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Help me solve an unstable ADC output

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rag_npa

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I have Constructed a precision Thermometer using RTD as sensor with range -100.0C to +400.0C & Resolution of 0.1C employing Look Up Tables .

But the problem is O/P of ICL7135 ADC increases gradualy at the rate of 0.1C per 10 minuts if we connect a fixed resistor to the I/P insted of RTD to test the instrument.

Could anyone help me how to make it stable with respect to time.
 

ADC output Unstable

So, does the reading stabilize after a while or does it continue to increase?

My guess would be something in your system is drifting due to self-heating. Check that your input clock and Vref is stable over temperature. After that... most electronic devices have temperature coefficients, so it could be almost anything, including the resistors you are using, that could have a lousy temp coef. The datasheet should tell you what kind of temperature drift you can expect from the ADC. You either have compensate for them, wait while the sytem comes to a stable operating temperature then recalibrate, or put the whole system in a temperature stable oven.

Good luck.
 

ADC output Unstable

If it's a thermal problem, you could try blowing a narrow stream of warm air or cool air at individual components to quickly locate the sensitive one.
 

Re: ADC output Unstable

Thank you Friends.
It was Voltage Reffrence from LM336 drifting.Could any one suggest how to correct it.
 

ADC output Unstable

Take a look at the datasheet for LM336. (It's usually the first place to look for an answer regarding a part.)

First, make sure the current you are using to bias the diode is low as possible to decrease self heating. Second, there is a circuit in there to minimize temperature drift. (Page 2 of this document)
**broken link removed**

Using the minimized temp co. circuit, the datasheet states typ. 1.8mV with max of 6mV drift over 0 to 70 degrees. If this is not enought for your application, you need to start looking for something that has better temp coef.
 

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