LTC2984 TC CJ Diode Overrange

Question

Hello, 
 I am using the Demo board for the LTC2983 to test both RTD and TC (Type K) circuits. I figured out what I was doing wrong to get the RTDs to work so that is fine. 
 However, I am trying to get the TypeK TCs to work to no avail. I have made sure I have COM grounded, I have a diode (1n4007) going from CH19 with the Cathode to the GND pin on the demo board (2210A) and the TC is connected between CH16 and GND as the spec for single ended TC is defined. 
 It seems to be wanting to respond, but is giving me a value of ~154 on the diode and is throwing a status of "Sensor Overrange" on the Diode which gives a CJ Soft Fault on the TC. I do not have open circuit detect on. 
 Is there something I am missing? I would have thought the demo setup would be straight forward enough but can't seem to figure out what is wrong. 
 Thanks in advance.

LoganC · Accepted Answer

Hi IsaacS , 
 The default non-ideality factor in the eval software (1.003) is very close to the ideal for Si, 1.0, this is the proper range for diode-connected BJT transistors. We use a 2N3904 on the demo boards. See the documentation for the DC2212A for an example. 
 Discrete diodes have a much higher non-ideality factor ( n or eta) - this can be edited in the channel configuration - try a value closer between 1.45 and 1.8 for the 1n4007. This is likely the cause of the overrange. 
 Note that discrete diodes also have a much higher variation in n than diode connected BJTs, so it is recommended to use a common NPN or PNP like the 3904 or 3906. 
 Hope it helps. 
 -Logan

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LTC2984 TC CJ Diode Overrange

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