Repurposing Unisense Picoammeter PA2000

[dagnum]

I have an old Unisense Picoammeter PA2000 at my university that I would like to use to measure sleeping current for a battery-powered sensor I built. The device is intended to be used in the laboratory setting for dissolved oxygen probes and there isn't a lot of documentation about building your own 'probes' or inputs to the BNC connectors. I have found a manual online: https://unisense.com/wp-content/uploads/2021/10/PA2000-manual_A5www.pdf

Some key specifications:
•    Polarization range: +/-2.5V
•    Measuring ranges: 200 pA, 2 nA, 20 nA, and200 nA
•    Signal range: 0 – +/-1,000,000 pA
•    Display range: 0 – +/-200,000 pA
•    Analogue output: 0 – +/-10V
•    Signal offset: > +/-1V
•    Resolution: 0.05 pA (approx. 0.1 µmol/L forstandard oxygen microsensor)
•    Maximum gain factor: >10

Can anyone help me understand how or if I can set this up to make a simple current measurement from a battery-powered device? I don't know what the inputs should be or understand the role of polarization range, signal offset, or gain factor.

[dobsonr741]

The Eevblog micro current adapter is much better suited for this job: https://www.eevblog.com/product/ucurrentgold/

The issue is your meter is providing of it’s own “polarization” voltage, whereas an IOT device would need to be on it’s own power.

What goes inside of your meter is about the same as in this DIY project (btw same range, too): http://circuitcellar.com/wp-content/uploads/2014/03/CC_042010_Lacoste_ReprintedwPermission-web.pdf

[Swainster]

Agree that this is not the right tool for the job, however for slightly different reasons. Polarisation is what the EE world usually call bias. For your application this would appear as a voltage source in series with the ammeter. Set it to zero and you could ignore it.

However, this instrument is intended to measure differences in current, not absolute current, and to maximise resolution it limits the dynamic range. For your application, I would guess that an absolute measurement would be wanted. The uCurrent fits this bill. Another option could be member Gyro's picoammeter project.

Lastly, is the question of what exactly you are measuring. If you just want to measure the minimum current then any of the above options could work. However if you want to estimate battery life i.e. total energy used, and you device is a type which mostly sleeps then occasionally wakes up with mA plus currents, then the above tools probably won't have enough dynamic range. It's possible to work around this by measuring the peaks separately (using a 'scope) and then manually adding the two contributions, but if you want a single instrument to do this for you then you need something equivalent to the rather pricy joulescope, or the more reasonably priced Nordic RF power profiler.

[dagnum]

Thank you both for the replies- I have a much better understanding of how the PA2000 works and how I want to go forward. I had been measuring current with a shunt resistor but didn't have the dynamic range to get a meaningful sleep current. The range is ~20 mA while measuring and logging down to sub uA during sleep. I'll follow Swainster's suggestion to measure the sleep and active currents separately using different valued shunts.