LM15851: Temperature diode reading problem

Robert Noe

Part Number: LM15851
Other Parts Discussed in Thread: TMP421, DXP

I'm using TMP421 with LM15851 for reading the temperature. It reads negative temperature (like -40 °C) for the remote sensor that is connected to TDIODE+/TDIODE- of LM15851. The local sensor is okay.

Also I have another TMP421 connected to a MMBT3904 and another one to a Xilinx FPGA, both remote and local temperatures of them are correct. So I'm sure that the problem is not the software.

How can I fix this problem? Should I use n-Factor Correction or something?

Regards,

Robert

over 7 years ago

0 Jim Brinkhurst1 over 7 years ago

TI__Mastermind 44560 points

Hi Robert

I haven't tried that particular temperature sensor with the thermal diode in the LM15851.

I'm going to do some investigation and will reply with more information as soon as I can.

Can you share the schematic details showing connections for the TMP421 and LM15851, including the ground connections for each device?

Best regards,

Jim B

0 Robert Noe over 7 years ago in reply to Jim Brinkhurst1

Intellectual 440 points

Hi

Thank for your answer
I don't have the schematics with me now but I can tell you that DXP/DXN of TMP421 is connected to LM15851 sensor with two series 100Ω resistor and a 100pF capacitor for filtering.

Edit: I checked the filtering capacitors and it seems that they installed a 10nF cap instead of 100pF, could it be source of the problem?

0 Jim Brinkhurst1 over 7 years ago in reply to Robert Noe

TI__Mastermind 44560 points

Hi Robert
Having a cap that much larger might cause problems. Try swapping to 100pF to see the effect.
I'm still working to access a comparable temperature sensor EVM I can try with my LM15851 EVM.
Best regards,
Jim B

0 Robert Noe over 7 years ago in reply to Jim Brinkhurst1

Intellectual 440 points

Hi Jim

Thanks, changing the cap solved the problem.

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LM15851: Temperature diode reading problem