May 24, 2020 Extremophile, Test Equipment

Building a Gyro Picoammeter

I am building a Picoammeter as described in a thread in EEVBLOG

Building a Gyro Picoammeter
Schematic (enlarge)

The things I changed from the original design are the capacitor (I used a coax cable) and the output protection (there is none).

Build photos


The environmental conditions in my lab are relatively stable. At the time of measurements: 20.5°C, 50%RH. The elevation is 625 m.

In all the tests the picoammeter was warmed up for at least 30 minutes.

For the tests I made the two boxes in the photo. One is a 1k - 1Ohm voltage divider and the other a 1 GOhm resistor.

If the picoammeter works properly, whatever voltage is fed through the 1GOhm box, should be measured to the output.

The voltage divider is used as a mV power supply (converts V to mV).

Test boxes

1) Step response

I am feeding to the picoammeter 4V through the 1GOhm resistor box.

Step response test setup (enlarge)
Step response (Slower than continental drift)

As expected from the values of the resistor and capacitor the bandwidth of the picoammeter is small. With a rise time of 636ms its bandwidth should be about 0.5Hz.

2) Noise

To measure the noise floor of the picoammeter I covered the input BNC with a BNC cup. I also turned off my crappy LED lights.

Noise measurement setup (enlarge)
Y: 5μV/div, Ref 0VDC, 10 PLC averaging (enlarge)
Y: 5μV/div, Ref 0VDC, 200 PLC averaging (enlarge)

Just for good measure this is the noise floor of the HP34970A

Y: 0.5μV/div, Ref 0VDC, 200 PLC averaging (enlarge)

3) Linearity

To test the linearity of the picoammeter I am feeding a 0 to 4V ramp and a 0 to -4V ramp from my DP832 through an 1 GOhm resistor. So in principle I should see the same voltage at it's output. This corresonds to the full ±4nA scale of the instrument.

Test setup (enlarge)
X: 8s/div, Y: 500mV/div, Ref 1.5VDC, 10 PLC averaging (enlarge)
X: 8s/div, Y: 500mV/div, Ref -1.5VDC, 10 PLC averaging (enlarge)

Here is the python code for the DP832 that makes the ramp. See also this article.

4) Resolution

Now lets see what is the finest measurement possible. For this test I am feeding a 0 to 100mV ramp through the voltage divider box and the 1GOhm resistor. This corresponds to 0 to 100fA.

Test setup (enlarge)
X: 8s/div, Y: (green)20μV/div, (white)20mV/div, Ref 0VDC, 10 PLC averaging (enlarge)

20μV correspond to 20 fA. I would say that the resolution of the instrument is ±20fA.