Cross talk when using ADG490 to mux thermocoules

Document created by analog-archivist Employee on Feb 23, 2016
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I switch thermocouples with the help of ADG409BR. In steady state i.e. when
channels do not switch, everything works well. But when attempting to switch
channels, beginning somewhere with ~10 Hz, there is influence between channels,
i.e. at heating one thermocouple, all four will be increased. When heating the
first thermocouple, the second channel of the indication will be a little bit
smaller, on the third it is even less, etc. After ADG409 there is an AD620 with
K=50 finally the signal goes to ADuC831 ADC input. Capacitors in a circuit of
the signal are not present, the first part of the signal chain is fed by
+/-15V. How could I override this problem? I need to receive data at minimum
~20Hz.

 

This is an interesting problem. There could be many possible explanations for
the behaviour. The most likely, from my point of view are:

- Cold-junction thermal problem
You don't mention how the cold-junction compensation (reference junction) is
done for the thermocouples.  input channels are referred to the same
cold-junction, but the temperature is slightly different for each thermocouple
then there is the possiblilty of crosstalk.

- Charge injection from the ADG409
Charge injection occurs when the multiplexer switches. If the AD620 inputs have
no path to ground or only a very high impedance path then the charge can build
up on the input, causing an error signal. Solution is to connect one input of
the AD620 to ground (if AD620 is on +/-15V supplies).

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