Precision Low Current Measurement w/ Feedback Ammeter

[@mjlorton]

Very fascinating, I certainly learnt something. Great production too. Cheers, Martin.

[@jjoster]

That is a good question. I can confirm that the output of my instrument, when fed an AC current is linear. I do not have a precision AC current source with which to calibrate it but I used a waveform generator and a 60Hz 20mV signal outputs 138uA while a 40mV signal outputs 276uA. It has the same reading up to 10kHz. Its -3DB performance is 124.5kHz.

[@Makenitso]

Thanks for the video. i wish every datasheet came with something like this! Just what i needed.

[@makeit2132]

Great video!... in the Corrosion Science field we call this kind of ammeter Zero Resistance Ammeter (ZRA). Next step hook it up to an arduino to do some data logging!

[@thijss1170]

Awesome vids you're uploading lately. This principle makes the Eevblog µcurrent work.

[@nityanandadas5575]

Excellent video. What is the input impedance in nano ampere range ?

[@jjoster]

Thanks, I am trying to pass on what I am learning in a useful, digestible manner. The uCurrent has it's op amp wired up backwards compared to mine (the current goes into the non-inverting input). Also it uses a current divider to change the ranges. I made mine much simpler by just changing feedback shunts to select the range. As far as I can tell effective it is just as effective.

[@cirdiam1800]

Very nice circuit and demonstration. Thanks.

[@leninchakravarthy1426]

Very curious and informative video. Thanks for making. I plan to make one. But as a begineer, i am confused how to connect this circuit in DUT. For example, if i want to measure the input current of a BJT amplifier, I-input is connected to output of BJT's voltage divider & I-gnd is connected to base of the BJT. Pls correct if i am wrong.

[@jjoster]

Actually it's better than I first realized. The output is linear, but I what I neglected to realize is that the pk-pk is not the RMS voltage. I don't work with AC much so I spaced that at first. If you watch the AC video you will see that it works great. You need to be mindful of the voltage though. You cannot feed it mains voltage... You can feed it the input voltage +- 1V. The max input voltage is +-18V.

[@clivebillson]

Thanks for the reply. I'm not too concerned about the frequency; it will all be at 50Hz. As I understand it then; what you're saying, is that with a variable a.c. current input, it will give a linear voltage output but not calibrated? I think that's fine for what I'm looking for. Thank you. I'm most grateful.

[@libervolucion]

I know this is an old video and that it has to do with really small values, but why is the current flowing into the output of the precision op amp? And then, closing the circuit through the positive terminal? I wasn't expecting that.

[@paulg.3067]

Do you have a source or name for those happy little switches? :)

[@seanalm3127]

Great video. One confusing thing though, you said this was a high-side current measurement (red wire) but is looks like you're taking a low-side current measurement (black wire) on the ground side of the load. Am I missing something?

[@clivebillson]

Seems just what I'm looking for to measure uA up to 50mA but this would be an ac current. Would this still work with ac. but give an ac voltage output I wonder?

[@leviterande]

really appreciated this video thx. one question: can this board handle nanoamp measurements even if the voltage of these low currents are low like in the uV range?

[@jjoster]

I never tried to measure voltages that low. The secret to it though is in the opamp. If you pick an opamp that has good enough resolution it ought to work.

[@leviterande]

craftycoder thx, actually, I dont need to measure the voltage. it is just the current as you should in the video. but it is just that the voltage of this current is going to be in the micro volts.. I am thining about measuring the current from an aerial inductor/antenna..

[@paulg.3067]

Hi, I used a OPA134 as transimpedance amplifier but without a current connected I get a voltage reading of 22mV. If I use that as Offset on the voltage measured with current connected I get quite exact current readings. Is this a problem if I have that offset voltage? My virtual ground is perfectly between -2.5V and +2.5V

[@jjoster]

There are trim pins on that part. 22mv sounds like a lot though. Up the voltage. You may be under volting it.

[@paulg.3067]

@@jjoster Thanks! You were right, OPA134 was undervolted. With +/-3.2V for OPA134 I get now 0V with no current connected.

