I would appreciate help with the following: we are using the above eval board, as a 2 channel sampler for IQ signals. The ADC there is AD7903:
1. Following testing with the eval board: we see severe distortions in sampling
low signals of 10 and 100 Hz. Is the 7903 specified for low frequencies at all?
In this case the same signal was introduced to both I and Q: the sampled signal
was identical for both channels yet distorted. What is guaranteed low freq (in
one of the data sheet's graphs the input freq starts at 10kHz)?
2. When working with channels A and B, using it for I and Q (the I- and Q- not
used), we observe a delay between the channels which is a function of the
signals' frequencies. At high frequencies the phase between the signals remains
pi/2, yet as the freq is reduced, the phase changes. We measured the signal at
the inputs to the ADC, for both channels, and found a perfect pi/2 delay,
however the sampled signals were of different phase.
Can you explain it or give us guidance as to how to configure the eval board.