Fully Differential Amp for Current Sensing - Page 1
Forum section: electronics / beginners

author: lnadolski
author_rank: Contributor
timestamp: June 01, 2016, 09:29:53 pm
content: Hello again,I am trying to develop a small current sensing module to measure currents in the 50nA to 30uA range (I know this may not be possible). A micro will be connected to the PCB and the current will be measured when the micro is in sleep mode. I am using a fully differential amplifier (TI THS4131) that has a shunt resistor (1 kOhm) connected to the PCB feedback network (see below/attached image). The output of the THS4131 will be connected to a LabJack U6 for measuring the differential signal from the THS4131.I would like to use the constant current sources (10uA and 200uA) to perform a two point calibration on the circuit in order to determine gain, zero-crossing.I have attempted current measurements and calibration with identical circuit topology except with smaller shunt resistors (0.1, 10 Ohm) and a different current source with good results.When using the 1 kOhm shunt and LabJack U6 200uA current source, I only see a fraction of the current (voltage drop) passing through the shunt. Originally I expected that all of the current supplied by the LabJack (200uA) would pass though the shunt (this was case when using smaller shunt resistors).I am obviously missing how I should be analyzing the circuit for this particular case. Should I be treating the input terminals of the THS4131 as a virtual short? If so, would I calculate the expected shunt current to be 200uA*Parallel Resistance/Total Resistance = 200uA * 800 / 1800 = approx 90uAI have experimented with a couple different shunt values (500, 333 Ohm) and found that the above formula holds based on measuring the voltage across the shunt and dividing by the shunt resistance.Note that the LabJack U6 is also the ADC.THS4131 VOCM pin is connected to LabJack U6 ground.Any help, suggestions and comments are greatly appreciated. Thanks for your time.L

author: kony
author_rank: Regular Contributor
timestamp: June 01, 2016, 10:12:54 pm
content: Have a read here:https://en.wikipedia.org/wiki/Instrumentation_amplifierYour differential input amplifier has nowhere near high enough input impedance, no wonder that it sinks considerable part of the current that ought to be flowing just trough the shunt.Also you accidentaly a word - you tie it to ADC, not DAC.You can get integrated instrumentation amp with diff output as well - for example this:http://www.ti.com/product/PGA281.

author: lnadolski
author_rank: Contributor
timestamp: June 02, 2016, 11:31:43 am
content: @Kony, Thanks for the reply/information.I corrected my original post...changing DAC-->ADC.Is it fair to say that when using the smaller shunt resistor values (0.1, 10 Ohm) as mentioned in my original post, the impedance of the shunt resistor is relatively small compared to the input impedance of the op-amp circuit?If I were to continue using the THS4131 as my op-amp of choice, what are some methods to increase the input impedance? If I increase the size of the gain (input) resistor and feedback resistor to maintain same DC gain of approx. 20 but use resistances of 10kOhm and 200kOhm how will this affect the output of the op-amp? Will I see a relatively large DC output from the op-amp when input is 0V due to non-ideal op-amp (bias current, offset voltage)?Thanks again for all your insight.L

author: Chris Mr
author_rank: Regular Contributor
timestamp: June 02, 2016, 12:22:20 pm
content: I suppose you are aware of thishttps://www.eevblog.com/projects/ucurrent/which might do what you want(invoice in the post for the advert Dave)

author: lnadolski
author_rank: Contributor
timestamp: June 02, 2016, 12:33:17 pm
content: @Chris Mr, Thank you.I am aware and we use a couple of the uCurrent at work.The need for a customized design led us away from using the uCurrent for this project. I used the uCurrent webpages and schematics to get a general idea of how to design a current sensing module. As a newbie it was super informative and obviously have a lot to learn.Thanks again for the post.