[@jjoster]

@@paulg.3067 That pleases me. I'm out of the hardware business at the moment. Cloud software is a gravity well difficult to escape. Good to know I still have a clue. Have fun!

[@lMAKAVELLIl]

Do you think it is possible to do a picoameter using this circuit. Main differences being the - i would use lmc662 from TI (which as ultra-low iput bias current of 2fA) - i would use a 1M resistor - i would use a shielding box

[@jjoster]

Miguel Costa Rodrigues not sure, would have to try it out I suppose. I haven't thought about this circuit in a while.

[@lMAKAVELLIl]

i meant 1G resistor

[@souvikkundu5992]

Hello, I am facing a weird problem after making this circuit exactly the same except instead of connecting the circuit to the MCP601P I have connected it to ground directly since I do have a constant 5V power source for the OPA227 opamp. The problem is that - while measuring the output voltage at node 6 of OPA277, If I am connecting the ground of the voltmeter common with grounds at pins 3 and 4 of the OPA277, I am getting a very low voltage (in mV range), whereas the voltage is as desired if I separate the ground while measuring the value at node 6. i.e. If I connect the voltmeter's ground with the ground of the current generating circuit it works as desired but not with the OPA277 's ground point. Why it is so ? Any reason ? I knew that grounds are mostly common in a given circuit. Any help is highly appreciated. Thanks in advance.

[@jjoster]

Souvik Kundu it is designed to have separate power rails. I used a battery to power the OPA as I recall.

[@souvikkundu5992]

Will the circuit not work without that MCP601P part ?? I have connected the ground instead of that part. I am not getting the desired value while measuring 100nA and below. Whereas 1uA and 10uA is giving an error of 12-13% approx. So is it important to build the circuit with MCP601P ?

[@jjoster]

I have not done an analysis of the circuit recently. I am not sure.

[@souvikkundu5992]

Okay. Thank you for your response..

[@seyedmarashi]

if possible test project with real load and then check how to current wil convert to voltage and voltage relation with Current Thanks for kindly video

[@Coco_drilo]

This circuit connect in series or parallel

[@libervolucion]

As far as I can see, it's in series. You realize it when he shows the breadboard. He speaks about the positive voltage that has to be connected on row 16, but there's no negative voltage. It goes straight into the circuit.

[@hulladek3]

Great vid!

[@hakonmoen3438]

Great! Some bright dude should compare this solution to Dave Jones' uCurrent. Just say'n it would be helpful that's all

[@textorijum]

great video! Just speak a bit faster... maybe if you prepare list of the things you want to say, you would go faster and still cover everything..

[@oshinprem9251]

Really great video..this is a transimpedance amplifier rite..i didnt understand the final voltage measuring part..if i m not wrong this circuit measure very low current in uamp to nano amp range current and convert it to voltage right..m sry m not a very technical person.. i was really hoping if i could get a little help in my project regarding this. in my project the current will be in nano amp range coming from laser based gas detector and will be using a tuning fork. i have to convert this small current into voltage and amplify.. and the gain should be maximum and this should work in frequency range 15-50 khz. can i use the exact same circuit for this purpose or how do i adjust the frequency range? or what modifications could i do to use it in my project? your help would be really really appreciated. it is very urgent so i thought u would be a great help...thankyou :) please reply asap @ [e-mail] Thankyou very much :)

[@jjoster]

You can use a circuit like this to do what you are doing. The amplified you use will need have the bandwidth you require, but 50kHz is not HF so you should have too many issues sourcing an amp. Build the circuit, test it out. The one "gotcha" is that it needs to be powered by a floating power source (battery or floating bench supply)

[@oshinprem9251]

craftycoder ohh cool..thanku thanku so much..i hope it works for my project.. your circuit only converts current to voltage right? and il be needing an extra amplifier in this i guess..?

[@jjoster]

oshin prem Yes, current to voltage is the purpose. One amplifier will do. Adjust the gain to the expected current you need to measure and select the amp based on testing and datasheet properties. Digikey sells purpose specific transimpedance amps. I've never used one but they may be worth a shot.

[@oshinprem9251]

craftycoder Alright...Thanku so much for giving me ur time :) this will surely do :)

[@jjoster]

I just posted up a video on this topic for you. Check it out.