EEVblog #1328 - uCurrent OP189 Measurements

[Kleinstein]

Using 2 x 2 AA is usually better than the virtual ground from the OP. It need less power and is usually lower impedance. The LMV321 allows to use just 1 coin cell or could work with 5 V from an LDO. So one does not absolutely need to remove the LMV321 if the OPA189 is used.
The MCP6H01 would allow using a 9 V block.

[Gandalf_Sr]

Using 2 x 2 AA is usually better than the virtual ground from the OP. It need less power and is usually lower impedance. The LMV321 allows to use just 1 coin cell or could work with 5 V from an LDO. So one does not absolutely need to remove the LMV321 if the OPA189 is used.
The MCP6H01 would allow using a 9 V block.

Thanks.  The MCP6H01 is good up to 16V (+/-8) but the TPS3809 voltage regulator used for the LED is rated at an absolute max of 7V so either we stay under 7V or that has to change too.

[EDIT] With 2 x (2xAAA) alkalines, the voltage would range from 6.4 down to 4.5 when the batteries were almost completely dead.  Although the MCP6H01 would work as a replacement for the LMV321, using a split battery supply means U2 and associated resistors could go away.

The TPS3809 could stay to give controlled battery level warning (LED goes out when battery is failing) but we would want the 4.55V version, part number TPS3809I50DBVR and we'd also want to change the value of the LED dropper resistor.

OK, I've ordered the parts from Digikey, I'll report back on the progress.

[Gandalf_Sr]

Well, I was about to perform this upgrade today when I realized that, if moving to a split rail supply using 2+2 AAA batteries, I'm going to have to figure out a way to switch both the +3V and the -3V rails or there will be a half-powered circuit drawing current all the time.  Either that or maybe I should find a replacement for the LMV321 that can handle 4 x AAAs like Dave suggests.

[Gandalf_Sr]

I figured it all out.  I did away with the short functionality which allowed me to switch the +3V and the -3V lines at the same time.  Here's what I did:
1. Used a 4 x AAA battery holder and added an extra wire (green) that's the center tap of the 4 batteries.
2. Cut away the ridges inside the plastic case using a chisel modelling knife and Dremel - the battery box just fits inside.
3. Removed U2, R6,R7,R10,C2
4. Replaced U1 and U4 with OP189IDBVRCT
5. Replaced U3 with TPS3809I50DBVR (4.55) volt version and changed R4 to 909 Ohms
6. Removed battery holder and cut tracks on PCB to remove shorting connection (see picture)
7. Soldered wires onto PCB (see picture)

On testing, I was able to set up a 0.5 uA current through the uCurrent and see 500 mV on the output (1mV/nA range) with my calibrated 34461A.  It looks like it was all successful.

[apoorv3in]

Modified the uCurrent based on V3 with OPA189 and LM321MF
running now with 9v Battery

[EEVblog]

I figured it all out.  I did away with the short functionality which allowed me to switch the +3V and the -3V lines at the same time.  Here's what I did:
1. Used a 4 x AAA battery holder and added an extra wire (green) that's the center tap of the 4 batteries.
2. Cut away the ridges inside the plastic case using a chisel modelling knife and Dremel - the battery box just fits inside.
3. Removed U2, R6,R7,R10,C2
4. Replaced U1 and U4 with OP189IDBVRCT
5. Replaced U3 with TPS3809I50DBVR (4.55) volt version and changed R4 to 909 Ohms
6. Removed battery holder and cut tracks on PCB to remove shorting connection (see picture)
7. Soldered wires onto PCB (see picture)

On testing, I was able to set up a 0.5 uA current through the uCurrent and see 500 mV on the output (1mV/nA range) with my calibrated 34461A.  It looks like it was all successful.

Have you done a full frequency sweep?

[Gandalf_Sr]

No, not yet Dave. Any tips on how to set that up, say with a Rigol MSO5074?

[SilverSolder]

No, not yet Dave. Any tips on how to set that up, say with a Rigol MSO5074?

[Andreas]

Hmm,

shure that the OPA189 is the right candidate for a (high impedant) current source input.
My measurements give a relative high (average) input bias current.
with a 10K source resistance I get a 23 uV offset voltage or 2.3 nA effective input bias current.

see also here:
https://www.eevblog.com/forum/metrology/emi-measurements-of-a-volt-nut/msg2819014/#msg2819014

with best regards

Andreas

[sourcecharge]

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.

