Bipolar voltage amplifier with floating power supply: Output current measurement - Page 1
Forum section: electronics / projects

author: Yansi
author_rank: Super Contributor
timestamp: December 29, 2020, 02:19:47 pm
content: Hello,For an application, I need a bipolar voltage amplifier, with the capability to measure output current. The classic scenario would be to just use an opamp and shove a current shunt in series with the output. Problem then is, that the shunt is floating with the output voltage, requiring very well balanced differential amplifier with very high CMRR.I thought I could go around not using the floating shunt resistor, by floating the power supplies instead. However, after I have verified the idea by simply simulating it to just get a quick sense if that works, I found that it works, but... there's an issue I did not think of.Here is the quick cobbled together circuit:It amplifies, all is fine, until you examine the current flowing through the current sense resistor. Voltage on the load is exactly what it should, but the current through the current sense resistor includes the current from the output of the opamp, and some other from bias imbalance (that's why the components were deliberately not made symmetrical - to look on their effect).If one thinks about it, it makes sense, that the currents through load and the sensing resistors can't be equal. Question now is, is there any other way to sense the output current from a bipolar amplifier, while avoiding to use a precision differential amplifier, let alone having to trim the CMRR manually in each amplifier?(The output voltages will be about +-20 to +-30V and 'd like to achieve at least 75dB dynamic range on the current sense, the CMRR hence must be at least that much).Is there a simple/cheap solution to it, that I forgot?

author: 2N3055
author_rank: Super Contributor
timestamp: December 29, 2020, 03:34:53 pm
content: Quote from: Yansi on December 29, 2020, 02:19:47 pmTake a look at Keithley 238. What you're trying to do is basically SMU. It's nothing to be ashamed of to do homework first.Hint, it is kinda similar to what you're trying here...

author: mag_therm
author_rank: Frequent Contributor
timestamp: December 29, 2020, 03:55:03 pm
content: LMP8645HV , a high side I_sense amplifier can do cmrr 95 dB.It can do that in a cm voltage range of +2 to +76 volt, while its supply voltage is in range +2.7 to +12 V.That does not meet the spec of your swing range, but maybe you can do a work-around, like a power supply at or below your negative rail?

author: djerickson
author_rank: Regular Contributor
timestamp: December 29, 2020, 04:07:35 pm
content: Hi YansiIt's a hard problem. Most hi-side current sensor ICs only operate from positive voltages. SMUs and current sources like the Keithley 236/7/8 use a similar approach but move the "ground" to the other side of the current sense resistor. That fixes the problem you describe *but* adds other problems. It means the whole amp circuit has to float with respect to the load, and it swaps a very hard CMRR current sense problem (High CMRR and High voltage) for a slightly easier (lower CMRR) but still complex differential voltage-sense amplifier. With an SMU it is complicated further since the sense amps have to be bootstrapped to achieve pA or lower loading.If you decide to go with a high-side current sense, you can use stable (10ppm or better) resistors or diff-amps and a common-mode-ratio adjust trimpot. To calibrae it, output voltage step changes (a square wave), and tune the trimpot for minimum change.At Teradyne, we built DC instruments with multiple channels and common grounds, so floating tricks were not an option: we had to sense hi-side. On one product, the high-side current sense amps used a complex and patented circuit with 6 bootstrapped precision op-amps, and bipolar, FET, high-voltage precision current mirrors. The instrument had 20 channels of these on two boards! For Semiconductor ATE, cost (and complexity) is no object.Someone needs to build an IC that solves the high-side problem. I suspect the market is small.Good luck,DaveCheck out my DIY-SMU project, similar to the Keithley 236http://www.djerickson.com/diy_smu

author: djerickson
author_rank: Regular Contributor
timestamp: December 29, 2020, 04:27:35 pm
content: Another crazy idea for correcting the CMRR of a high-side current sense circuit: Fix it in software.If you can drive and measure the output voltage, you could measure the change in current with no load current. Then apply that correction as an offset as a function of the voltage. If you're lucky it will be linear with V. The drift (over temp. and time will determine how often you need to do this cal. Somewhere between once a second and once ever:-)This won't work if you are applying the current in real-time, or need fast response, such as a precision current limit circuit. But it could work for slow stuff...Dave

author: 2N3055
author_rank: Super Contributor
timestamp: December 29, 2020, 04:49:19 pm
content: Why don't you simply float whole amperemeter circuit and make it isolated from source. It is digital isn't it?Simply float whole front end (make it differential) and A/D and speak with it trough isolation. All you need is small isolated power to power it.

author: Yansi
author_rank: Super Contributor
timestamp: December 29, 2020, 06:09:30 pm
content: Quote from: 2N3055 on December 29, 2020, 03:34:53 pmBuilding a diy SMU would be a good exercise, this time however all I need is a bipolar voltage supply of little current (under 50 mA maximum). Then I have decided would be interesting to add current measurement capability. Probably a bad idea, because it makes stuff awful more complex, than what was needed in the first place.Only hoped there may be some simple solution.And by simple bipolar PSU of 50mA tops, I mean simple. No range switches, no precision current limits (at least I do not need a constant current source here).Quote from: djerickson on December 29, 2020, 04:27:35 pmSoftware fixing of the current measurement is certainly possible, but I would take it as a last resort only if it can't really be "solved enough" by HW. Of course the basic gain/offset calibration will be done, but I don't plan to go any further.Quote from: djerickson on December 29, 2020, 04:07:35 pmSure that would fix it, no whackers! But that only means, you have just a different differential sensing problem.For this case it seems would be likely easier to just buy a high CM voltage differential opamp, with integrated and trimmed resistors, like INA132, LT1991 or similar. These seem to not be that expensive after all, considering it is likely even cheaper to use the INA132, then to wrestle with a floating power supply.Quote from: 2N3055 on December 29, 2020, 04:49:19 pmThat would add yet another floating supply and isolators and separate ADC, while making it impossible to extend this topology with precision current limiters in the future. Idon't think this is worth it. Floating supply's don't come cheap and separate ADC with digital isolator doesn't either.In this case the more cost effective way indeed seems use a monolithic difference amplifier, that includes trimmed/matched resistors inside. I have never used those, so there may be some other catches I don't know of.So yes, I should have done my homework first.

author: Yansi
author_rank: Super Contributor
timestamp: December 30, 2020, 01:59:02 am
content: Quote from: djerickson on December 29, 2020, 04:07:35 pmTook my time to go through it. I think I am quite impressed. Good study material you have there!And btw, don't be afraid of getting custom mains transformers even for one-offs. Considering the price of the rest and that you hardly need a 100 VA transformer, you could expect like $50 a piece. At least that's about what a custom 100 VA toroidal one would cost here locally made. There are suprisingly a lot of motor/transformer winding shops around in Europe, not sure about the US though.

author: David Hess
author_rank: Super Contributor
timestamp: December 31, 2020, 03:47:54 am
content: The conventional high-side current sense circuit which mirrors the output current can be offset by adding a current into the summing node so that it responds to positive and negative currents. Then the output offset can then be removed with a matching current at the load resistor to produce a bipolar output centered at common. In a bipolar application though common would have to be the negative or positive supply which would be inconvenient.