RE: Microamp precision when measuring current with a scope

awneil — Fri, 10 Jun 2016 14:22:52 GMT

Just happened to arrive in my mailbox today.

Other brands are, of course, available ...

RE: Microamp precision when measuring current with a scope

Eliot Stock — Fri, 10 Jun 2016 14:17:54 GMT

Interesting! Thank you.

RE: Microamp precision when measuring current with a scope

awneil — Fri, 10 Jun 2016 13:24:17 GMT

In today's mailbox:

www.rs-online.com/.../affordable-precision-current-sensing

RE: Microamp precision when measuring current with a scope

Eliot Stock — Thu, 02 Jun 2016 09:51:29 GMT

Excellent, thank you. I'll have a go with a few different values and see what I can do.

RE: Microamp precision when measuring current with a scope

Stian Røed Hafskjold — Thu, 02 Jun 2016 09:39:52 GMT

Generally it is not easy to get high-precision measurements with an oscilloscope (this of course is dependent on how good the scope is). The tutorial is based around measuring BLE events and then the currents can reach somewhere around 10mA. Having a larger resistor will force you to use a lower sensitivity mode in order to avoid clipping, which will give you lower accuracy. Another reason to not have a larger resistor is to avoid having a too large voltage drop which will result in wrong measurements. Using a lower resistor value the common-mode currents, which can be caused by both external and internal factors, are more significant, hence also a lower accuracy. So, if you are certain that the current is low, for instance when only measuring system OFF, you can try with a higher resistor value, still using the highest sensitivity setting on you scope. It all depends on the dynamic range of your equipment. So it is a trade-off here. Feel free to try different resistor values to find the one that fits your measurements and equipment best.

As described in the tutorial, you should "calibrate" your measurements by connecting both probes to the same node (at the voltage level you are going to measure) to see how accurate the differential voltage is. If you get other than 0 diff voltage it will give you an indication on the accuracy limits.

If you have a "high bandwidth" setting on your scope you should enable this. Also using 1X probes is pretty critical to avoid too much noise. If the signal is periodic you can consider using averaging mode on the scope, which will reduce the noise factor.

I think using a ammeter is a better way to measure low currents, since they use integrating opamps over a specified time interval to integrate all the current that goes through. So as long as the "events" on the chip that draw current is periodic with an interval shorter than the integration interval on the ammeter, you will get a very accurate measurement. Testing here shows that "event" intervals shorter than 100ms will give good measurements on general purpose multimeters. You can consider changing your code to sample the accelerometer at shorter intervals than 100ms just for measurement purposes and then do some calculations.

RE: Microamp precision when measuring current with a scope

awneil — Wed, 01 Jun 2016 12:30:56 GMT

Use a Current Sense Amplifier (CSA) to amplify the signal.

But note that scopes are not high-precision measuring instruments ...