References/links:
- EEVBlog thread
- Low level measurements handbook
- LMC662 Datasheet
- The 1GOhm resistor used
- Coax cable for the capacitor
![](https://btbm.ch/content/images/2020/05/Gyro_Picoammeter_002.jpg.png)
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
![](https://btbm.ch/content/images/2020/04/2020-04-28-16.26.44.jpg)
![](https://btbm.ch/content/images/2020/04/2020-04-28-16.27.30.jpg)
Tests
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).
![](https://btbm.ch/content/images/2020/05/test-boxes.jpg)
1) Step response
I am feeding to the picoammeter 4V through the 1GOhm resistor box.
![](https://btbm.ch/content/images/2020/05/TestSetup1.jpg)
![](https://btbm.ch/content/images/2020/05/Risetime-1.png)
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.
![](https://btbm.ch/content/images/2020/05/noise-setup-1.jpg)
![](https://btbm.ch/content/images/2020/05/noise-floor.png)
![](https://btbm.ch/content/images/2020/05/noise-floor-200plc.png)
Just for good measure this is the noise floor of the HP34970A
![](https://btbm.ch/content/images/2020/05/HP34970A-noise-floor.png)
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.
![](https://btbm.ch/content/images/2020/05/test-setup-linearity.jpg)
![](https://btbm.ch/content/images/2020/05/Linearity-1.png)
![](https://btbm.ch/content/images/2020/05/Linearity-2.png)
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.
![](https://btbm.ch/content/images/2020/05/Test-setup-Resolution.jpg)
![](https://btbm.ch/content/images/2020/05/Resolution.png)
20μV correspond to 20 fA. I would say that the resolution of the instrument is ±20fA.