# Measuring low currents with oscilloscope - does the current really flow backwards this much?

I have an RF mcu intended to run on a coin cell and I'm going to need to measure current consumption as it transitions from state to state.

As a quick test I put a 2 ohm resistor in series with the circuit and attached that to the oscilloscope with the probe on one side of the resistor and he probe ground on the other side of the resistor. Here is what I'm getting on the oscilloscope:

What is curious is how the current appears to flow back wards as much as it went forwards. I assume my test setup is bad, and I either need isolation somewhere or I need to implement a differential probe.

Is this normal and expected, or do I need to resolve some issues before trying to use this for measuring current consumption?

Note that the processor is running at 24MHz which closely matches the pulses in the waveform.

Here is the test setup:

• have you set your scope to DC or AC coupling? – Marcus Müller Jan 18 '17 at 20:26
• @MarcusMüller DC. The average is right around 2mA so the scope seems to be capturing the right area. – Adam Davis Jan 18 '17 at 20:27
• Keep the loop area small (loop area from probe to probe ground ) otherwise your making an antenna. – Voltage Spike Jan 18 '17 at 21:29
• You have a resonance in the 20 MHz ballpark. Likely caused by inductance of wiring combined with inductance of your 'scope ground clip, resonating with 'scope + cable capacitance. Describe your 2-ohm current sense (and its wiring) and ALSO describe how you connect it to the 'scope. A photo might help a lot. – glen_geek Jan 18 '17 at 22:27
• What bypass capacitors do you have on the MCU, and where are they connected relative to the current sense resistor? – Bruce Abbott Jan 18 '17 at 23:26

For the sake of discussion, let's assume that the MCU absorbs around 1mA (I suspect much less, actually). This means 2mV across the sensing resistor, which is pretty low. I think you are just measuring common mode noise, and your DC component is swamped by that.

You could try to activate an averaging acquisition mode, averaging over several waveforms. You should see some improvement.

You could also try to deactivate all possible sources of interference in the room (if you have LED lighting fixtures their power supply could contribute to the radiated and conducted noise, for example).

Try also to avoid using the ground clip of your scope probe: that acts as an antenna! Locate a ground point near your resistor and connect to it using this kind of probe accessory:

EEVBlog #441 - "How to track down common mode noise" gives additional hints and I think it's relevant.

Anyway to measure such tiny currents you may need some additional amplification. Dave Jones, from EEVBlog, has designed his μCurrent amplifier just for this kind of things.

1. Learn how to use and calibrate a probe for low level high speed square waves without the ground wire (inductive ringing >>1 MHz) and remove probe clip using the pin and barrel gnd.

• There should be no ringing on the scope test square wave at high sweep rate.
• repeat with 2nd probe.
• When they are matched, you can put then anywhere and A-B is a flat line < 1mV noise.
• Then use for Current sense R (50mV~100mV shunt) in differential mode or on ground side with single probe.
2. The ESR of a coin cell can be several hundred ohms to >1kOhm so 1 Ohm is pretty small for current sensing, so size accordingly such as 100 Ohm.