Electronics > Projects, Designs, and Technical Stuff
1A current source but still not stable
Dr. Frank:
This circuit usually is extremely stable, I have designed exactly that topology, up to 100mA.
See here: https://www.eevblog.com/forum/metrology/recycling-of-precision-current-source-noise-reduction-for-low-burden-shunts/msg1432410/#msg1432410
The stability @ 6V supply seems to be reasonable, whereas at 12V, it should be the same stability, as this circuit does not depend on the supply voltage of the current path.
This excessive drift does not fit to the parameters (T.C.) of the high quality components.
Make sure, that the FET does not heat up the VPR221. Therefore separate both heatsinks.
The different batteries used, and their inter connection is also not precisely drawn in your schematic. Please provide an excact schematic, how these 3 / 4 batteries are connected to the circuit.
The reference and the OpAmp and the GND sense side should best be supplied by the same battery, to avoid any ground loops.
Please check, if you have used a correct Kelvin / 4 wire connection for the current shunt VPR 221. See datasheet of the shunt.
The current connections I+ I- should be connected the FET, and to - of the Battery VS / 6V, whereas the sense lines S+, S-should be connected to the + of the OpAmp and to GND of the Opamp supply.
There must be no other connection between GND and the - of the VS battery!
Then the OpAmp should be checked, if it's still ok.
If I remember right, a similar OpAmp, AD706 is used also in hp 34401 DMMs, and that caused a lot of failures after years, in the Ohm range, due to breakdown of the input transistors.
That was a systematic error, happening on many instruments.
So I wouldn't trust this salvaged part at all .. check if the reference voltage @ + and feedback voltage @ - are stable and always identical to a few µV, or if the difference voltage drifts.. that would imply an excessive bias current.
Maybe you choose another Opamp which has rail-to-rail voltage characteristics, or use a chopper OpAmp.
Concerning both 34470A, before you start measurements, without current, let them warm up 1h at least, and do an ACAL on both, then NULL both instruments in the 1A range.
That might improve the accuracy.
Current measurement is not so precise by specification, anyhow, so the absolute difference in reading seems to be all right.
If one of both instruments already has FW 3.0, you can use the DIG feature for DCI @ 20µs to check if the current output is oscillating.
I've also implemented a stabilizing capacitor C6 of 10nF between - and the direct output of the OpAmp, and this is very important for stability over different loads, and compliance voltages by the FET.
Frank
spec:
--- Quote from: Dr. Frank on January 17, 2019, 07:13:10 pm ---This circuit usually is extremely stable, I have designed exactly that topology, up to 100mA.
See here: https://www.eevblog.com/forum/metrology/recycling-of-precision-current-source-noise-reduction-for-low-burden-shunts/msg1432410/#msg1432410
--- End quote ---
The OPs circuit is almost guaranteed to oscillate because it has two unit slopes active at 0dB. Your circuit has compensation to sort out this problem.
dmm2018:
--- Quote from: Dr. Frank on January 17, 2019, 07:13:10 pm ---The different batteries used, and their inter connection is also not precisely drawn in your schematic. Please provide an excact schematic, how these 3 / 4 batteries are connected to the circuit.
The reference and the OpAmp and the GND sense side should best be supplied by the same battery, to avoid any ground loops.
Please check, if you have used a correct Kelvin / 4 wire connection for the current shunt VPR 221. See datasheet of the shunt.
The current connections I+ I- should be connected the FET, and to - of the Battery VS / 6V, whereas the sense lines S+, S-should be connected to the + of the OpAmp and to GND of the Opamp supply.
There must be no other connection between GND and the - of the VS battery!
--- End quote ---
Thank you Dr. Frank. The schematic and the PCB layout are attached. Three batteries are used, i.e. Battery A (12V), Battery B (12V) and Battery C (6V or 12V).
For a VPR221 with Kelvin / 4-wire connection, I'm not sure whether it's correct to connect the trimming circuit to Pin-2 and Pin-3 of the VPR221 as shown in the schematic.
Kindly advise.
dmm2018:
Hello Zero999, Rerouter, strawberry and spec,
We have checked the voltage across the VPR221 (between Pin-2 and Pin-3). Several waveforms with different timebases (10ms/div, 1ms/div, 100us/div, 10us/div, 1us/div and 100ns/div) were recorded (using Tekronix TPS2014, AC coupling). The measured waveforms are quite stable (except the one with 10us/div).
Kindly advise.
Doctorandus_P:
I find it a bit strange that you're "complaining" about what looks like a bit of drift in the last 3 digits of a 7 1/2 digit EUR 1500 meter with such a simple circuit.
Is your MOSfet designed for lineair applications, or is it optimised for fast switching, which most are?
This can be a source of instability (local heating in the FET) and even destruction.
I think power transistors are easier to tame for a current sink.
For better stability the circuit mentioned by Zero999 looks good.
From memory:
I think Davey Jones put some beefy capacitors over the shunt resistors to improve stability. But because the shunt resistors are so small you would need some relatively big caps (Few 100 uF) to get a usable time constant.
Also:
It is not possible to check stability with a DMM. You need an oscilloscope for that, and preferably some rapidly changing setpoints to measure step response. Opamp's do not like the capacitive loading of power MOSfet's and step responce is probably lousy without further compensation.
For these measurements it also does not matter if you just turned your fancy DMM on, or if it has been accumulating dust or warming up for months.
Opamps are not ideal gain blocks you can use randomly.
There are thousands of different opamps, and they all have different limitations and flaws, and it is the job of a decent engineer to select the right opamp and work around it's limitations to make a circuit with it that works reliably and to specifications.
Talking about specifications:
What kind of figures are you expecting for?
Such a wide open circuit will probably drift a few miles if you touch it or blow in its direction if you measure it with a 7 1/2 digit DMMM.
(P.S. Don't take it badly, i'm jealous, but please learn to interpret what you see on such a meter more accurately).
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