Gas station without pumps

2015 July 15

PteroDAQ board calibration

Filed under: Circuits course,Data acquisition — gasstationwithoutpumps @ 19:02
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Yesterday, while I was in the circuits lab, I checked the calibration of the voltage references on the KL25Z boards and the 4 Arduino boards I had with me.

What I did was to measure the power-supply voltage on the board with a good multimeter and make several of the PteroDAQ self-calibrations of the reference voltage (v0.2b1 does a new calibration every time the “pause” button is pressed).

For the KL25Z board, the voltage regulator on the board was well calibrated—I got a reading of 3.3001V with the bench multimeter.  The 33 PteroDAQ calibrations I recorded gave an average reading of 3.3095V with a standard deviation of 400.6µV. That means that the PteroDAQ reported voltages will be about 0.28±0.01% too high (much better than the ±3% specification for the bandgap voltage reference on the chip).   This is probably better than any of the cheap meters I have at home.

For the Arduino boards, the reference is normally the USB 5V power supply, which was not stable enough to do these comparisons with—I couldn’t get a constant reading on the good voltmeters but saw fluctuations of almost 10mV.  I should have had a 9V wall-wart power supply with me, so that I could get a more stable voltage source from the on-board regulators, but lacking that, I used a bench power supply directly connected to the +5V and Gnd pins of the Arduino to force specific voltages around 5V and did the same comparisons as for the KL25Z board.

board measurements voltage reading
KL25Z 33 +0.28±0.01%
Sparkfun Redboard 10 +0.08±0.10%
Duemilanove  11 –0.26±0.15%
Leonardo  10 –1.81±0.13%
Uno  8 +1.68±0.05%%

The greater fluctuation for the Arduino boards is probably due to the lower resolution of the ADC—the 10-bit ADC should have a reading around 225 at 1.1V with a 5V reference, so ±0.15% is only ±1/3 LSB.  The ATMega chips are guaranteed to have ±10% accuracy on the bandgap reference, but that is over the full temperature range, so ±2% seems about right for room temperature.

The USB power-supply is not a constant voltage, and the fluctuation in the USB power-supply voltage (which can be as much as ±10%) is a problem when using the Arduino boards, so powering them off of a wall wart is a good idea when trying to measure signals accurately.

The voltage measurements are as good as with super-cheap handheld voltmeters (which generally have a specification of about ±1%), so the PteroDAQ system is good enough for first electronics courses and hobbyist labs.

1 Comment »

  1. […] glitches.  I decided to investigate that a bit more today, using the new version of PteroDAQ. The voltage calibration was reasonably good on the KL25Z board (about 0.28% too high), so I decided to use that one with […]

    Pingback by FG085 function generator bugs | Gas station without pumps — 2015 July 16 @ 17:50 | Reply


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