So after watching this video, it leads me to think that the OP189 is better, but the following post has me scratching my head:

Hmm,

shure that the OPA189 is the right candidate for a (high impedant) current source input.
My measurements give a relative high (average) input bias current.
with a 10K source resistance I get a 23 uV offset voltage or 2.3 nA effective input bias current.

see also here:
https://www.eevblog.com/forum/metrology/emi-measurements-of-a-volt-nut/msg2819014/#msg2819014

with best regards

Andreas

Does this effect uA and mA measurements?

sorry to bring this topic up so late, but I've been waiting for those that were testing it to post their results.

I think I want to change this to the OP189 if uA and mA measurements are unaffected.

[Kleinstein]

The input bias current effectively adds to the measured current, a little like additional offset. One would see the sum of the voltage offset and the bias current. 23 µV (2.3 nA) looks like quite high. The typical current should be lower.
The bias current can vary between units and can also vary with the input impedance (in the higher frequency range).

In most cases on can tolerate a small offset and just subtract it. Some extra 2 nA should not be unnoticed in the mA range and hardly visible in the µAs.

One does not have to change both of the AZ OPs. Only the OP at the input is critical for the noise.

[sourcecharge]

The input bias current effectively adds to the measured current, a little like additional offset. One would see the sum of the voltage offset and the bias current. 23 µV (2.3 nA) looks like quite high. The typical current should be lower.
The bias current can vary between units and can also vary with the input impedance (in the higher frequency range).

In most cases on can tolerate a small offset and just subtract it. Some extra 2 nA should not be unnoticed in the mA range and hardly visible in the µAs.

One does not have to change both of the AZ OPs. Only the OP at the input is critical for the noise.

Ya, it seems that would make sence about the voltage offset, but why didn't dave see that voltage offest in his testing?

The decrease in noise is great improvement for it, but I think I want to change both ops because of their frequency range and the higher frequency of the chopper.

I figured it all out.  I did away with the short functionality which allowed me to switch the +3V and the -3V lines at the same time.  Here's what I did:
1. Used a 4 x AAA battery holder and added an extra wire (green) that's the center tap of the 4 batteries.
2. Cut away the ridges inside the plastic case using a chisel modelling knife and Dremel - the battery box just fits inside.
3. Removed U2, R6,R7,R10,C2
4. Replaced U1 and U4 with OP189IDBVRCT
5. Replaced U3 with TPS3809I50DBVR (4.55) volt version and changed R4 to 909 Ohms
6. Removed battery holder and cut tracks on PCB to remove shorting connection (see picture)
7. Soldered wires onto PCB (see picture)

On testing, I was able to set up a 0.5 uA current through the uCurrent and see 500 mV on the output (1mV/nA range) with my calibrated 34461A.  It looks like it was all successful.

I've also got a question about this design change.

First, I think I'm going to use dual PS's at +/-10V instead of batteries because I'm using the PS's in my FG isolator anyway, so that would eliminate U2,R6,R7,R10,C2,U3,R4, the battery holder, and the LED.  I'm thinking because the PS's would show the input voltages and therefore I don't really need a low voltage indicator. 

What I don't understand is why you attached the negative to the switch and then cut the shorting traces to the switch.  Doesn't this take the function of the switch away from Off/On/On(Shorted), to Off/On/On(Not Shorted)?

Can't the negative voltage wire simply go the negative battery terminal pad and leave the switch and traces alone?

Just in case, I included the schmatic of the uCurrent


Edited:

I figured out why you did that, because you are using batteries and not PS's that can be turned off.

I'm guessing that the ops will still drain the negative supply side of the batteries because their is no virtual ground.

I wasn't thinking about that because I had PS's in mind with their own on/off switch.

[Kleinstein]

With more changes to the circuit one could consider to build an independent circuit (e.g. with adapter and raster board or dead bug style) instead of starting from a µCurrent board and replacing much of the parts. A modified circuit with a compound stage instead of 2 stages in series could get away without the second Az OP and use a more conventional one.

Keep in mind the µCurrent uses low tolerance resistors for the gain and the shunts to get the gain right without any adjustment.

