Additional Information
The NI 670x devices are software-timed devices that are updated by software commands. In other words, they have a “static” update rate because the analog output channels are updated based on software, not a hardware clock.
The maximum update rate of your device depends on whether you are using the NI 6703 or the NI 6704.
Maximum Update Rate of the NI 6703:The NI-6703 has one 16-bit DAC that is time-division multiplexed to create 16 voltage output and 2 calibration channels (18 channels total). Each of the channels has a sample-and-hold that maintains the channel’s output levels between DAC updates. The NI-6703 continuously updates each of the analog channel’s sample-and-hold circuits to maintain the proper value for the output. The channels are scanned at a rate of 50 µs per channel, thus a channel value will change anywhere from 0 to 0.9 ms (50 µs/channel * 18 channels = 0.9 ms) after it has been updated by software. If the channel that the software updates is the next channel to be updated in the 18-channel update cycle, the output will be updated almost instantaneously. On the other hand, if the channel was updated in the 18-channel update cycle just before the software updated that channel, it will be another 0.9 ms before the channel will be updated again. If we consider the maximum update rate of the device to be the longest possible time it takes for a channel to update, the maximum update rate for the NI-6703 is 0.9 ms.
Maximum Update Rate of the NI-6704:The NI-6704 operates the same way as the NI-6703 except that the NI-6704 has 16 voltage output, 16 current output, and 4 calibration channels (36 channels total). The channels are scanned at a rate of 50 µs per channel, thus a channel value will change anywhere from 0 to 1.8 ms (50 µs/channel * 36 channels = 1.8 ms) after it has been updated by software. If we consider the maximum update rate of the device to be the longest possible time it takes for a channel to update, the maximum update rate for the NI-6704 is 1.8 ms.
Settling Time:The maximum update rates for the NI 6703 / 6704 do not reflect the settling time for the channels. If you acquire at speeds that do not allow for enough settling time, the measurement accuracy will decrease significantly. The typical settling time for the NI 6703/6704 (including channel latency) is:
- 5.6 ms to +/- 0.1% accuracy
- 11.2 ms to +/- 0.01% accuracy
- 14.4 ms to +/- 0.001% accuracy
If high speed and/or high timing accuracy are required, you should use a PCIe/PXIe-6738 or PCIe/PXIe-6739 hardware timed, analog output device instead.