simple to figure out what the worst case is (plus or minus the datasheet maximum, signs chosen so they all add up.
>
>Cheers
>
>Phil Hobbs

Too much work.

--

John Larkin         Highland Technology, Inc

lunatic fringe electronics

```
```On Sat, 15 Apr 2017 07:43:30 -0700, Jim Thompson
<To-Email-Use-The-Envelope-Icon@On-My-Web-Site.com> wrote:

>On Sat, 15 Apr 2017 07:03:16 -0700, John Larkin
><jjlarkin@highlandtechnology.com> wrote:
>
>>
>>I have a fairly bizarre 4-opamp circuit with differential output, and
>>I need to calculate the worst-case DC offset. The math is a nuisance,
>>and I'll have to repeat it if I change anything. So, Spice.
>>
>>https://dl.dropboxusercontent.com/u/53724080/Circuits/Amps/E2_Offsets.jpg
>>
>>The issue is now how to program the four opamp DC offset sources. One
>>way is to use four asynchronous sine waves, and run it and see how
>>things eventually spike. Sort of a cheap Monte Carlo.
>>
>>There must be some number-theory choice of frequencies that's best,
>>but a few guesses seem to make peak DIFF offsets just below +-8 mV, so
>>that seems to be the answer. Eyeballing the graph sort of suggests the
>>probability that any given offset could ever really happen. I doubt
>>that we'd ever have all four amps at 2 mV in the worst-case
>>directions; the typ offset is 0.5 mV. So I guess I'll never see more
>>than, say, +-5 out.
>
>Binary.  Assign each offset to a bit position, then count.
>
>                                        ...Jim Thompson

Too much work.

--

John Larkin         Highland Technology, Inc

lunatic fringe electronics

```
```On Apr 15, 2017, John Larkin wrote
(in article<b994fc9ci7539tubi7ui2950as70ibk8s2@4ax.com>):

>
> I have a fairly bizarre 4-opamp circuit with differential output, and
> I need to calculate the worst-case DC offset. The math is a nuisance,
> and I'll have to repeat it if I change anything. So, Spice.
>
> https://dl.dropboxusercontent.com/u/53724080/Circuits/Amps/E2_Offsets.jpg
>
> The issue is now how to program the four opamp DC offset sources. One
> way is to use four asynchronous sine waves, and run it and see how
> things eventually spike. Sort of a cheap Monte Carlo.
>
> There must be some number-theory choice of frequencies that's best,
> but a few guesses seem to make peak DIFF offsets just below +-8 mV, so
> that seems to be the answer. Eyeballing the graph sort of suggests the
> probability that any given offset could ever really happen. I doubt
> that we'd ever have all four amps at 2 mV in the worst-case
> directions; the typ offset is 0.5 mV. So I guess I'll never see more
> than, say, +-5 out.

This approach is an application of multisine test signals, where one
synthesizes a fairly random wideband signal using a finite set of sine-wave
test tones.

There are many ways to choose tone frequencies and their amplitudes. There is
a large literature. Lots in IEEE Trans on Instrumentation and Measurement.

One common dodge is to make the amplitudes of the test tones vary with
frequency such that the SNR of all tones are the same despite the
differentiating effect of say a transformer or the like.

NIST uses multisines generated from a Josephson-junction voltage source to
measure known-property near-gaussian noise to calibrate Johnson-noise based
absolute thermometers.
For your application, I&rsquo;d chose at least five test tones whose periods are
mutually prime.

Joe Gwinn

```
```On Sat, 15 Apr 2017 11:09:07 -0400, Joseph Gwinn
<joegwinn@comcast.net> wrote:

>On Apr 15, 2017, John Larkin wrote
>(in article<b994fc9ci7539tubi7ui2950as70ibk8s2@4ax.com>):
>
>>
>> I have a fairly bizarre 4-opamp circuit with differential output, and
>> I need to calculate the worst-case DC offset. The math is a nuisance,
>> and I'll have to repeat it if I change anything. So, Spice.
>>
>> https://dl.dropboxusercontent.com/u/53724080/Circuits/Amps/E2_Offsets.jpg
>>
>> The issue is now how to program the four opamp DC offset sources. One
>> way is to use four asynchronous sine waves, and run it and see how
>> things eventually spike. Sort of a cheap Monte Carlo.
>>
>> There must be some number-theory choice of frequencies that's best,
>> but a few guesses seem to make peak DIFF offsets just below +-8 mV, so
>> that seems to be the answer. Eyeballing the graph sort of suggests the
>> probability that any given offset could ever really happen. I doubt
>> that we'd ever have all four amps at 2 mV in the worst-case
>> directions; the typ offset is 0.5 mV. So I guess I'll never see more
>> than, say, +-5 out.
>
>This approach is an application of multisine test signals, where one
>synthesizes a fairly random wideband signal using a finite set of sine-wave
>test tones.
>
>There are many ways to choose tone frequencies and their amplitudes. There is
>a large literature. Lots in IEEE Trans on Instrumentation and Measurement.
>
>One common dodge is to make the amplitudes of the test tones vary with
>frequency such that the SNR of all tones are the same despite the
>differentiating effect of say a transformer or the like.
>
>NIST uses multisines generated from a Josephson-junction voltage source to
>measure known-property near-gaussian noise to calibrate Johnson-noise based
>absolute thermometers.
>For your application, I&#2013266066;d chose at least five test tones whose periods are
>mutually prime.
>
>Joe Gwinn

