01-28-2010 05:31 PM
I am using the PCI-6120 board in warp mode, at 1 MHz. Everything is working fine but there seems to be a significant discrepancy between the measured noise performance (with 50 ohm or short) and quoted performance in the 6115/6120 spec sheet. Below is my comparison.
AI Range Quoted N+Q (uV) Measured N (uVrms)
+/-42 V 550 1410
+/-20 V 220 600
+/-10 V 110 280
+/-5 V 69 140
+/-2 V 31 60
+/-1 V 18 30
+/-0.5 V 10 20
+/-0.2 V 5.1 15
It seems to me that the N+Q data is shifted by one analog input range as the numbers agree well just not on the right ranges. For example the 42 V range quoted N+Q agrees with the 20 V range measured noise. Has anyone else witnessed this?
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01-28-2010 05:57 PM
The specifications for the PCI-6115 and PCI-6120 are not applicable when using Warp Mode. Warp Mode causes the board to sample faster than the specified ADC settling time, as a result your accuracy will suffer. The specifications give are valid in non-warp mode sampling up to 800 kS/s maximum. I apologize for any inconvenience this may cause.
Regards,
01-28-2010 06:17 PM
Seth, thanks for the reply but it is not a warp mode issue. I checked the device under normal sample rates using the onboard sample clock from 10 kHz - 800 kHz and the noise levels change less than 10 %. I also tested the device using the onboard counter as the timing source just like in warp mode but at sample rates from 10 kHz - 800 kHz and got similar results. The latter was the lowest of the two configurations. ~ Mike
01-28-2010 06:19 PM
01-28-2010 06:22 PM
02-02-2010 11:41 AM
Hello Mike,
Sorry for the delayed response. Your measured N values, are they averaged or single point values? Also, has the card been self-calibrated recently?
Thanks,
02-02-2010 01:58 PM
02-02-2010 06:37 PM
I tired the self-calibration. The noise levels did not change. Here is some additional data I took at a 500 kHz sample rate.
Range N(single point) N(averaged)
42 11.3 mV 1365 uVrms
20 4.82 mV 589 uVrms
10 2.22 mV 272 uVrms
5 1.16 mV 142 uVrms
2 0.44 mV 55 uVrms
1 0.25 mV 30 uVrms
0.5 0.16 mV 20 uVrms
0.2 0.13 mV 15 uVrms
02-03-2010 04:30 PM
So what you've measured here is actually system noise, which we spec on page 4 of the specifications, not noise uncertainty, which we spec on page 3. The system noise for the PCI-6120 is shown below. To calculate system noise from these specs, you using the following formula:
(System Noise in LSBrms) * (Input Range)/(2^Resolution) = System Noise in Vrms
So, for the 42V range, the specified noise in LSBrms is 1.2 LSBrms. The input range is 84 (±42V is 84 volt total). The resolution is 16 bits. Thus:
1.2*((84)/(2^16)) = 0.001538 V = 1,538 uV
For each of the ranges, the system noise is:
Range | LSBs | System Noise (µV) |
42 | 1.2 | 1538.09 |
20 | 1.2 | 732.42 |
10 | 1.2 | 366.21 |
5 | 1.5 | 228.88 |
2 | 1.7 | 103.76 |
1 | 2 | 61.04 |
0.5 | 2.2 | 33.57 |
0.2 | 2.8 | 17.09 |
Hope this addresses your question.
02-03-2010 04:39 PM