I am testing the chip with an "MSOP8 To DIP8 Test IC Socket Programmer Adapter" on a breadboard, with Vs supply at 5V, Vd=ground. I want to use it as a setpoint controller. It passes the tests recommended with and without hysteresis, as long as the input pins (1 and 8) are both grounded. As soon as they are not, with or without a J thermocouple output, the chip output (pin 6) goes to 4.5V, no matter what the setpoint voltage at pin 5 is (I am using an opamp buffer with a voltage divider pot at the positive input to set the setpoint voltage). The problem seems to be that the input voltage to the 8494 is 16-21 mV for some reason without the thermocouple, and about 40mV with the thermocouple, even if I disconnect the filtering capacitors (and they are not polarized), so if the gain is about 122, it makes sense that the output rails. I am using values for the filtering recommended here:https://www.analog.com/en/resources/analog-dialogue/articles/measuring-temp-using-thermocouples.html. Hopefully the same for 8495 as 8494? Also, I am using the values for 60Hz filtering recommended at that link, at the output, and I have that 1K resistor between the opamp output and setpoint pin on the 8494 (recommended for filtering).
Why is the input voltage so high?
