Perfect, thanks for clarifying.
As someone else noted the MCP3204 looks to be the chip you're referring to.
And the code is here: https://github.com/dc42/RepRapFirmware/blob/6b835cb3c8e9aa10113ae750151d9f9065116cd4/src/Heating/Sensors/CurrentLoopTemperatureSensor.cpp#L23
I'll report any more findings as I implement.