We are now trying to build a multi-channel ultra-sonar receiving system. For the sake of power consumption and budget consideration, each 8 channels are multiplexed into one after the band pass stage; The BP has a central frequency of 300kHz and a band width of about 30kHz. We are using 4/3fc sampling and the AD sampling frequency should be 300k*4/3*8=3.2MHz.
Figure1, multi-channel ultra-sonar receiving system
We are using ADG608 as the multiplexer and the static crosstalk seems OK.
Figure2, the schematic and layout of the Mux (only top and bottom layer are shown)
Figure 3, the PCB board
Right now A1~A2 of ADG608 are tied to GND and A0 is supplied by an output of Agilent wave generator. If A0 is configurated as a 3.2MHz square wave (to let the MUX to switch between ch1 and ch2), we get jamming everywhere on our board, no matter whether our board is powered on or not. The unwanted jamming will be amplified by VGA and dominate the input of AD.
The jamming still exists even when the output of the wave generator is not connected to our board, as long as the output wire (BNC cable) is near our board (or maybe the oscilloscope).
If the square wave is replaced by a sine wave, then the jamming goes away.
How to generate A0~A2 and feed them safely into the MUX at such a high switching frequency?
Figure 4, the jamming when a square wave is used as A0