I am using a E Series DAQ Device, PCI-6024E with the Sample-and-Hold amplifiers, SC-2040. The system works very well with time-based sampling (scan clock controlled internally). Now my application needs to have scan clock control externally by a shaft encoder, which gives 1024 TTL pulses per revolution. I found that the analogue signals sampled were much noisier than they were using time-based sampling. Therefore I started troubleshooting by unplug all the inputs and ground a channel + and - to ground (AIGND) and measure the noise level. I saw a DC offset of few mV and a rms noise level of 20 mV. The actual level of noise depended on the frequency of the TTL pulses, i.e. the rotational speed of the shaft encoder. However, I found that if
I only shorted the channel + and - but did not connect them to ground, I got virtually no noise, at least within +/- 1 LSB. Any sugguestions are welcome.
