Dear Sirs,
Dear Bob (Benjamin),
while I am setting up the management of the ADS1248 (see other thread), another curious issue has come up.
We are manufacturing hundreds of boards using the schematic (attached) based upon the Lmp90079.
It works very well and the 7 "K" type TC are correctly and precisely read.
However it happened that one TC has gone in short to GND, and instantly the reading from the Lmp90079 has become crazy.
Referred to the schematic, is same as (any) pin of X6 becomes shorted to GND. I know that the Lmp90079 has a limit of reading low input voltages which should not be lower than 0.4V (vs. GND). In fact, in LAB we tested a simulated short to ground of X6 (any pin) through a diode (0.5Vdc Vf) and readings still are perfect.
As soon as the common point (connected to VIN7) or a TC "+" wire goes shorted to GND, the ADC becomes crazy.
Crazy means that our readings become all high positive (e.g.: with TC in ambient temp. we read something around 160ºC or more), or low negative (like all readings become -270ºC).
Testing a number of identical boards, 60% go high positive, and 40% go low negative!
And there is no way to change this. The circuit is exactly the same for all the boards.
Questions are:
a) is this "normal" behavior for Lmp90079 if any VINx goes shorted to GND?
b) any capability to detect what's happening? (Burn Out Current is enabled but we see no help from this)
Thanks so much in advance for any help.
My best regards
Paolo Bozzola - CJB
(file with schematic: uploaded)