I only need four offset sources. I want them to occasionally peak
simultaneously, positive and negative.

--

John Larkin         Highland Technology, Inc

lunatic fringe electronics

```
```On 04/15/2017 10:03 AM, John Larkin wrote:
>
> I have a fairly bizarre 4-opamp circuit with differential output, and
> I need to calculate the worst-case DC offset. The math is a nuisance,
> and I'll have to repeat it if I change anything. So, Spice.
>
> https://dl.dropboxusercontent.com/u/53724080/Circuits/Amps/E2_Offsets.jpg
>
> The issue is now how to program the four opamp DC offset sources. One
> way is to use four asynchronous sine waves, and run it and see how
> things eventually spike. Sort of a cheap Monte Carlo.
>
> There must be some number-theory choice of frequencies that's best,
> but a few guesses seem to make peak DIFF offsets just below +-8 mV, so
> that seems to be the answer. Eyeballing the graph sort of suggests the
> probability that any given offset could ever really happen. I doubt
> that we'd ever have all four amps at 2 mV in the worst-case
> directions; the typ offset is 0.5 mV. So I guess I'll never see more
> than, say, +-5 out.

Make the offset sources Gaussian noise uncorrelated for each trial, run
a bunch of parallel simulations and assume the Central Limit Theorem to
find the mean and variance of the Gaussian distribution of the sum of
random variables to your required confidence, then work it "backwards"
to find the best fit mean and variance for the unknown distribution
you're pulling "samples" from

I will assume that's too much work, though. :(
```
```On 04/15/2017 12:23 PM, bitrex wrote:
> On 04/15/2017 10:03 AM, John Larkin wrote:
>>
>> I have a fairly bizarre 4-opamp circuit with differential output, and
>> I need to calculate the worst-case DC offset. The math is a nuisance,
>> and I'll have to repeat it if I change anything. So, Spice.
>>
>> https://dl.dropboxusercontent.com/u/53724080/Circuits/Amps/E2_Offsets.jpg
>>
>> The issue is now how to program the four opamp DC offset sources. One
>> way is to use four asynchronous sine waves, and run it and see how
>> things eventually spike. Sort of a cheap Monte Carlo.
>>
>> There must be some number-theory choice of frequencies that's best,
>> but a few guesses seem to make peak DIFF offsets just below +-8 mV, so
>> that seems to be the answer. Eyeballing the graph sort of suggests the
>> probability that any given offset could ever really happen. I doubt
>> that we'd ever have all four amps at 2 mV in the worst-case
>> directions; the typ offset is 0.5 mV. So I guess I'll never see more
>> than, say, +-5 out.
>
> Make the offset sources Gaussian noise uncorrelated for each trial

Or multitone sine wave signals if you want to strictly limit bandwidth
or whatevs

```
```On Saturday, April 15, 2017 at 10:03:27 AM UTC-4, John Larkin wrote:
> I have a fairly bizarre 4-opamp circuit with differential output, and
> I need to calculate the worst-case DC offset. The math is a nuisance,
> and I'll have to repeat it if I change anything. So, Spice.
>
> https://dl.dropboxusercontent.com/u/53724080/Circuits/Amps/E2_Offsets.jpg
>
> The issue is now how to program the four opamp DC offset sources. One
> way is to use four asynchronous sine waves, and run it and see how
> things eventually spike. Sort of a cheap Monte Carlo.
>
> There must be some number-theory choice of frequencies that's best,
> but a few guesses seem to make peak DIFF offsets just below +-8 mV, so
> that seems to be the answer. Eyeballing the graph sort of suggests the
> probability that any given offset could ever really happen. I doubt
> that we'd ever have all four amps at 2 mV in the worst-case
> directions; the typ offset is 0.5 mV. So I guess I'll never see more
> than, say, +-5 out.
>
>
> --
>
> John Larkin         Highland Technology, Inc
>
> lunatic fringe electronics

Add 'em all up, worse case, then take the sqrt of that number.
(I guess that doesn't work.)

I use a lot of dual opamps, I've always wondered if the
offset's would be better if I made one inverting and the
other non.
(usually after the first stage one care's less about the offset,
unless it's a filter.)

George H.
```