EEVblog #1328 - uCurrent OP189 Measurements

[EEVblog]

Part 3 of designing a better uCurrent series.
Measuring the noise and consumption of the OPA189 compared to the MAX4239 using a dynamic signal analyser and an oscilloscope.

[Kleinstein]

The test with the split rail shown +-2.3 V. This is quite close to the lower 4.5 V limit for the OPA189 - noise may be a little higher there. It may be more the higher supply with the 2x3 V supply instead of the rail splitting circuit.

Ideally the noise of the OPA189 should be about equivalent to the noise of a 1.6 K resistor. It is kind of convenient to add noise sources as the corresponding resistance, as the take care of adding the powers and not the votlages. The  1 K resistor in the feedback network would be another small contribution to the noise. There is still a bit missing to the measured 10 nV/Sqrt(Hz) that corresponds to some 6 K.

For modding an existing µCurrent it would be enough to replace the OP at the input - the noise of the 2 nd OP would not matter that much.  Some filtering at the output would be a good idea, as not all scopes have BW limits much lower then 20 MHz.

[EEVblog]

The test with the split rail shown +-2.3 V. This is quite close to the lower 4.5 V limit for the OPA189 - noise may be a little higher there. It may be more the higher supply with the 2x3 V supply instead of the rail splitting circuit.

No difference at 6V.

[graybeard]

The chopping frequency I measured was ~300 KHz with a 150KHz subharmonic as well as many high order harmonics.

I would move your fixture and cabling away from the front of the CRT on your FFT.  I have had the magnetic fields from the sweep coils couple into circuits.   Even though they are in your shielded box, some of the magnetic fields can still make it in.

[BrianHG]

The chopping frequency I measured was ~300 KHz with a 150KHz subharmonic as well as many high order harmonics.

I would move your fixture and cabling away from the front of the CRT on your FFT.  I have had the magnetic fields from the sweep coils couple into circuits.   Even though they are in your shielded box, some of the magnetic fields can still make it in.

Oh yeah, that nasty 15KHz all out omnipresent signal which I also find in around 75% of the CDs I own as the mic and their preamps pick up that signal from the semi-modern CRT console on the studio audio mixers of the day.

It used to drive me nuts as my ears used to be good up to 25KHz when I was a teenager and no matter how small the signal, at times it had enough db for me to make out at high volume.

[graybeard]

I can't hear 15750 Hz any more

[iteratee]

How is the OPA WRT bandwidth vs supply voltage? Dave mentioned in the last video that the OPA189 is "wide supply". I have seen "high speed" opamps give banner specs for bandwidth that only really applies to the highest supply voltages that can get massively nerfed towards the low end.

[Kleinstein]

The supply current does not change much with the supply voltage, except a little fro 4.5 V to some 5 .5V.  So I would not expect much effect on the bandwidth and noise, except for the very low end, below some 5 V.

Because of noise one would often like a limited bandwidth (e.g. up to some 100-200 kHz, so that much of the chopper noise is left out).

If one would need high bandwidth of the whole amplifier, one would no longer use 2 stages x 10 in series, but a compound amplifier: one AZ OP and a normal higher speed OP inside the loop. The AZ OP would than do something like a gain of 3 or 5 and the other OP would do the rest.  This would also avoid possible interaction between the 2 chopper frequencies, may get away with less power and only needs 1 pair of precision resistors for the gain. So the compound amplifier is also attractive for a more normal version.

[iteratee]

I can't hear 15750 Hz any more

When I was little I could type "beep(23000)" into quickbasic (or whatever) and only I could hear it through the crappy PC speaker. 

30 years later, probably not. I'd test on this phone but I know the headphone output rolls off at 19 khz or so.

Nowadays I hear 20khz at all times whether it's there or not. 

[EEVblog]

The chopping frequency I measured was ~300 KHz with a 150KHz subharmonic as well as many high order harmonics.

I would move your fixture and cabling away from the front of the CRT on your FFT.  I have had the magnetic fields from the sweep coils couple into circuits.   Even though they are in your shielded box, some of the magnetic fields can still make it in.

Oh yeah, that nasty 15KHz all out omnipresent signal which I also find in around 75% of the CDs I own as the mic and their preamps pick up that signal from the semi-modern CRT console on the studio audio mixers of the day.

25KHz in this case.
I noticed this and did a quick video on it:

[SilverSolder]

[...]
Nowadays I hear 20khz at all times whether it's there or not. 

Think of all the electricity you're saving! 

[SilverSolder]

How much of the noise from the uCurrent actually comes from the rail splitter, rather than the op amps?  Is there anything to be gained by using two batteries instead of a rail splitter, from this perspective?

