Tinycurrent noise

[AndreKR]

I have a tinyCurrent (Sorry Dave, but the uCurrent was out of stock for ages.) but I'm having some noise issues with it and I wonder if this is the normal noise that is to be expected or if I'm doing something wrong or maybe my tinyCurrent is broken.

I tested a bunch of setups with an ESP32 as the load (about 35 mA current with some spikes).

When I use a BNC cable (this would be my preferred way of connecting it) I consistently get this amount of noise:



With a probe I can sometimes get less noise, but I think this happens only when I have a dodgy connection to the probe (the banana plug isn't the greatest of connectors) as it seems to change when I knock the tinyCurrent or fiddle with the probe:



I tried another type of connection but it's even worse (disconnected for the photo):



For comparison, here's the setup with the 1 Ohm resistor I use for best performance (but higher burden voltage):



After watching video MXbDf8cCsNg on YouTube I tried it with a 10 µF capacitor across the output, but that gave unusable results:

[AndreKR]

The noise is still there when no load is connected:



With BNC as well:



But when the device is off, the noise is gone:

[Rx7man]

Dave did a great video about noise in a ucurrent, and it was dependent on what kind of load was on it.  IIRC it was some differences in chip manufacturers for the same part number, and the root cause was the split rail generating op amp was unstable

[Kleinstein]

The extra "noise" when using the BNC connection may actually be oscialltion of the amplifier due to capacitive loading. The test with the large capacitor at the ouput goes in the same direction.  Dave's µCurrent in some versions also had a similar problem.
Chances are the circuit is missing a resistor in series with the ouput.  For some OPs just the capacitance of the BNC cable ( ~ 100 pF / m) can be enough to make them oscillate.

[AndreKR]

Dave did a great video about noise in a ucurrent

Can you link me to it? I actually looked but couldn't find any that seemed related judging by the titles.

[Kleinstein]

The video one the µCurrent problem is #1057:
https://www.eevblog.com/forum/blog/eevblog-1057-current-murphy/msg1424097/#msg1424097

There is a newer video on the option to use lower noise OPs, but that would be more like a different step. Here it is likely more about an oscillation problem. A resistor (e.g. some 50 or 100 Ohms) in series to the output should likely fix the problem.

[AndreKR]

Tried it with this setup: (tinyCurrent off and on)

[Kleinstein]

The extra resistor may help at the BNC connector or when connecting a higher capacitance meter. There is no need for it with a 1:10 sope probe, but it would not hurt.