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To detect surface temperature of heating foil, I decided to use thermocouple of type K. To amplify the small signal I choose the thermocouple amplifier with cold junction compensation integrated, AD8495 with the configuration present in the following image.

Thermocouple amplifier AD8495

As thermocouple probe I chose GK11M and it's connector is IM-K-PCB (the green one). As you can see from the picture of the green connector, there is positive symbol, which in the schematic is the pin 1 of IM-K-PCB, where is connected to the the negative input of AD8495.

Thermocouple probe IM-K-M thermocouple connector

The output "Thermocouple1" will be read with a multimeter.

At room temperature it seems to work. In fact at the output of the amplifier I measure 128mV, which is around 25.6°C (5mV correspond to 1°C). The problem is when I place the thermocouple to a warm surface, the output from the amplifier decreases, where it should increase instead. When I touch the thermocouple, I measure 17mV, which I suppose is the lowest output that the amplifier can give me at the output.

At first, I thought that the problem might be because I connected the positive pin of the connector to the negative input of the amplifier. So I unscrew the thermocouple probe lead, swap the pins and tried to measure the temperature, but the problem is still present. The output is still lower than 17mV even though the temperature is around 45°C.

I also tried to touch the thermocouple probe leads directly to the screw of the connector IM-K-PCB. If I connect the positive lead to the positive screw, I still get 17mV, but when I connect the negative lead to the positive screw of the connector I measure 0.212V, which is 42°C.

I don't understand, where I'm doing wrong. Any ideas of I could solve the problem?

Should I try to create antother PCB connecting the positive pin o fthe connector to the positive input of the amplifier? But even though I swapped the pin, is not working....

Edit 1----------------------------------------------------------------------

I removed the capacitor C11 and C12 anc connected the pin -Vs directly to ground. Now I measure room temperature of 27.6°C, which is equal to 138 mV. I still have the problem, that when temperature increase the output voltage decrease. Decoupling capacitor on +Vs supply are present near the amplifier.

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  • \$\begingroup\$ FYI: Thermocouple color codes: en.wikipedia.org/wiki/Thermocouple#Types For K-Type --> IEC: (+):Green (-):White; ANSI: (+):Yellow, (-):Red (those silly physicists!). \$\endgroup\$ Aug 10, 2023 at 14:39
  • \$\begingroup\$ You can try to short the input pins to ground to get the ambient temperature sensor configuration of the AD8495 to see if the chip alone would be working correctly. See the datasheet under debugging. \$\endgroup\$
    – Arsenal
    Aug 17, 2023 at 8:15

1 Answer 1

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Any ideas of where the problem might be? Did I do something wrong?

Yes, you have to wire DIRECTLY the "-Vs" pin of the AD8495 to "ground", as stated in Figure 33 of the datasheet.

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  • \$\begingroup\$ In fact, I wired the -Vs to ground through capacitors... \$\endgroup\$
    – stefyanna
    Aug 10, 2023 at 12:31
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    \$\begingroup\$ "In fact, I wired the -Vs to ground through capacitors..." Well, there's your problem. Near Fig 33: "Decoupling capacitors should be used to ensure clean power supply voltages on +VS and, if using dual supplies, on −VS, also." You're not using dual supplies (as shown in your diagram). Forget the caps (C11, C12), and directly connect Pin 3 to ground. \$\endgroup\$ Aug 10, 2023 at 13:13
  • \$\begingroup\$ OMG, I didn't realize the part of conditional... Thanks for showing me. In the meantime I desolder the capacitor and connect direct to GND the -Vs pin. I can measure correct room temperature, but once temperature increase, the output voltage still decrease. Any idea of the problem? \$\endgroup\$
    – stefyanna
    Aug 17, 2023 at 8:08
  • \$\begingroup\$ The inputs probes are swapped .. \$\endgroup\$
    – Antonio51
    Aug 17, 2023 at 9:21
  • \$\begingroup\$ @Antonio51 realized about this mistake, so I tried to swap the pin of the thermocouple probe unscrewing the connector of thermocouple probe (yellow connector). Is this enough or should I really swap the inputs from the PCB? \$\endgroup\$
    – stefyanna
    Aug 17, 2023 at 11:47

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