Troubleshooting Unstable ADC Reading for PT100 Signal Circuit

In summary: This can cause the readings from the ADC to be wildly inaccurate, especially if the noise levels are changing rapidly.You should aim to have the supply voltage as close to zero as possible when taking the ADC reading, to minimize any noise contribution.In summary, you may be seeing fluctuations in the ADC readings which are not due to the PT100 sensor itself, but due to noise in the system. You may be able to reduce this noise by using a pre-amplifier, or by ensuring that the supply voltage is as close to zero as possible when taking the readings.
  • #1
Chandra214
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We have a four wire PT100 signal conditioning circuit, which transpires resistance to voltage.
7mV constitutes one degree celsius. we are using a 12bit ADC to decode the analog input.
When measured from a true RMS multi meter, the ADC input voltages are stable upto millivolt ( 4 1/2 digit multi meter). But upon reading the ADC values in software, we are unable to see the same stability in the voltage. the ADC shows a fluctuation of few millivolts.
Can somebody throw some light on the issue.

Thanks,
Prathap Chandra
 
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  • #2
You may be seeing several (different) effects here...

1) What is the sampling rate of your DMM? Probably lower than that of your ADC.

2) How much bits of instability do you see? (least significant bit, least significant two bits, three?) Are you sampling at an appropriate rate (greater than the ADC settling time, for the given system impedance?) Given the extrema, is this more than a degree or two? Is this acceptable for your application?

With that in mind, you can buy ICs or modules (meant to work with PT100 platinum sensors) that have guaranteed accuracy of 0.1C or better (and either a digital output, or amplified analog output).
 
  • #3
I must admit, Am feeling like a novice now..
The sampling rate of the DMM is 3times/sec, where as ADC sampling rate is in MHz range.

We see around four bits of instability.

how do i calculate an appropriate sampling rate for a given impedance?

right now we are getting acceptable values, but we are trying to achieve accuracy.
 
  • #4
With an ADC you will normally have to integrate to get any kind of stability(the multimeter is already doing this internally). I am note sure if you are referring to the input of a commercial DAQ (e.g. from National Instruments) when you write "ADC", if so there should be a setting for this in the drivers.
Otherwise you simply have to do it manually, there are all sorts of "tricks" one can use to e.g. get rid of 50/60 Hz noise etc; but one can usually get away with simply calculating the mean value of a few hundred samples (depending on sampling rate).

Btw, a fluctuation of a few mV is actually reasonably good and don't expect a huge improvement no matter what you do; ADCs are inherently noisy so one should always try to feed them a signal that is as high as possible.
In practice this means that one should always use a pre-amplifier (with a suitable BW) when dealing with low-level signals and get the signal into the 0.1-1V range or so before sending it into the ADC.
 
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  • #5
with sampling hundred times and taking its mean is serving the purpose.
we are getting readings with +/- 0.25 degC.
Thanks for your inputs.
 
  • #6
Is this for a thermocouple or RTD? Thermocouples are by their nature very noisy, 0.25 deg C is pretty good for any regular thermocouple especially with just a 12-bit ADC.

A multimeter isn't going to tell you much in terms of fluctuation or the quality of the signal. For that you need an oscilloscope. Otherwise you won't know if its the signal or the ADC that's generating the majority of the noise.
 
  • #7
PT100 is a RTD
Measurement using scope showed ambient noise level of 20mV
 
  • #8
Also, check the power supply for the ADC. Some of these effectively use the power supply as the reference voltage for the A to D conversion.

If the supply voltage has quite small hum or noise levels on it, it may be different at different times when the conversion is taking place.
 

1. How can I troubleshoot unstable ADC readings for a PT100 signal circuit?

To troubleshoot unstable ADC readings for a PT100 signal circuit, first check all connections and make sure they are secure. Next, check the power supply and make sure it is stable. You should also check for any interference or noise in the circuit. Additionally, ensure that the ADC is properly configured and calibrated.

2. What are some common causes of unstable ADC readings for a PT100 signal circuit?

There are several potential causes of unstable ADC readings for a PT100 signal circuit. Some common causes include poor connections, unstable power supply, interference or noise in the circuit, and incorrect ADC configuration or calibration.

3. How can I ensure accurate readings from my PT100 signal circuit?

To ensure accurate readings from your PT100 signal circuit, it is important to properly calibrate the circuit and ADC. Additionally, make sure all connections are secure and there is no interference or noise in the circuit. Using a stable power supply can also help to improve accuracy.

4. What steps should I take if my ADC readings are fluctuating or inconsistent?

If your ADC readings are fluctuating or inconsistent, first check all connections and make sure they are secure. Next, check for any interference or noise in the circuit. You may also need to recalibrate your circuit and ADC to improve accuracy. If the issue persists, there may be a problem with the ADC itself and it may need to be replaced.

5. Are there any specific troubleshooting techniques for a PT100 signal circuit?

Some specific troubleshooting techniques for a PT100 signal circuit include checking for any damage or wear on the components, using a multimeter to test connections and voltage levels, and using a signal generator to simulate the PT100 sensor. Additionally, make sure to follow proper grounding techniques and use shielded cables to reduce interference.

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