I'm using a couple LTC2990 parts and just realized I connected the polarity for V3 and V4 back wards. i.e the chip wants to subtract V3 from v4 meaning V3 should be a greater value than V4. In my case V4 is a greater value then v3. When I configure the chip to show the the result of v3-v4 I would of expected to see a value and bit 6 of the msb reg set to say the value is negative. What I see is the resister always seems to show full scale negative value meaning -0.3v. The value I measure across the pins v3 and v4 is -12mv way below the -0.3v limit.
I did rework my board and swapped the pins so V3 is greater than V4 and everything worked great and I get the right values. So my question is why wont the chip read negative values correctly even though the data sheet shows it can support -0.3v to 0.3v.