I am using xPC Target to read data from an external A/D board. I notice that, when I read consecutive channels from the board, the second channel that is read is erroneous in that it is different than what would be expected. I observe this different voltage only when I use the A/D input block in xPC Target to read data from my board.
An experiment that demonstrates this is the following:
1) Feed some signal, perhaps produced by a sinewave generator, into channel 1 and leave channel 2 disconnected.
2) Create a model that reads and displays both channels 1 and 2. You will see that channel 2 shows almost the same voltages that channel 1 shows.
3) Connect a resistor from channel 2 input to ground. Now the signal you see on channel 2 is suppressed and depends on the value of the resistor. A smaller resistor does more suppression of the extraneous signal on channel 2. If the source resistance of the circuit under test is large, then clearly, adding a resistor to ground is not acceptable since it will change the measured voltage.
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