ChrioN

I'm toying arond with a precision dc circuit that requires something like a jfet op amp -
choppers and such are out of the question.

I've searched around and found nothing. How hard could it be?
Vos at unity gain is basically: Voltage at the output of the opamp - voltage at the input of the op amp.
Atleast in this case, because I don't have to worry about ibias and such.
So lets say we built a simple circuit with a couple of op amps in order to determine that difference.
We now know the offset voltage Then what? Input and output impedance should be kept alone.
Maybe applying a variable voltage to the feedback node? I have no clue.
« Last Edit: June 26, 2020, 12:38:18 PM by ChrioN »
"All PCB traces are curved to emulate the electrical response of point-to-point wiring" -Drip


Matador

I think you are looking for a 'DC servo':  for example:

https://sound-au.com/articles/dc-servo.htm

abbey road d enfer

I'm toying arond with a precision dc circuit that requires something like a jfet op amp -
choppers and such are out of the question.

I've searched around and found nothing. How hard could it be?
Vos at unity gain is basically: Voltage at the output of the opamp - voltage at the input of the op amp.
Atleast in this case, because I don't have to worry about ibias and such.
So lets say we built a simple circuit with a couple of op amps in order to determine that difference.
We now know the offset voltage Then what? Input and output impedance should be kept alone.
Maybe applying a variable voltage to the feedback node? I have no clue.
This is the exact description of a DC servo, as Matador said.
https://seventhcircleaudio.com/assets/J99B/J99BR11/docs/j99b_sch-194077baab99b50340100df094a2b74bacd4b939170c573984c975dc9930d807.pdf
Check what both sections of U3 do. One compensates offset for null output offset, the other for null input offset.
Who's right or wrong is irrelevant. What matters is what's right or wrong.
Star ground is for electricians.

moamps

This is the exact description of a DC servo, as Matador said.
https://seventhcircleaudio.com/assets/J99B/J99BR11/docs/j99b_sch-194077baab99b50340100df094a2b74bacd4b939170c573984c975dc9930d807.pdf
Check what both sections of U3 do. One compensates offset for null output offset, the other for null input offset.

Interesting design (with a drawback, IMO), an implementation of Jensen twin servo with another DOAs. As I see it, first servo sets the zero DC at output of the first DOA, second is used for second DOA. Both DOAs work as DC amplifiers so a servo is needed (two are maybe overkill).
The problem I see is that two decks of the switch should be BBM, last should be MBB. But all decks are BBM.

JohnRoberts

I'm toying arond with a precision dc circuit that requires something like a jfet op amp -
choppers and such are out of the question.
There are modern precision op amps (without choppers) that deliver good DC performance.
Quote
I've searched around and found nothing. How hard could it be?
How hard did you search...? I wrote about DC servos back in the 80s.
Quote
Vos at unity gain is basically: Voltage at the output of the opamp - voltage at the input of the op amp.
Atleast in this case, because I don't have to worry about ibias and such.

? with NF voltage at output should roughly equal + input voltage +/- input offset voltage.
Quote
So lets say we built a simple circuit with a couple of op amps in order to determine that difference.
differential..? but that differential amp can also have errors.
Quote
We now know the offset voltage Then what? Input and output impedance should be kept alone.
Maybe applying a variable voltage to the feedback node? I have no clue.
There is a well vetted simple servo approach using high quality film caps and op amp to buffer impedances. Perhaps moot in light of modern op amps with improved DC precision. (last time I used a DC servo was in the 80s but at the time it was a merchantable feature.)

I hope this helps.

=======
{TMI} now for today's TMI... My criticism of the DC servo(s) schematics posted is the topology. While this gets a little esoteric, I prefer a topology using a passive LPF RC feeding the servo op amp input. This way it is impossible to slew limit the servo op amp with step impulses. Even if the servo is fed from an existing circuit output, that output can be exposed to RF and high edge rate interference.  [/TMI]

JR

 
Don't only half-ass tune your drums. Visit https://circularscience.com to hear what properly "cleared" drums sound like.

