Forums

opamp circuit offsets

Started by John Larkin 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 Innovations | et | | 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. F and 5F seem like a nice pair to start with. -- 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.