author: Fgrir
author_rank: Regular Contributor
timestamp: June 02, 2016, 03:18:43 pm
content: A few thoughts:You could just get rid of the shunt resistor and rely on the input resistance of your amplifier. You were correct when you said that the inputs to the THS4131 will appear as a virtual short, so the sum of the two input resistors would be your effective shunt resistance. Or you can accept the parallel combination of the shunt resistor and the two input resistors as your effective shunt resistance. In either case I'd probably put a couple of diodes across the amplifier inputs to protect it against too much differential voltage when it saturates.The input bias and offset current specs for the THS4131 are a bit high for the current range you are trying to measure - a proper FET-input instrumentation amp will do a much better job.If the Labjack is part of the final design and not just being used to test your circuit, then you can probably just tie the 1k shunt across the labjack inputs and skip the amplifier entirely. Looking quickly at the datasheet for the Labjack it appears to have a programmable gain instrumentation amplifier as it's front-end.

author: danadak
author_rank: Super Contributor
timestamp: June 02, 2016, 03:40:09 pm
content: Use the selector guide and tools here for some possibilities.http://www.analog.com/en/products/amplifiers/instrumentation-amplifiers.htmlRegards, Dana.

author: lnadolski
author_rank: Contributor
timestamp: June 02, 2016, 07:59:01 pm
content: Thanks for the help all.@Fgrir, thanks for the explanation of the resistance see by the current source.Regarding the input bias and offset current of the THS4131...I fully agree with you. I had reservations when selecting the THS4131 for this particular application. Some of the other project requirements kind of narrowed down my options...The LabJack in fact is part of the final design but I was using the op-amp circuits to allow for configurable gain, filtering and the ability to connect different op-amp outputs to the LabJack differential input via some DPDT relays.Thanks again.L

author: Marco
author_rank: Super Contributor
timestamp: June 02, 2016, 08:23:46 pm
content: What requirements? The bandwidth? It's wasted, Johnson noise limits you to around 10 Hz for 50 nA resolution (you'll need a really good shunt at that). Also the low frequency voltage and current noise of this opamp isn't even specced, so good luck.

author: lnadolski
author_rank: Contributor
timestamp: June 02, 2016, 09:11:25 pm
content: @Marco, Thanks for the reply.I am unfamiliar with Johnson noise limits. I will do some research.Thank you for the 'good luck' wish. Same to you with all your endeavors.L

author: tszaboo
author_rank: Super Contributor
timestamp: June 02, 2016, 09:18:59 pm
content: The THS4131 is a very bad choice for current sensing. You want something like an INA195 or INA128 or similar current shunt amplifier or Instrumentation amplifier. Also, with the high value resistor, your micro will never even boot up, because it draws too much current, so voltage drop.

author: lnadolski
author_rank: Contributor
timestamp: June 02, 2016, 09:50:24 pm
content: @NANDBlog, thanks for the input. I am realizing that the THS4131 was 'a very bad choice'.The micro will already be in sleep mode when the shunt resistor is connected to the micro.Thanks again and I will look into a different IC if possible to use.L

author: Fgrir
author_rank: Regular Contributor
timestamp: June 06, 2016, 06:20:07 pm
content: Quote from: Marco on June 02, 2016, 08:23:46 pmCan you explain your thinking on this statement? By my calculations a 1K shunt would exhibit 50nA RMS Johnson noise at 150MHz bandwidth, not 10Hz.

author: Marco
author_rank: Super Contributor
timestamp: June 07, 2016, 02:42:47 pm
content: Oops, forgot to divide by R

author: Cerebus
author_rank: Super Contributor
timestamp: June 07, 2016, 10:33:09 pm
content: When you say:Quote from: lnadolski on June 01, 2016, 09:29:53 pmDo you mean a full scale current of 30 uA with a resolution of 50 nA, or are you asking for an instrument with multiple ranges including 0-50nA and 0-30uA?And, is the current source to be measured floating with respect to your instrument or is it, as in your diagram, sharing an earth reference with the instrument? What you have drawn looks like it just needs to sink an earth referenced current; if that is the case the answer might be super simple.Also, crucially, what kind of sample rate are you looking at needing? Your use of an ADC with a differential input and a 50MHz amplifier suggests you're looking at quite a fast ADC. High speed would turn it from a potentially almost trivial design into possibly quite a tricky one.