The OPA189 is not that much faster than the MAX4239. There is however the different copper frequency which may be a factor when using it with a scope. One may need to change the second OP, as the Max4239 has a limited maximum supply. It still works with 5 V total.

[Gandalf_Sr]

Sourcecharge
Yes, you figured it out.  Because I moved from a single supply (where the center voltage is created by the electronics) to a V- 0 V+ battery setup, I changed to switch both sides of the battery supply.  You're right that I lose the shorted position but that's easy to replicate, just turn it on and short the inputs.

If you were not using batteries then you could keep the shorted position and not modify the tracks around the switch.

[SilverSolder]

Just for fun, I tested the uCurrent background noise level using a venerable old spectrum analyzer, as an exercise in first principles.



The results:




On the 3571A we see a uCurrent noise floor of -93dBV, if we ignore the chopper noise.

We must add 2.5dB to that due to logarithmic averaging, taking us to -90.5dBV

Then we convert dBV to RMS, so  -90.5dBV -> 30uV RMS

Now we have to adjust RBW since the noise power bandwidth of the filters are about 12% wider than the 3dB bandwidth.  So 100Hz RBW --> 120Hz RBW for noise.

Finally we divide the RMS signal by rt-hz, i.e. RMS/sqrt(AdjRBW).
Presto,   30uV/10.95 = 2.75uV/rt-hz



This number is lower than Dave's measurement of 3.5uV/rt-hz, but still on the same planet...   there could be a hole in my methodology, this analyzer is not exactly user friendly!  Comparing it to a car, it is like driving a 1965 Mustang, with manual steering and transmission, without power brakes...

[sourcecharge]

With more changes to the circuit one could consider to build an independent circuit (e.g. with adapter and raster board or dead bug style) instead of starting from a µCurrent board and replacing much of the parts. A modified circuit with a compound stage instead of 2 stages in series could get away without the second Az OP and use a more conventional one.

Keep in mind the µCurrent uses low tolerance resistors for the gain and the shunts to get the gain right without any adjustment.

The OPA189 is not that much faster than the MAX4239. There is however the different copper frequency which may be a factor when using it with a scope. One may need to change the second OP, as the Max4239 has a limited maximum supply. It still works with 5 V total.

So here is where I am.
I did the mods that I described plus some, but there is a couple of problems.
First, the virtual ground of the circuit is actually required for the op amps to be biased correctly.  You can do this with the balenced comparitor's output to virtual ground or you can use two 100nF caps from +V and V to virtual ground.
Second, the unconnceted voltage readout on 3 of my mastech meters for both of my ucurrents (2 of them) are about 0.4mV and 0.12mV in mA and uA settings but the nA setting seems to be way off which I'm guessing is the problem that was previously described.
They are both on the same + and - V PS with the same vitual ground.
I double check the resistor networks and found them in spec.
I've also replaced the mA 0.01 ohm shunt resistors in both of them so they are new too just in case I overloaded the first ones.
I replaced the 270 ohm resistor on the output of the last opamp and put a 0 ohm jumper in it's place, while scratching off the masking near the output terminal and cutting the trace and jumping that cut with the 270 ohm resistor.
I've spiced this and it shouldn't change anything really, but it always nagged me on the idea that the resistor network is using 0.05% resistors on the last op amp resistor network but it has a whoping 270 ohms in series with the 9k ohms.  0.05% of 9k is 4.5 ohms, and 270 ohms included into it just doesn't make any logical sence.
The amout of current (voltage readout) under load is offset by the same amount that they are reading under unconnected conditions.

Anyways, if anyone has any ideas with whats going on, let me know....

[Kleinstein]

The 270 Ohms at the output of the last OP are not directly on series to the 9 K,  but the output is taken from behind the resistor.
The idea behind the 270 ohms resistor is to avoid capacitive load to the OPs output, that may lead to oscillation. AFAIR the resistor was at a slightly odd place, not where it ideally should be. There may be slight differences in circuit versions.

The virtual ground driver had some problems with oscillation, depending on the exact OP type used. Here capacitive loading is part of the problem. So it is either the OP or passive with resistors and capacitors. Combining the OP with capacitors is tricky.

