Sensor Calibration Problem? Help needed

Question

I have a system which has a arduino, a xbee, a gas sensor ,a temperature sensor and a custom shield which do the power distribution a lippo battery is ued to powers everything. The temperature sensor gives value which is higher than the actual value by 10 to 12 degree.Also the gas sensor reading is higher . But when i power the xbee and arduino from the USB cable and power the rest of the things from the battery the temperature value is correct. I am unable to understand why is this happening.

neilk · Answer

deepankarmaithani Hi Deepank I don't know the answer to your precise problem, but I can try and shed some light on overall accuracy: If your MAXIMUM temperature is 60 celsius, then your MAXIMUM measured voltage is only 600mV With the default Analog Reference Voltage of 5V, you are only using just over 10% of the ADC range. This link: http://www.arduino.cc/en/Reference/AnalogReference Explains how you can use the internal Analog Reference Voltage to increase the accuracy of your measurements. I have used the INTERNAL (ie: 1.1V) when working with a UNO - this increases accuracy by a factor of just under 5. However, note this: " After changing the analog reference, the first few readings from analogRead() may not be accurate." I was initially caught by that problem. Finally - and sticking my neck out here - is it possible that the Vcc derived from USB power and the Vcc derived from the power jack have slightly different values - but still within spec.? If so, how might that impact on the default 5V ADC reference? Perhaps someone else in the community knows the answer..... Hope this helps Neil

Robert Peter Oakes · Answer

As far as the 5V reference for the UNO, yes it is using the 5V supply and therefore inherently unstable and prone to interference from other things including the CPU itself as it varies its load, coupled with the fact that USB is rarely 5V and can be as low as 4 it should be relegated to experimenting only. Switching to the internal BandGap 1.1V reference will increase the resolution of your measurements but should not be confused with accuracy. Im not sure how accurate the BandGap reference is but that plus the base accuracy of the ADC and then the tolerances of the sensor all together will define your accuracy and linearity. In most cases ADCs don't need to be accurate, nor does the VRef. But they do need to be stable over temperature, time, load and supply changes. The rest can be compensated for in software. Neil is correct in suggesting the 1.1V Internal reference though as it will increase your resolution to ~1mV instead of ~5mV and as the band gap is more stable than the supply it will provide more consistent readings over time (It will mostly ignore the changes to running the CPU on 5V vs 4V for instance). If you want to increase the resolution even further then (As you dont need fast samples per second) you can try over sampling where you take say 16 measurements, add them all together and then shift the bits 2 bits to the right. This will increase your effective resolution to 12Bits instead of 10 (300uV instead of 1mV) resulting in a resolution of 0.03dec C per ADC count. you can read all about over sampling here:- http://www.atmel.com/images/doc8003.pdf

Sensor Calibration Problem? Help needed

Top Replies