4072 accuracy when measuring 7uA DC

I need to understand the accuracy of 4072 card, for an error budget study.
1) when measuring 7uA DC current, please let me know the details of error stackup. not just the result, I need the calculation details for each error component.
on page 3 of attached specification sheet, bottom table, first row, it states:
DC Current* ± (ppm of reading + ppm of range)
- Range: 20 mA
- Resolution: 10 nA
- Burden Voltage (typical): <20 mV
- Noise (ppm of range rms): 20
- 2 Year (0 °C to 55 °C): 400 + 150
- Tempco/°C (0 °C to 55 °C): 8 + 1
my error calculation is as follwing (7uA DC/ temperature 40C):
- Noise (ppm of range rms): 20 * 10^-6 * 20,000uA = 0.4uA
- 2 Year (0 °C to 55 °C): 400 * 10^-6 * 7uA + 150 * 10^-6 * 20,000uA = 3.0028uA
- Tempco/°C (0 °C to 55 °C): (8 * 10^-6 * 7uA + 1 * 10^-6 * 20,000uA) * 40C = 0.80224uA
so total error = 0.4 + 3.0028 + 0.80224 = 4.20504uA @ 7uA DC
error % = 4.20504/7 = 60.072%
Is my calculation right or wrong?
2) on page 2 of the attached specification sheet, under "DC System Speeds", it says "Trigger latency ..... 2uS". Does this mean 4072 can catch the trigger condition 2uS after the event?
Say I have a step function DC current drops from 1A to 0 at T0. I set trigger as 0.5A. so 4072 will detect the event @ T0+2uS??
Thanks!!!!!!
Attachments:
NI PXI 4072 DMM Specifications.pdf ‏665 KB

Hi QWERT1 and welcome to the discussion forums.
Since the lowest range of the 4072 is 20mA, reading 7uA will prove to be a challenge and I do not doubt you will be seeing an error in the realm of 50%. Comparatively, the 4071, which has a 10uA range, would allow for much higher accuracy readings at this level.
I see that you contact National Instruments directly and seem to have closed out your service request with an Applications Engineer.
For all other viewers reading this, QWERT1 calculated the accuracy correctly except for one aspect: the tempco. The only mistake here was that this needed to be multiplied by the number of degrees difference from the last calibration temperature. QWERT1 multiplied by the temperature, 40, rather than the temperature difference from the last external or self-calibration temperature. So if the calibration temperature is 23degC and the current temperature is 25degC, then you would multiply the error by 2.
Thanks again for using the forums QWERT1!
PBear
NI RF

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