Some of the AZ OPs react to the capacitance or more accurately the impedance at some 10-500 MHz for the input nodes. The modern ones include some EMI filtering, but it may still need more external filtering to avoid odd effects from the higher frequency bands, like capacitive loading or cable length. For the higher current ranges the shunt resistors essentially short the input, but not the 10 K for the nA range. 
Filtering maybe something like 100 pF+100 Ohms in series in parallel to the 10 K shunt.  A ferrite bead at the input may also help.

[sourcecharge]

so the rev5 boards have the 270 ohm resistor in series with the 9k ohm resistor and it also goes to the output terminal.

I simply couldn't leave it alone, although all spice sims show no difference in any output.

What's really got me confused is the 0.4mV to 0.5mV and the 0.1mV to 0.2mV output of the ucurrents when they are unconnected.  These seem to be affecting the mA and uA measurements, although the mA and uA measurements are within 0.5%.

So, I guess the question is, can this be improved or is there something wrong with both of my ucurrents?  Come to think about it, they had the same problem with the max4239 at about 0.2mV each.

I measured 0mV on the input of the 1st opamp for both, and then 0.05mV on the input of the 2nd opamp for the 0.4mV ucurrent, and 0.01mV input on the input of the 2nd opamp for the 0.1mV ucurrent.  But when I measure current I'm within 0.5% even though the offsets are included in both the mA and the uA measurement ranges.

I'm only using matched caps to make the virtual ground, and I'm wondering if an opamp virtual ground would be better.

Edit, I think I just figured out where the measurement was comming from.

If the output of the 1st opamp has a bias voltage of somewhere between 0uV  and 5 uV, then the 10x amplification makes it 0 to 50uV?  Then the next op amp, amplifies it to 0.4mV and 0.1mV for the unconnected measurement?

Can anyone verify this logic? I'm not sure.

[Kleinstein]

The max4239 can have an offset of some 1 µV, maybe a little more with unequal impedance at the inputs. This amplified 100 times and this some 100 µV = 0.1 mV range offset at the output is well possible. It may be a little more if the input offset is higher.
In the nA range there is also bias current from the OP.

The OPA189 has more bias current and may react more sensitive to unequal impedance.

The 270 Ohms from the OP to the output and the Feedback from behind the 270 Ohms, has little effect. It essentially adds the 270 Ohms to the open loop output impedance. So the OP will compensate for it. The resistor may help a little with EMI, but does not help to prevent oscillation from capacitive loading. So be careful with a capacitive load (some DMMs).

[SilverSolder]

The max4239 can have an offset of some 1 µV, maybe a little more with unequal impedance at the inputs. This amplified 100 times and this some 100 µV = 0.1 mV range offset at the output is well possible.[...]

Is the compound amplifier talked about earlier in the thread less sensitive to the offset issue -  i.e. does a compound amplifier have only the offset of the input amplifier, whereas the offset of the second op-amp inside the loop disappears due to feedback?  - if so, it might be a good reason to try the compound amplifier idea for this application?

[sourcecharge]

The max4239 can have an offset of some 1 µV, maybe a little more with unequal impedance at the inputs. This amplified 100 times and this some 100 µV = 0.1 mV range offset at the output is well possible.[...]

Is the compound amplifier talked about earlier in the thread less sensitive to the offset issue -  i.e. does a compound amplifier have only the offset of the input amplifier, whereas the offset of the second op-amp inside the loop disappears due to feedback?  - if so, it might be a good reason to try the compound amplifier idea for this application?

If the only "significant" amount of offset is from the 1st opamp, can this be nulled by using some other opamp with manual pots which can be set by the user while "zeroing" the unconnected measurement?

[SilverSolder]

The max4239 can have an offset of some 1 µV, maybe a little more with unequal impedance at the inputs. This amplified 100 times and this some 100 µV = 0.1 mV range offset at the output is well possible.[...]

Is the compound amplifier talked about earlier in the thread less sensitive to the offset issue -  i.e. does a compound amplifier have only the offset of the input amplifier, whereas the offset of the second op-amp inside the loop disappears due to feedback?  - if so, it might be a good reason to try the compound amplifier idea for this application?

If the only "significant" amount of offset is from the 1st opamp, can this be nulled by using some other opamp with manual pots which can be set by the user while "zeroing" the unconnected measurement?

The easiest is to zero the voltmeter that the uCurrent is driving (or add a small offset if you're using a scope).

So an offset isn't a big deal in most normal use cases.