Interesting design (with a drawback, IMO), an implementation of Jensen twin servo with another DOAs. As I see it, first servo sets the zero DC at output of the first DOA, second is used for second DOA. Both DOAs work as DC amplifiers so a servo is needed (two are maybe overkill).
The problem I see is that two decks of the switch should be BBM, last should be MBB. But all decks are BBM.

I've always thought that the twin servo thing was complete overkill, the name sounds great thou "Twin Servo"

ChrioN

Quote from: JohnRoberts
There are modern precision op amps (without choppers) that deliver good DC performance.

There definitely are! But I want to be cocky for now and say even they're not good enough.

Quote from: JohnRoberts
How hard did you search...?

Again, not quite good enough I suspect. DC servos I off course know of, but I was thinking maybe there was another way (and for sure there are if you want to involve digital solutions. But I find elegant analog ones to be the most elegant.

Quote from: JohnRoberts
? with NF voltage at output should roughly equal + input voltage +/- input offset voltage.

I think we are saying the same thing.

Quote from: JohnRoberts
differential..? but that differential amp can also have errors.

I was thinking of a good chopper for that part.
"All PCB traces are curved to emulate the electrical response of point-to-point wiring" -Drip

JohnRoberts

There definitely are! But I want to be cocky for now and say even they're not good enough.
I can't guess how good you need...
Quote
Again, not quite good enough I suspect. DC servos I off course know of, but I was thinking maybe there was another way (and for sure there are if you want to involve digital solutions.
I once considered using flash based microprocessors to read a DC offset and drive DPOTs to correct. Unclear how often you want to update. For low noise analog designs you may not want a micro running in the background. 
Quote
But I find elegant analog ones to be the most elegant.
Elegant is not a technical term.
Quote
I think we are saying the same thing.

I was thinking of a good chopper for that part.
I haven't looked at choppers for a long time and never used one myself. There is some concern about them leaking switching noise, but I suspect modern ones are quite good.

Perhaps share what your application is and design criteria?

JR
Don't only half-ass tune your drums. Visit https://circularscience.com to hear what properly "cleared" drums sound like.

ChrioN

Coming back to DC-servos - I always looked at them as something you used when you didn't want big dc offsets in your ac waveform = not something you use in dc circuits. I have very limited knowledge in these, I think I'll have to look them up further.


Quote from: JohnRoberts
I can't guess how good you need...[/qoute]
I can't say because I don't know yet. It would be nice to come close to a choppers 0.5-5uV.

Quote from: JohnRoberts
For low noise analog designs you may not want a micro running in the background. [/qoute]
If you do you're pcb layout right and use isolation ics, they're hardly noticable. Of course this becomes harder as circuit complexy grows.

Quote from: JohnRoberts
Elegant is not a technical term.
Typo, otherwise a wierd because the word comes up two times in a short sentence.

Quote from: JohnRoberts
I haven't looked at choppers for a long time and never used one myself. There is some concern about them leaking switching noise, but I suspect modern ones are quite good.

They are not quite good, some of the LT/AD ones are a amazing!

Quote from: JohnRoberts
Perhaps share what your application is and design criteria?

Basically an analog front end.
"All PCB traces are curved to emulate the electrical response of point-to-point wiring" -Drip

Coming back to DC-servos - I always looked at them as something you used when you didn't want big dc offsets in your ac waveform = not something you use in dc circuits. I have very limited knowledge in these, I think I'll have to look them up further.

DC servos are useless in DC circuits because the whole point of a servo is getting rid of the DC component, think of a servo as an alternative to a coupling capacitor. If you want to use something with a low offset without using chopper amps, you should try something like the LT6018 or LT1115 with 50uV offset voltage, thats basically as low as it gets without using choppers, also the noise voltage density is a stellar 0.9 nV/rootHz.  Do you need more DC precision than this? what for?

You mentioned you need FET opamps, is input bias/offset current an issue? are you going to be using medium to high resistances? you haven't told us your application.  FET opamps usually have much higher offset voltage than bipolars but better bias/offset current, the LT1112 has a slightly higher offset voltage (60uV) but a much lower input current 250pA, with its 0.3V/usec SR and 750 KHz. GBP it is a DC rather than audio opamp.  Is the DC component relevant in your design or not? if not, then a DC servo will do, but still, you have to take into consideration the offset voltage of the servo, how much offset voltage is allowed/tolerable at the output? all these questions would have more meaning if you tell us what you want to do.
« Last Edit: June 28, 2020, 09:42:10 AM by Dualflip »


Coming back to DC-servos - I always looked at them as something you used when you didn't want big dc offsets in your ac waveform = not something you use in dc circuits. I have very limited knowledge in these, I think I'll have to look them up further.


