Other Parts Discussed in Thread: ADS1248,
I am using an LMT85 as a cold junction sensor (actually two) for thermocouple measurements. The LMT85 is interfaced to an ADS1248. It is required, that the PGA AINN or negative input of the differential measurement, be at least 100 mV above AVSS. I posted a similar question on the Precision Data Converter Forum. Joseph Wu was kind enough to reply with his thoughts and suggestions.
I suggested basically lifting of floating the ground of the LMT85 with, let's say a divided down voltage of 200 mV through a buffer op amp. Heck, the LMT85 needs 10 uA of supply current. An op amp should sink that easily, times two.
However, Joseph actually suggested placing a diode between the LMT85 ground pin and the circuit/system ground. I quickly tired this using a 1N4148, and a couple LMT85s. Appears to be working fine. The differential measurement of each LMT85, represents room temperature fairly closely. Has anyone tried this? Is it a good approach as interfaced to an ADC with integrated PGA?
Operational note: the LMT85 will be used to measure ambient between -20 °C and 55 °C.