AD594: Voltage changews with supply voltage

Document created by analog-archivist Employee on Feb 23, 2016
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We are currenty evaluating the AD594 Monolithic Thermocouple Amplifier with
Cold Junction Compensation. We notice that the output voltage from the device
varies when we increase the supply voltage. The output also changes when we
touch the device (heat it up). We assumed that the device would give us a
correct temperature reading if the supply voltage was in the correct range. We
also assumed that the cold junction compensation would keep the output constant
and the device would not be affected by ambient temperature. Could you advise
us on how to use the device to measure temperature accurately.


With the AD594 is connected to a J type thermocouple and the thermocouple at
25degC, you should get an output of 250mV.

First thing to check is that you have provided a path for the input bias
current of the input amplifier. If possible, connect pin1 to ground ( as shown
by the dotted line in figure 1) or if you require the thermo couple to be at a
voltage other than 0V, connect a 10k resistor between pin 1 and ground.

The most frequent cause of the errors in the AD594 is that the junction between
the thermocouple wires and copper tracks is not at the same temperature as the
IC, you must ensure that the cold junction (where the thermo-couple metals meet
the copper traces) are at the same temperature and mounted close to the AD594
IC ,such that the IC and the cold junction are at the same temperature.

Other sources of problems can be sensitivity to EMI… remember that long
thermocouple wires make an excellent ariel and will quite happily pick up any
RF (or mains) interference in your lab. It be necessary to add a filter between
the TC and the AD594.