PXI
PXI-e 4137 20Mohm measurement at small voltages
Dear community,
We are having and PXI-e 4137 card and try to achieve a good U-R characteristic measurement for volume production, later on. In order to get the software and hardware setup right, we are using a 20Mohm metal layer resistor to test the U-R line (the 20Mohm sense resistor is going to be used later in production). As it is for a volume production line, time is critical.
Here is one of our latest measurement results we can achieve:
Measurement parameters:
- 2 wire configuration
- calibration before measurement
- very short 20cm cable from PXI-e 4137 card
- voltage sweep from -4V to 4V in 12 points
- smallest current limit of 1µA
- Auto range for voltage and current
- source delay 200ms
- aperture time 100ms
It is clear to us, that the measurement at 364mV is a challenge for the system. The parameters at this point are approximately:
U = +364mV
R = 21.075Mohm
I_real_mean = 364mV/21.075Mohm = 17.272nA
I_measured = 364mV/21.050Mohm = 17.292nA
I_offset = 20pA (which is below the +-5°C spec: 0.03% + 100 pA and therefore fine)
What is annoying though is the current noise. Even with 100ms aperture and 200ms source delay, the data points are quite noisy. We'd be able to trim out the offset, but the noise is an issue for us and using longer source delay times and apertures are a challenge. The rms noise can be calculated to:
I_max = 364mV/21.065Mohm = 17.280nA
I_min = 364mV/21.035Mohm = 17.304nA
I_noise_range = 24pA/6 = 4pA (assuming +-3sigma)
The expected rms noise from spec would be 0.8pA, so 5x smaller. See Figure 4, 100ms aperture: https://www.ni.com/docs/de-DE/bundle/pxie-4137-specs/page/specs.html
The voltage accuracy and noise appear to be sufficient according to specification.
What could be the failure for the noise in this measurement? The measurement is critical for us and the noise a challenge. Are we potentially missing out settings? Which settings should we consider apart form aperture and source delay time?
Help appreciated.