[EEVblog]

How much of the noise from the uCurrent actually comes from the rail splitter, rather than the op amps?  Is there anything to be gained by using two batteries instead of a rail splitter, from this perspective?

Watch the whole video 

[SilverSolder]

How much of the noise from the uCurrent actually comes from the rail splitter, rather than the op amps?  Is there anything to be gained by using two batteries instead of a rail splitter, from this perspective?

Watch the whole video 

I did!   Re-watching now, as I obviously missed something! 

[Edit] Senior moment...   I did see that section the first time, but somehow failed to conclude that the noise of the rail splitter was inferred to be very low even if not directly measured...   Going to get another cup of coffee, maybe that will help, but might need something stronger!

[Unixon]

Dave, did you have any issues with offsets/biases of U2 LMV321 in your uCurrent circuit?
According to the schematic U2 inputs are not impedance balanced. Isn't it a problem?

In one particular design I've used LMV321 to create a 2.5V reference (actually, to follow TL431) with unipolar 5V or 3.3V power supply and I got huge ~100-200mV negative output offset.
Without changing schematic, I solved this issue by replacing LMV321 with TSX561A (ST part), but this made me look at LMV321 and its relatives with a little apprehension.

[EEVblog]

Dave, did you have any issues with offsets/biases of U2 LMV321 in your uCurrent circuit?
According to the schematic U2 inputs are not impedance balanced. Isn't it a problem?

In one particular design I've used LMV321 to create a 2.5V reference (actually, to follow TL431) with unipolar 5V or 3.3V power supply and I got huge ~100-200mV negative output offset.
Without changing schematic, I solved this issue by replacing LMV321 with TSX561A (ST part), but this made me look at LMV321 and its relatives with a little apprehension.

Nope, no issues.
But any offset in the virtual ground reference won't matter, because it's the actual ground reference point.

[David Hess]

If one would need high bandwidth of the whole amplifier, one would no longer use 2 stages x 10 in series, but a compound amplifier: one AZ OP and a normal higher speed OP inside the loop. The AZ OP would than do something like a gain of 3 or 5 and the other OP would do the rest.  This would also avoid possible interaction between the 2 chopper frequencies, may get away with less power and only needs 1 pair of precision resistors for the gain. So the compound amplifier is also attractive for a more normal version.

Or make a compound amplifier with the chopper stabilized one correcting the low frequency noise and drift of the faster wideband amplifier or discrete differential pair.  I have done this many times for spectacular results.

[SilverSolder]

If one would need high bandwidth of the whole amplifier, one would no longer use 2 stages x 10 in series, but a compound amplifier: one AZ OP and a normal higher speed OP inside the loop. The AZ OP would than do something like a gain of 3 or 5 and the other OP would do the rest.  This would also avoid possible interaction between the 2 chopper frequencies, may get away with less power and only needs 1 pair of precision resistors for the gain. So the compound amplifier is also attractive for a more normal version.

Or make a compound amplifier with the chopper stabilized one correcting the low frequency noise and drift of the faster wideband amplifier or discrete differential pair.  I have done this many times for spectacular results.

How big is the problem of two chopping frequencies beating up on each other - could that lead to low frequency misbehaviour?

I like the compound amplifier idea.  This should make it possible to increase the bandwidth significantly?  OPA189 might do a super job in a "supporting role"?   I seem to remember seeing application notes with some fairly straightforward designs, but can't just recall where...

[Kleinstein]

The composite amplifier with the 2nd fast OP inside the loop is the simple solution, but the overall BW is limited to the GBW of the AZ OP (preferably a little lower to get extra phase reserve) and the noise would be from the AZ OP. For the compound amplifier I still have an old appl. note around that is attached.

An extra, fast low noise amplifier and the chopper OP only for stabilization can be faster and lower noise than the AZ OP for the higher frequencies. However the low frequency region may have additional noise and the bias current of the 2 amplifiers add. This was definitely the method of choice in the old days when the AZ OP were slow and relatively noisy. The circuit tends to be a little more complicated though. LT apply note 21 has a lot of such examples.

[David Hess]

How big is the problem of two chopping frequencies beating up on each other - could that lead to low frequency misbehaviour?

I have seen it happen with older parts which did not dither their chopping clock; the symptoms typically include low frequency tones or high offset voltage drift which may be indistinguishable from low frequency noise.  (1) Some of the early parts provide a way to synchronize the chopping clock to avoid problems and it is not only with other chopper stabilized amplifiers.  You might want to synchronize with a sampling analog-to-digital converter, switching power supply, or microprocessor clock.