 'Designing Audio Power Amplifiers". by Bob Cordell has a chapter on the design and implementation of DC servos that's worth a read.
Covers the basics and also goes into more advanced schemes such as adding a 2nd pole.   
Note to self:  Don't let c**ts wind you up



 'Designing Audio Power Amplifiers". by Bob Cordell has a chapter on the design and implementation of DC servos that's worth a read.
Covers the basics and also goes into more advanced schemes such as adding a 2nd pole.

+1 on Bob's book, a great one indeed!

+1 on Bob's book, a great one indeed!

This. 

 :)
Note to self:  Don't let c**ts wind you up

clintrubber

Again, not quite good enough I suspect. DC servos I off course know of, but I was thinking maybe there was another way (and for sure there are if you want to involve digital solutions. But I find elegant analog ones to be the most elegant.

Curious to your application, or to which specs that need to be met.  Or an exercise in how far things can be brought?

I patented a 'digital' system for offset reduction in multi-channel amps, in the presence of signal.
In essence a digital DC-servo if you will, but no uP.
Intended for integration; for a discrete realization it's obviously unwieldly. 

ChrioN

Thats or the replies guys.

Hm. I thought I mentioned the application, but I may very well forgot to(o), as well. Its basically an analog front end for higher bit adcs. Most likely single ended for now, but for real white coat performance you need differential, in order to cancel out stuff like Seebeck effects. For now I'm trying to get the most precision/accuracy and stability that I can make, with the knowledge I have so far. Off the shelf parts is a must and I ain't trying to invent something new.
Input current bias/offset is very important.
I'm really having a blast doing this.
« Last Edit: July 05, 2020, 03:14:42 AM by ChrioN »
"All PCB traces are curved to emulate the electrical response of point-to-point wiring" -Drip

JohnRoberts

Thats or the replies guys.

Hm. I thought I mentioned the application, but I may very well forgot to(o), as well. Its basically an analog front end for higher bit adcs. Most likely single ended for now, but for real white coat performance you need differential, in order to cancel out stuff like Seebeck effects. For now I'm trying to get the most precision/accuracy and stability that I can make, with the knowledge I have so far. Off the shelf parts is a must and I ain't trying to invent something new.
Input current bias/offset is very important.
I'm really having a blast doing this.

What kind of time duration do you need this accuracy for? 

Operating in the digital domain with some rudimentary logic (maybe a small microprocessor) you can use the output of the ADC itself to perform a self calibration with input (shorted). Using digital pots you can correct a decent range of DC offset errors. This will be fine for relatively short term measurements, but long term stability has other variables, like temperature coefficient of components and even devices.

For serious stability you can detect the temperature of the circuit components and heat or cool as needed. Peltier cooling devices are a bit much so perhaps easier to heat everything to some nominal temperature above expected ambient that is easier to maintain.

If you don't need continuous  long term stability you could program an automatic self-calibration cycle (perhaps use a FET to short the input for these calibrations).

 JR
Don't only half-ass tune your drums. Visit https://circularscience.com to hear what properly "cleared" drums sound like.

I'm toying arond with a precision dc circuit that requires something like a jfet op amp -
choppers and such are out of the question.
I claim that choppers and such are not out of the question. The Analog ADA4522-1 or -2 or -4 are quite wonderful amplifiers for DC servos, and I have been using them successfully as integrators in DC servo circuits for the past few years with a variety of amplifier circuits. They are low cost zero drift (chopper) amplifiers, they have very low residual offset (~7µV), practically unmeasurable output spurious tones, and great basic linearity to start with.

Because of the zero drift "chopper" system, they also have extremely low 1/f noise, far less than any of the typical JFET amplifiers that get pressed into integrator service for a DC servo. The integrator stage can inject its 1/f noise into the controlled amplifier stage because of the loop gain structure of the servo and the controlled amp. So, it makes sense to select an integrator amplifier that has low 1/f noise, and low noise in general. The ADA4522-2 (the dual that I'm using to control a balanced amplifier) works quite well in regards to this noise.

