measuring current sense resistors [closed]

I have a reference design circuit with 4 power rails. Already designed into the circuit are 4 high side current sense shunt resistors. What's the best way I can continuously measure the voltage across each of these shunts simultaneously? I already have a multi-channel scope, but the inputs share GND. The circuit to be measured is a low voltage, low current circuit (<5V, 1mA to 1A with 500uA resolution).

Is there some off-the-shelf multi-channel amplifier system that i can buy and plug into my scope inputs? I'm looking to avoid having to do any circuit design or soldering for the measurement system.

Thanks.

Edit: (1) The voltage rails are at 5V, 3.3V, 1.8V, & 1.2V (2) Sampling rate is >=20KHz

closed as off-topic by Leon Heller, Daniel Grillo, Ricardo, Nick Alexeev♦Sep 1 '15 at 4:59

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• Your last paragraph turns this into a shopping question, which is off-topic here. (BTW, yes, there are many off-the-shelf data acquistion systems that have differential inputs.) – Dave Tweed Aug 31 '15 at 14:32
• Please add more information to your question. 1) Are the voltage rails the same voltage? 2) What voltages are you trying to measure the current of? 3) What bandwidth do you need? – Dwayne Reid Aug 31 '15 at 16:00
• Hi Dave - thanks for the note. I'm new to this and trying to learn practical, quick solutions to be able to qualify a circuit and measure current draw across multiple rails with high side monitoring. I don't need a SKU or P/N, more a general idea for off-the-shelf approaches to solve this which don't require a PCB to be fabricated or individual components to be assembled. Perhaps there's a better way I could phrase the question? – user2856006 Aug 31 '15 at 18:35
• Hi Dwayne - I responded to your questions in an edit, above. Thanks for your time/help. – user2856006 Aug 31 '15 at 18:36

You could use current monitor ICs that level shift the high-side sense current to ground.

For example, the AD8218. There are other chips available, and you should search for something that is appropriate for your application. That particular chip has a bandwidth of several hundred kHz and provides a gain of 20 as well as level shifting.

Or use an oscilloscope with independently floating channels like I do (not cheap).

You did not specify what the intention of the current measurement was. In particular if your aim was to monitor the current real time to watch for current spikes and fast changes or if rather the goal was simply a long term monitoring with a relatively slow sample rate. If you just need the latter then I think you have a number of options. The choice will depend upon whether you need to have the measurement data logged for later use in data analysis.

For the simplest case just use four separate battery operated digital multimeters. You can monitor and take down readings into an engineering log book.

The next step up is to acquire digital multimeters that have an optically isolated serial interface into a computer or galvanically isolated USB interface. Using this will allow automated data gathering without having to take manual readings. You can find this type of digital meter on Amazon for less than a hundred bucks.

The third option is to acquire some Lascar USB data loggers. These run on batteries and are setup by plugging into a PC serial port. Then once armed they operate independently in data logging mode storing the data internally. After the test session is complete the logger is once again plugged into the PC for data download. These are more convenient than multiple digital meters but cost about the same. Depending upon the application the voltage resolution of the linked Lascar may be a limitation so investigate thoroughly before purchasing.

• Thanks for the detailed response Michael! I'd like to monitor all 4 traces of current in real time, with at least 20KHz sampling. Ideally I'd want to plug it into my existing scope, which has the bandwidth & # of channels but shares GND unfortunately. Are there any similar solutions you'd recommend for this real time measurement? I need a little help coming up with a quick, practical solution that doesn't require any board design or soldering. – user2856006 Aug 31 '15 at 16:31
• If you have a firm commitment to not build anything to get your real time measurements then I would suggest that you get yourself a set of four differential scope probes. These would convert your scope to four channels with capability to support voltage measurement that are floating with respect to the scope ground. The common mode range of differential probes is of course limited by the design of the probe so select ones that are suitable to your measurement requirements. (continued) – Michael Karas Sep 1 '15 at 1:25
• (continued from above) Check in eBay to get an idea of what is available. You can find products over a wide price range. – Michael Karas Sep 1 '15 at 1:27