Reading K-Type Thermocouple with nRF52840 on Coin Cell (2.3V to 3V)

[Luqman Naseer]

I’m using an nRF52840 powered by a coin cell (3V nominal, dropping to 2.3V) and need to read a K-type thermocouple. The challenge is finding an amplifier that:

1. Operates reliably from 2.3V to 3V.

2. Provides an output compatible with the nRF52840’s ADC range (0V to 2.8V).

3. Includes cold junction compensation (CJC).

4. Consumes minimal power.

Challenges

1. Most thermocouple amplifiers (e.g., MAX31855, AD8495) require ≥2.7V, which may not work at 2.3V.

2. Level shifting adds complexity and power consumption.

3. Integrating CJC while keeping the design simple and low-power.

1. Are there amplifiers that work down to 2.3V?

2. What’s the best way to implement CJC in a low-power design?

3. How can I ensure the output signal is within the nRF52840’s ADC range?

4. Any tips for minimizing power consumption?

[snoopy20]

1. You need minimal current, take a look at a dixon charge pump en.wikipedia.org/.../Voltage_doubler, you can do it with the nRF52 hardware (pwm etc) and minimal external components or just buy a very cheap IC to do it. More efficient then dc-dc boosts and not much noise.

2. Voltage divider. If the thermocouple output is relative to it's input voltage you can either regulate the output of the voltage doubler (most accurate approach) or measure the voltage at the voltage doubler (another voltage divider) and then proportion the read from the thermo.

3. Not sure what this is.

4. Done. For NRF52 general power saving read the datasheet, turn stuff off when not needing it, use interrupts and sleep modes etc etc.