Additionally, the two amplifiers in the ADA4522-2 are run with the same switching clock, so there is no chance of 'birdies' caused by slight inaccuracies in the switching rate of one amplifier relative to the other. In short, there is no way to make these amplifiers present measurable switching tones into an amplifier when used as a DC servo as an integrator amplifier. I have experimented with passive RC post filters, and they do not have any effect at all - there are no output spurs to eliminate with post filtering.

The biggest drawback of these amplifiers is a synthetically high input "bias current" caused by charge switching effects from the rapidly switching input stage. This is a CMOS amplifier with an extremely low inherent input bias current, but leakage currents are generated by the switching action of the zero-drift switching mechanism as it switches the input stage at the chopping frequency. These charge glitches, when impressed upon the input passives of an integrator, get lowpassed and act more like an input bias current than a switching noise.

These currents therefore require some caution when scaling the impedance of the components around the input stage, to prevent the total offset voltage from being increased excessively as this bias is impressed upon the input resistors. This means that larger capacitors and smaller resistors should be used around the input stage to minimize the effect of these "bias" currents. I'm using 100nF caps and 66K5Ω resistors in a noninverting integrator circuit, and this seems to increase the offset by only  a handful of microvolts - an acceptable range to me. This behavior seems to be modeled realistically by the SPICE model for the amplifier, so you can use SPICE to see how your overall design will perform relative to these "bias currents".

If you've designed filters before, you're probably thinking that the time constant provided by an integrator using 100nF and 66K5Ω is not far enough below the audio band (-3dB is around 24Hz). So, how does this work out in a DC servo, where the -3dB point ought to be only a few Hz at greatest? This trick is done by attenuating the "force" output of the DC servo, so that the integrator amplifier is attenuated into the input of the controlled amplifier. This attenuation factor increases the servo's time constant by an amount proportional to the attenuation factor. With this 24Hz -3dB integrator, I typically use a 20:1 output attenuation and then end up with a ~1.2Hz corner frequency of the controlled amplifier, and a generally acceptable time response of the controlled amplifier.

The DC servo drives an inverting amplifier with a single resistor into the controlled amplifier's inverting input node. The inverter uses a 10kΩ feedback resistor, so I feed the DC servo integrator output signal into the amplifier's inverting node with a 200kΩ resistor to get the 20:1 attenuation.

The tradeoff of this output attenuation is that the correction range of the servo is diminished by the servo output attenuation factor. With ±15V supplies, the 20:1 attenuation means a correction range around ±749mV. Still, for these amplifiers in this application, this is completely sufficient.

On the positive side, the noise of the servo integrator stage is also reduced by the same factor. With a 20:1 attenuation, this results in no appreciable added noise from the servo, even with extremely low noise stages.

A final and significant benefit of all of this is that the integrator requires only a 100nF integrator capacitor. These capacitors can then be a small 3216 size C0G SMD capacitor, which is extremely tiny, extremely linear,  non-microphonic, and extremely reliable compared to a film integrator cap.

Overall, using a modern zero drift amplifier as an integrator with carefully sized passive components can result in a very high performance integrator at low cost and small PCB area. IMHO you should indeed consider using them.

Thanks Monte, good stuff.
Note to self:  Don't let c**ts wind you up

Don't forget you can always trim the offset of an already low offset opamp, not very practical for large production but I don't think thats your case.
« Last Edit: July 07, 2020, 11:17:56 PM by Dualflip »

gyraf

..I raise you with a single-node servo cct, attached...

/Jakob E.
..note to self: don't let Harman run your company..


 

Related Topics

  Subject / Started by Replies Last post
11 Replies
3114 Views
Last post October 20, 2004, 02:47:38 AM
by Samuel Groner
4 Replies
3934 Views
Last post February 11, 2005, 03:20:38 PM
by volta
1 Replies
1603 Views
Last post November 28, 2005, 02:35:42 AM
by PRR
5 Replies
2849 Views
Last post July 10, 2006, 02:43:58 AM
by gyraf