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