LMT70: unstable (TAO) output

Stevan Stepanov

Part Number: LMT70

Hi!

I use a closedcube board with LMT70.

I connected LMT70 to a 12-bit ADC input to the NUCLEOF410RB board.

The problem is that for a constant temperature, I get an unstable value on the ADC and thus an unstable measured temperature.

The board on which the LMT70 is placed has 2 capacitors on it. (one decoupling 100nF and one filter capacitor 0.1nF).

I tried to power the whole system with a battery, also, I stabbed the sensor directly into the NUCLEO board (without wires), but none of that helped.

Can someone help me, how can I get a stable output?

over 3 years ago

0 Ren Schackmann over 3 years ago

TI__Mastermind 35855 points

Hi Stevan,

Are you sure you have LMT70? I don't think you could 'stab the sensor directly into the board without wires' if you truly had our DSBGA device. Can you show me a picture of what you're doing?

thanks,

ren

0 Stevan Stepanov over 3 years ago in reply to Ren Schackmann

Prodigy 55 points

Hi Ren,

Thank you for your quick response.

Yes of course, I can share pictures:

I am using boards:

I use this formula (from the datasheet):

My code:

See the result:

The temperature in my room is constant (for example 25*C).

The problem is that "adc_val" varies, and should not for a constant temperature.

0 Stevan Stepanov over 3 years ago in reply to Stevan Stepanov

Prodigy 55 points

Something is wrong with uploading images:

Boards:

Code:

Results:

I hope it will load now.

Thanks in advance.

0 Ren Schackmann over 3 years ago in reply to Stevan Stepanov

TI__Mastermind 35855 points

Hi Stevan,

The 5C noise you observed is about 25mV of noise. You could have this much noise in your ADC's reference voltage, particularly if it uses its supply for reference. The ST ADC may need more input capacitance to work well, or may not be suited to making measurements at this accuracy at all. The LMT70 is specified for 1.1nF maximum output capacitance, so keep this in mind if you add capacitance.

thanks,

ren

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LMT70: unstable (TAO) output