Help with current shunt amplifier - Page 1
Forum section: electronics / projects

author: teslinius
author_rank: Regular Contributor
timestamp: September 03, 2021, 10:02:27 pm
content: I am designing a circuit that's gonna measure currents up to 25A and i want the resolution of 1mA. I have a 16 bit adc (ads1115). Current shunt resistance is 0.01 ohm and i am using op07 opamp powered by +12V. The thing is i did some calculations and i found out that with this setup i wouldn't be able to measure very low curretns like a few mA. So i'm not sure if i choose a different opamp with higher gain or do i add a separate higher resistance shunt for measuring lower currents and a relay for switching ranges(i only care about the 1mA resolution at lower currents(<5A)and not for whole 25A region)? or if there's a simpler waydon't mind the component values because i haven't adjusted them yet

author: tszaboo
author_rank: Super Contributor
timestamp: September 03, 2021, 10:24:33 pm
content: 1. What's your common mode voltage on your shunt?2. Why don't you just place an INA194 or similar on it?

author: David Hess
author_rank: Super Contributor
timestamp: September 04, 2021, 12:53:43 am
content: Quote from: teslinius on September 03, 2021, 10:02:27 pmWhat calculation leads you to believe that it will not work at low currents?0.01 ohms and 1 milliamp is 10 microvolts, which is reasonably consistent with the best precision operational amplifiers, although there are better parts than the OP-07 now. The low source resistance makes this a good application for a chopper-stabilized operational amplifierI am not a fan of using the instrumentation amplifier configuration because of common mode rejection limitations but this is not important if your current shunt is referenced to ground limiting the shift in common mode voltage. I preferthis circuitbut it requires the operational amplifier to operate at the shunt voltage and it only reads current in one direction.

author: Kleinstein
author_rank: Super Contributor
timestamp: September 04, 2021, 05:22:05 am
content: The 100 pF at the output of the OP are usually a bad idea. OPs don't like capacitive load and tend to show more ringing and may oscillate if too much.The datasheets may show the performance with some capacitove load, but this is more like the upper limit on how much could be tolerated, not a min value.It depends on the circuit if an INA is usefull or suitable. There is a difference if the shunt is near / at ground, somewhere floating or at the supply.INAs are good with common mode rejection, but many have a rather limited common mode range. So they may just not work at the positive supply.The ciruit shown also has a limited common mode range - though still better than many INAs.The simple positive side current sense amlifier is made specially for the postive side, but does not work below a certain voltage.A shunt at ground does not need a good common mode rejection and an INA has little advantage.10 µV resolution seems reasoneble with the OP07.

author: Terry Bites
author_rank: Super Contributor
timestamp: September 04, 2021, 04:45:40 pm
content: 25A through 0.0R generates 0.25V . Your diff amp has a gian of 76 so the output would be 19V! The OP07 isnt a single supply opamp and won't work in this configuration. Even with matched resistors, the drift will be too difficult to eliminate. Resovling 1mA in 25A needs 12bit precision, rolling your own will not do! Go for the ready rolled item, eg LTC6102 analog or MAX9612 (with ADC) for a couple of euros.

author: Vovk_Z
author_rank: Super Contributor
timestamp: September 04, 2021, 08:42:14 pm
content: OPA189 is still not very expensive but super-accurate, possibly it can help.Possibly OP07/op37 will work fine too bit it'll need more wide calibration range.

author: David Hess
author_rank: Super Contributor
timestamp: September 05, 2021, 12:24:03 am
content: Quote from: Terry Bites on September 04, 2021, 04:45:40 pmUsing a dual supply solves the problem if an OP07 is used, and any single supply operational amplifier will have problems generating low output voltages. This application really requires a negative supply no matter what.

