Unstable LM92 question

Hi all,

I've been experimenting a while with an LM92 temperature sensor:

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And I'm a bit dissapointed, is it normal that a sensor with 0.33°c accuracy and a resolution of 0.0625 °C has its lower 2 to 3 bits jumping around? Resulting in a 0.2°C jumpy display? I would have thought it would be more stable than that? Or am I missing something? I have the lm92 connected as followed :

SDA & SCL with a 4k7 pull up to a PIC T_CRIT and INT connected to a 10K resistor to V+ A0 and A1 to V+ V+ = 5v

22nF over V+ and GND

I'm trying to build a reasonably accurate ambient air sensor, i've tried using an LM35 but gave this up because of the difficulty to get a good linear amplification to boost the signal to a usable level for A/D and thought a digital sensor would be the best option... Anybody got a better idea for a sensor that has got a good accuracy, not to pricy and has good availability or can fix my LM92 problem?

Thanks!

Reply to
Jurgen Priem
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22nF capacitor for power supply decoupling might me insufficient, try 100nF or 220nF instead.

Is +5V supply voltage stable and clean? Is the supply of microcontroller properly decoupled? Poorly decoupled microcontroller could introduce disturbance to supply voltage. You might try to add LC-filter (ferrite bead + small capacitor) in the power supply of microcontroller and/or temperature sensor.

Reply to
Mika Lindblad

properly

Decoupling the sensor with a series resistor from V+ to a big capacitor may also help, if the noise is high frequency enough. With lower frequency noise a separate regulator to the temperature sensor V+ would be a help.

I'd have a scope on that V+ line right at the sensor chip, to see how noisy it is.

If worse comes to worst you could always oversample and average. It's a band-aid, but if it works...

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Tim Wescott
Wescott Design Services
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Reply to
Tim Wescott

why don't you simply average the readings. that will smooth it out.

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Reply to
Jamie

Maybe your domain is more in the digital area. If an amp was difficult and you can't get it all quiet with the bypassing that the others have suggested I'd advise to seek help from a consultant. Preferably local so he/she could come in. Must be experienced in low-noise analog/mixed stuff.

Or just post schematic and layout.

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Regards, Joerg

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Reply to
Joerg

Thanks all for the tips, turned out that the bypass on the pic had a bad solder joint, also changed the bypass over the LM92 to 100nF Now I have a much more stable temp reading!

Reply to
Jurgen Priem

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