National Instruments Data Acquisition Device NI USB-621x - Manual (Page 155)

Appendix B Troubleshooting
NI USB-621x User Manual B-2 ni.com
are various methods of achieving this reference while maintaining a high
common-mode rejection ratio (CMRR). These methods are outlined in the
Connecting Analog Input Signals on USB-6210/6211/6212 Devices and
Connecting Analog Input Signals on USB-6215/6216/6218 Devices
sections of Chapter 4, Analog Input .
AI GND is an AI common signal that routes directly to the ground
connection point on the devices. You can use this signal if you need a
general analog ground connection point to the device. Refer to the
When to
Use Differential Connections with Ground-Referenced Signal Sources
section of Chapter 4, Analog Input , for more information.
How can I use the AI Sample Clock and AI Convert Clock signals on a
USB-621x device to sample the AI channel(s)?
USB-621x devices use AI Sample Clock and AI Convert Clock to perform
interval sampling. As Figure B-1 shows, AI Sample Clock controls the
sample period, which is determined by the following equation:
1/sample period = sample rate
Figure B-1. AI Sample Clock and AI Convert Clock
AI Convert Clock controls the convert period, which is determined by the
following equation:
1/convert period = convert rate
This method allows multiple channels to be sampled relatively quickly in
relationship to the overall sample rate, providing a nearly simultaneous
effect with a fixed delay between channels.
Channel 0
Channel 1
Convert Period
Sample Period