I get different results in AI voltage when i sample the data with an generated signal (DAQmxCreateCOPulseChanFreq) or an external clock signal - Encoder signal on PFI8. This effect happens with a M-Series 6220 and a X-Series DAQ card. I measure differential in the +- 1V range. When the max. signal ampltude is ca. 20 mV it seems, that there is an offset of ca. -1mV on the signal. The sampling rate is 40kHz. I need to use the external clock, because in the other case i have problems with the capacity of the data buffers. Additional the quadrature encoder is counted.
Can anybody explain me this strange behaviour?