But obviously the smaller the offset of the uCurrent itself, the better!

[sourcecharge]

The max4239 can have an offset of some 1 µV, maybe a little more with unequal impedance at the inputs. This amplified 100 times and this some 100 µV = 0.1 mV range offset at the output is well possible.[...]

Is the compound amplifier talked about earlier in the thread less sensitive to the offset issue -  i.e. does a compound amplifier have only the offset of the input amplifier, whereas the offset of the second op-amp inside the loop disappears due to feedback?  - if so, it might be a good reason to try the compound amplifier idea for this application?

If the only "significant" amount of offset is from the 1st opamp, can this be nulled by using some other opamp with manual pots which can be set by the user while "zeroing" the unconnected measurement?

The easiest is to zero the voltmeter that the uCurrent is driving (or add a small offset if you're using a scope).

So an offset isn't a big deal in most normal use cases.

But obviously the smaller the offset of the uCurrent itself, the better!

ya you're probably right, but it was just a thought.

[Kleinstein]

The relevant offset if from the 1st OP only.  It is somewhat tricky to use the trim pins at one OP to compensate more than the OPs own offset.  At least for BJT based OPs the offset trim also effects the drift. In the ideal picture the offset is proportional to the absolute temperature. For a very small offset, like a few µV at the input this may still be acceptable. It would not be zero drift, but could still be low drift. However modern OPs rarely have the  trim pins and something like an OP27 needs quite a lot of supply current and more than 3 V.

The compound amplifier idea does not help much with the offset. It helps with needing only 1 AZ OP and only 1 pair of precision resistors. This can help reducing the overall power needed (a non AZ OP may be lower supply current at the same speed) and speed as the overall BW could reach something like 1/2 the GBW of the AZ OP.  However this would be quite some change in the circuit - so nothing for the existing board, more like some a thing for a new version with the OPA189. Because of noise, a high BW version may be better with the AZ OP to stabilize a low noise amplifier.

The OPA189 will be tricky for small currents, as it may need added filtering to avoid an effect from external capacitance at the input. This would interfere with a high speed. Because of this a OPA189 version would likely be for the 2 higher current ranges only, maybe with an extra option as a µV amplifier (e.g. for thermocouples or an external higher current shunt). If needed, the very low currents (e.g. < 10 µA) would be better done with a TIA like configuration anyway, with a different OP.

[SilverSolder]

The relevant offset if from the 1st OP only.  It is somewhat tricky to use the trim pins at one OP to compensate more than the OPs own offset.  At least for BJT based OPs the offset trim also effects the drift. In the ideal picture the offset is proportional to the absolute temperature. For a very small offset, like a few µV at the input this may still be acceptable. It would not be zero drift, but could still be low drift. However modern OPs rarely have the  trim pins and something like an OP27 needs quite a lot of supply current and more than 3 V.

The compound amplifier idea does not help much with the offset. It helps with needing only 1 AZ OP and only 1 pair of precision resistors. This can help reducing the overall power needed (a non AZ OP may be lower supply current at the same speed) and speed as the overall BW could reach something like 1/2 the GBW of the AZ OP.  However this would be quite some change in the circuit - so nothing for the existing board, more like some a thing for a new version with the OPA189. Because of noise, a high BW version may be better with the AZ OP to stabilize a low noise amplifier.

The OPA189 will be tricky for small currents, as it may need added filtering to avoid an effect from external capacitance at the input. This would interfere with a high speed. Because of this a OPA189 version would likely be for the 2 higher current ranges only, maybe with an extra option as a µV amplifier (e.g. for thermocouples or an external higher current shunt). If needed, the very low currents (e.g. < 10 µA) would be better done with a TIA like configuration anyway, with a different OP.

As seen in the spectrum plot, the noise from the auto-zero amps is "significant" (not a problem for most use cases, but definitely visible).  So, having a "normal" op amp disciplined by an auto-zero servo sounds like a good idea to explore, just to get rid of the noise.   Furthermore, if we now don't have to worry about the offset at all any longer, perhaps there are a lot more options for which op amps to use.

The downside with any servo type arrangement is that now, a DC current will "fade out" over time, as the servo corrects it away.  So perhaps this fact alone makes that idea still born?

Funny how much thinking goes in to analog electronics, even when the problem you are trying to solve is well understood!