08-20-2008 02:43 PM
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??
08-22-2008 12:16 PM
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!
12-04-2013 09:17 AM
How would one calculate the accuracy of the 4072 Isolated digitizer. I'm looking at page 12 of the spec and am baffled
by the specs in DB. Help please.
12-05-2013 08:08 AM
Here is a great KnowledgeBase article that outlines the process for determining the accuracy of NI DMM's. The article also includes an example accuracy calculation, which is specific to the NI 4071 DMM, but the exact same process applies to the NI 4072 DMM. Please let me know if you have further questions regarding the process described in the KnowledgeBase article. Thanks!
NI KnowledgeBase Article: How Do I Determine the Accuracy for NI DMMs?
12-05-2013 08:12 AM
Yea, I've been throught that article and have done the calculations. I was specifically asking about page 12 and 13 of the spec showing acuracy of the isolated digitizer accuracy. What is this and how does it relate to the other measurements.'
12-05-2013 11:50 AM
It all depends on the signal that you are reading. The calculations for the DC signal accuracy will still be the exact same process as outlined in the KnowledgeBase article that I sent you. If you are measuring an AC signal in isolated digitizer mode, then you will need to take into account the additional specifications listed in the "Isolated Digitizer Accuracy Specifications" table, which deal with the frequencies of an alternating signal.
12-05-2013 01:37 PM
You'll need to add the additional noise uncertainty listed on the bottom of page 4 to the DC specification when using the DMM in digitizer mode. Note that NI does not specify 4072 uncertainty when using the DMM at full speed (1.8MS/s). If you need guaranteed uncertainty, take a look at the 4071, which includes a more complete additional noise error section on page 3. You add that value - times the range multiplier - the the range ppm and then perform a normal DC uncertainty calculation (measurement*measurementuncertainty+range*(rangeuncertainty+Additionalnoiseerror*multiplier).
Tell us more about your application and maybe we can find you a better instrument to suit your accuracy needs at high speed. Have a great day!
12-05-2013 01:54 PM - edited 12-05-2013 01:55 PM
Another thought: take a look at the PXIe-4141, which has a 10uA range and can be used as a true current sense (i.e. no external voltage drop). It's technically an SMU, but by setting the output voltage to 0V, you can watch the current flowing through it. Just a thought. The 4141 can only measure at 600kS/s, but the accuracy is guaranteed at all sample rates (aperture times). At full sample rate, the specified additional range uncertainty caused by aperture time is 100nA, which brings overall measurement uncertainty of a 6uA measurement to ~104nA in the 10uA range @ Tcal±5°C.