EEVblog #1328 - uCurrent OP189 Measurements

[Kleinstein]

The AZ OP to correct an normal amplifier would not be a normal DC servo to bring the DC output to zero, essentially simulation AC coupling, but a DC servo to force the input difference to zero, so correcting the offset. There is no principle problem with this - it just needs extra parts and in most implementations there will be a little more low frequency noise than just the AZ op. This is from extra resistors used for isolation to keep out the chopper spikes from the fast amplifier.
The AZ amplifier plus some resistors from the filter (some 10-100K) would set the noise at low frequencies like < 10-100 Hz and the fast amplifier would set the higher frequency noise.  Compared to just an AZ amplifier one usually has a little more noise at low frequencies, but less noise at higher frequencies (e.g. > 100 Hz).
Just an AZ OP may also need some filtering, though this can often be lower resistance.

The AZ OP to correct the error of a normal amplifier is nothing new and standard technique.

[SilverSolder]

[...]
The AZ OP to correct the error of a normal amplifier is nothing new and standard technique.

I found this example of...  something! -  in the LTC2057 data sheet.  Is that the kind of technique you are referring to?

[David Hess]

I found this example of...  something! -  in the LTC2057 data sheet.  Is that the kind of technique you are referring to?

Yes, that is the idea.  The integration constant controls the breakpoint frequency between the two amplifiers.  If you know the sensitivity of the offset null terminal, then you can calculate the optimum breakpoint frequency.

Many years ago I extended it to differential operation using a pair of LT1028s and an LTC1151 dual.  (1) First I measured the sensitivity of the LT1028's offset null and then calculated the optimum frequency breakpoint, which ended up being right on.  Noise performance was somewhere between incredibly good and impossibly good.

Linear Technology also published several examples of this with low noise JFET differential pairs which would be suitable if you also want low input bias current for higher source impedance.

(1) I used the dual LTC1151 because separate chopper stabilized amplifiers might have resulted in intermodulation between the clocks.  Modern automatic zero and chopper stabilized operational amplifiers lack any ability to control or synchronize their clock which suggests that this problem has been solved.

[SilverSolder]

[...] Noise performance was somewhere between incredibly good and impossibly good. [...]

So the chopper noise is completely suppressed in this design?

[David Hess]

[...] Noise performance was somewhere between incredibly good and impossibly good. [...]

So the chopper noise is completely suppressed in this design?

Yes, but the operating conditions for the chopper stabilized amplifier are ideal with very low AC impedance.  It is more like the broadband noise of the chopper is completely suppressed, or the flicker noise, including drift, of the main amplifier is completely suppressed.

I absolutely saw an improvement over either amplifier alone, and as I adjusted the frequency breakpoint between the amplifiers, I could directly measure an increase in 0.1 to 10 Hz noise, which is how I calibrated it.

[SilverSolder]

Any gotchas?   Bias current perhaps?

[Kleinstein]

Both OP still contribute to the noise: the chopper (+ a little from the filter) at low frequency and the other OP at higher frequency.

However the current noise of both OPs add. So the current noise will be the sum of both, possibly with some filtering for the chopper OP at really high frequencies. The bias current also adds - so one may want more like a JFET based OP.

The chopper still sees the input impedance direct. So ideally there should be some additional filtering at the positive input of the chopper (e.g. some 1 K to the input and some 100 pF to round or/end the neg side input). How the filter should look like can depend on the OP. I am still not sure about the choice here.

A slight difficulty is that many modern OPs don't have offset trim pins anymore. So the choice of OPs is not that large.

[sourcecharge]

Both OP still contribute to the noise: the chopper (+ a little from the filter) at low frequency and the other OP at higher frequency.

However the current noise of both OPs add. So the current noise will be the sum of both, possibly with some filtering for the chopper OP at really high frequencies. The bias current also adds - so one may want more like a JFET based OP.

The chopper still sees the input impedance direct. So ideally there should be some additional filtering at the positive input of the chopper (e.g. some 1 K to the input and some 100 pF to round or/end the neg side input). How the filter should look like can depend on the OP. I am still not sure about the choice here.

A slight difficulty is that many modern OPs don't have offset trim pins anymore. So the choice of OPs is not that large.

https://www.eevblog.com/forum/projects/modern-opamps-with-null-offset/

Seems you guys were trying to figure this out before.

Here are the ops that were listed in that thread:
OP277
LT1636
OPA192
OPA2192
Are any of these able to be used?

[SilverSolder]

OP277 - Rare, expensive
LT1636 - Slow
OPA192 - Does not actually have pins for offset adjustment
OPA2192 - Same as OPA192 (dual version) - no pins for offset


The LT1037 / LT1007 is still available... not cheap, but not stupid expensive either.

Maybe it is possible to design a composite amp that doesn't rely on offset adjustment pins?

[Kleinstein]

The simple circuit uses the offset trim pins, so the only OP from the list would be an OPA277 (not sure if there was an OP277.  With this type I am no so sure it would make much sense, as it is not that fast and low noise to start with.

It would be more like something like OP27, OP37, LT1037, AD8675 or LT1028 as relatively fast low noise BJT based ones or a low noise JFET based one. However many of the modern low noise ones don't have the trim pins. The AD8610 and ADA4627/ADA4637 are a few candidates that still have trim pins.
There are also some plans around to use a discrete JFET based amplifier with an AZ OP for zero stabilization.

[David Hess]

Maybe it is possible to design a composite amp that doesn't rely on offset adjustment pins?

In the inverting configuration, the non-inverting input can accept the correction signal.