author: teslinius
author_rank: Regular Contributor
timestamp: September 06, 2021, 09:05:55 pm
content: Quote from: David Hess on September 04, 2021, 12:53:43 amIsn't this circuit differential amplifier?, because i am trying to use a differential amplifier not instrumentationQuote from: David Hess on September 05, 2021, 12:24:03 amI added a negative supply circuit, this is my first time making it, is it correct?, for supply i am using a 50hz transformer.Also can i put the polyfuse at the ac input?, i don't know how it behaves in ac.And i don't 100% understand operational amplifiers yet, so on single supply it can't reach perfect 0v on the output there's allways some offset and dual supply solves that?

author: David Hess
author_rank: Super Contributor
timestamp: September 07, 2021, 02:12:26 am
content: Quote from: teslinius on September 06, 2021, 09:05:55 pmA differential amplifier has differential inputs and outputs. What you have drawn is the 4 resistor version of an instrumentation amplifier that uses a signle operational amplifier. It is really more of a "difference" amplifier because real instrumentation amplifiers have high impedance inputs to limit errors from high source impedance, but that does not matter in your application because the source impedance of a current shunt is very low.QuoteYou have the right idea but you cannot make a dual supply that way. Half wave rectify the output from the transformer to get plus and minus supplies.QuotePolyfuses work fine for AC.QuoteThat is right, so even if you used a "single supply" operational amplifier, a negative supply would be needed to allow the output to reach ground.

author: teslinius
author_rank: Regular Contributor
timestamp: September 07, 2021, 07:44:17 pm
content: I decided to use a dual supply transfomer so it's easier, it's correct now right?So now what about the opamp?, do i change it, the circuit?, as someone said my only problem is i don't know how to make it measure through all the 25A range, since at 25A it will generate around 0.25V which by gain of let's say 80 is more then 12V, unless i change the shunt to a lower resistance.

author: David Hess
author_rank: Super Contributor
timestamp: September 07, 2021, 09:20:01 pm
content: Quote from: teslinius on September 07, 2021, 07:44:17 pmThat will work fine. I am not sure what D2 is doing.QuoteOr lower the gain so that the output fits within the supply voltage range.A different operational amplifier will only make a small improvement in the output range unless the supply voltages are increased. The chief advantage of the OP-07 is its low price for a precision operational amplifier.

author: Terry Bites
author_rank: Super Contributor
timestamp: September 08, 2021, 07:14:07 am
content: I've just looked at the data sheet for the ADC, figure 44 seems to show the very thing you are after.I'm still curoius about the common mode voltage you are working with. What voltage ranges can you expect Vq to be with respect to circuit ground?.If you are making a low side measurement it doesnt matter, however if its a high side measurment you may have problems.https://www.allaboutcircuits.com/technical-articles/resistive-current-sensing-low-side-versus-high-side-sensing/

author: teslinius
author_rank: Regular Contributor
timestamp: September 08, 2021, 08:31:35 am
content: Quote from: David Hess on September 07, 2021, 09:20:01 pmThat's a tvs diode if some high voltage ripples appear, so i calculated the needed gain is around 48, but there's one more problem the you can set max input adc to 1024, 2048, 4096 and 6000mV but since i'm powering it from 3.3V the max input voltage is 2048mV, so i need a voltage divider right?, or i can change the adc supply voltage to 5V so max input is around 6V.

author: GigaJoe
author_rank: Frequent Contributor
timestamp: September 13, 2021, 10:58:17 pm
content: Probably, this OK. but I would do 2 cascades .First stage OP07 1:1 with all resistors of the same nominal let say 10K , to increase rejection rate , with 0.01% (select nominal . or alter resistor to all nominal the same) it would be around 70dB (i'm right ?)a second stage to amplify and adjust to required levela negative you can do by ICL7660 , LT1054 or similar, an important thing , cut the noise on output, some 79L33 need in place , + capacitors.OP07 don't RRO opamp - you need to calculate ( measure \ adjust , if positive let say LM317 + adjustable) a precise rail voltage to limit input on ADC. (output from opamp)you need something RC in front ADC to average , if it noisy. (or in software level )