Electronic – Measuring low currents with oscilloscope – does the current really flow backwards this much

current measurementmicrocontrolleroscilloscopeRF

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:

Best Answer

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.

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Electronic – Cannot get a digital square wave on oscilloscope from the GPIO of the Raspberry Pi

There are a couple of issues with your code that can be easily fixed. First, you are calling wiringPiSetup which means you will need to refer to the pins by their WiringPi designation which can be found on this page:

https://projects.drogon.net/raspberry-pi/wiringpi/pins/

You are then setting pin 7 (wiringPi designation) to be an output which is actually GPIO 4 (physical pin 7) and not the CE1 pin as you mentioned. CE1 would actually be wiringPi pin 11.

The next problem is in the while loop. You are doing a digitalWrite on wiringPi pin 4 (not 7 as you configured) and are always setting it low so it would never toggle high even if configured correctly. The following code should be what you are looking for but is not tested. This assumes you want to use CE1 which is normally used for SPI but should work with WiringPi as an general I/O pin:

  if (wiringPiSetup () == -1)
    exit (1) ;

  pinMode(11, OUTPUT);

  while(1) {
    digitalWrite(11, 0);
    digitalWrite(11, 1);
  }

Oscilloscope measuring of PMT in photon counting mode

I'm not sure what your 13 kHz ripple is, but it seems more like power supply ripple than a ground loop. If your HV supply is a positive supply, then the PMT 'ground' is going to be ripple-y. The floating ground lead on your scope presumably has a lot of parasitic capacitance, since it goes to the chassis and everywhere inside. When the ground lead is connected to PMT ground, there's not much capacitance on the anode, but when the ground lead is on the anode, there's a big chunk of capacitance to charge and discharge with the ripple. That's my speculation.

In general, what you want when dealing with pulses is a "pulse discriminator", which lets you put conditions on the height, length, etc. of the pulses you're interested in. Since you're pulse counting, you probably don't need anything fancy, a simple comparator will do, but it will need to have sufficient negative input range. You can build a simple one using an op-amp, but you can get ones off the shelf that are faster and with logic level output. Alternatively you could use an inverting amplifier topology to flip the signal, but you will still need negative input range and sufficiently high bandwidth to pass pulse waveforms through.

Edit: One more thing, isn't the max voltage on the R1166 1250V?

Best Answer

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Electronic – Cannot get a digital square wave on oscilloscope from the GPIO of the Raspberry Pi

Oscilloscope measuring of PMT in photon counting mode

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