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# Ltc 1250 differential thermocuple circuit analysis and questions

Category: Datasheet/Specs
Product Number: LTC1250

I'm trying to understand this thermocouple circuit made with ltc 1250. As far as I understand, the output should be 10mV/C, but there was an error in the datasheet its writed (100mV/C).
A difference amplifier was designed for the K type thermometer, assuming the Seebeck effect was 40uV per C degree. The gain of this difference amplifier was set as 250, so we can obtain 10mV at 1 degree change. I understand so far.
The other thing is the connection of LTC1025. LTC 1025 is set as the reference point of the 3k resistor. Adding a voltage reference to this point in the difference amplifiers creates a constant dc bias at the output and it is not multiplied by the gain. The voltage coming from the LTC1025 thus adds 10mV/C offset to the output.

What I don't understand here is the connection of LTC1025 to the negative power supply with 33k. I think the LTC1025 will be 0 C when measuring (-) degrees lower than 0. And it will switch to sinking mode. However, in order not to create an offset while sinking, the more negative V-supply is connected as the sinking source. Is this true?

My other question is what is the purpose of capacitors C1 and C2?

Another question is how were Resistance values determined when multiplying by 250 times? I think it was calculated from the noninverting and inverting gain coefficients, but I couldn't understand the exact purpose of 5k.

Another question is, did we also apply the offset voltage (ltc1025 Vout value) that we applied to the 3k resistor as a reference to the input of the 5k resistor?

http://tinyurl.com/2bgv3x6d

I think it's a kind of trimming circuit. I saw a trimming circuit like the one in the photo I took, but this one looked interesting. Is there a source explaining a similar trimming circuit?

## Top Replies

• It seems that there's a typo in the output, the type K amplifier with a 100mV/°C output, needs a closed loop gain of ≈2,500 and for 10mV/°C output, needs a closed loop gain…

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