(1) In the past, high noise prevented chopper stabilized amplifiers from replacing precision bipolar parts even in low frequency applications.  Maybe the OPA189 is different in medium impedance applications but I would sure want to empirically test it; the lack of a current noise graph and DC or 0.01 Hz noise specification makes me wonder if TI is hiding something.

I like the compound amplifier idea.  This should make it possible to increase the bandwidth significantly?  OPA189 might do a super job in a "supporting role"?   I seem to remember seeing application notes with some fairly straightforward designs, but can't just recall where...

Usually it was about noise instead of bandwidth.  Chopper stabilized amplifiers have flat flicker noise while linear parts have increasing flicker noise at lower frequencies and wide bandwidth parts can have astonishingly high flicker noise, but there is not much overlap between the requirements for low flicker noise and wide bandwidth except maybe in test instrumentation; I would not mind having an FFT signal analyzer with both but the common solution is just to use a separate low noise preamplifier.

That OPA189 is much better than past chopper stabilized parts I have used but total noise could still be improved by perhaps 4 times in a low impedance, 100s of ohms, application.

[Kleinstein]

There is a parallel thread about the low frequency noise of the chopper OPs. This includes some data on the OPA189.
https://www.eevblog.com/forum/metrology/low-frequency-noise-of-zero-drift-amplifiers/msg3094181/#msg3094181

So I don't think there is hidden flicker noise. The nasty parts is more like the input impedance in the higher frequency region (e.g. input capacitance, but may be more) can have an effect on the offset / input bias current. The new OPs often include some EMI filtering, but this seems to be not sufficient to fully suppress these effects. Worst case one may need RF gear like a VNA to build a precision circuit.

With the chopper OPs there is the tendency that the low noise parts (e.g. OPA189, ADA4522, MCP6V91, max44250) also have relative high bandwidth. I think this because low noise needs a high chopper frequency as the internal capacitors are limited in size. So they need higher speed anyway. With higher currents amplifiers also tend to get faster.

[Gandalf_Sr]

Dave,

I have a uCurrent Gold Rev 5 that looks like it is fitted with two MAX4239s (part marking 'ABAA').  A few quick questions for us existing owners:

1. Is the OPA189 going to be lower noise than the MAX4239?
2. Can we just replace the 4238/4239 with the OPA189 or are there other components that will need (or you recommend) changing too?
3. What's required as far as the power supply goes? My uCurrent Gold is fitted with a coin cell holder that would not allow 2xCR2016s as the side contact would touch both batteries' +ve terminal; replacing that holder with either a double coin cell holder or 3 x AAAs is fine but what is the specific max/min voltage spec?

Thanks in advance

[EEVblog]

Dave,

I have a uCurrent Gold Rev 5 that looks like it is fitted with two MAX4239s (part marking 'ABAA').  A few quick questions for us existing owners:

1. Is the OPA189 going to be lower noise than the MAX4239?
2. Can we just replace the 4238/4239 with the OPA189 or are there other components that will need (or you recommend) changing too?
3. What's required as far as the power supply goes? My uCurrent Gold is fitted with a coin cell holder that would not allow 2xCR2016s as the side contact would touch both batteries' +ve terminal; replacing that holder with either a double coin cell holder or 3 x AAAs is fine but what is the specific max/min voltage spec?

Thanks in advance

All answered in my latest video. I haven't done any further testing yet though.

[Kleinstein]

The OPA189 has 4.5 V min for the supply. The max4239 and LMV321 (or similar) have some 5.5 V as maximum for the supply. So there is only a relatively small window for the supply.

3 alkaline cells (e.g. AA) may fast run below 4.5 V and 4 x alkaline are usually more than 5.5 V when new, so it would need an LDO to get some 5 V.

Besides the supply limits the rest should be OK with changing the OP(s). The noise is relevant mainly at the input.
If all 3 OPs are exchanged (something like MCP6H01 instead of the LMV321) a voltage like 6-9 V could be OK.

[Gandalf_Sr]

Thanks guys

I went back and watched the whole video.  Therefore I think the answer is that the OPA189 is going to:
a. Be a drop in replacement except that it will be missing the enable pin but that is not going to affect anything
b. Lower the noise level and increase the bandwidth
c. Need a minimum Vbat of 4.5V so some way of upping this needs to be implemented
d. The LMV321 needs to be changed or removed and a split-rail power setup used.

Am I right?

Thanks for suggesting the MCP6H01 as an LMV321 replacement Kleinstein, I'll go check that out.  What are the pros and cons of just removing the LMV321 and going with a split rail with 2 x (2 x AAAs)?