Nuts and Volts - June 2017

use to write to the serial port and still read all the data into Excel without losing anything was 15,000 baud. This particular version of PLX-DAQ can read any baud rate, as long as it matches the Serial.begin() value in the void setup() function. The trick to use PLX-DAQ v2.8 effectively to suck the data into Excel even at this slow rate is to turn off the auto calculation. This way, we can still set up graphs and other analysis which will not be executed in real time, but only after all the data is brought in.

If data is written to the serial port slower, it can be brought in and plotted in real time. This is why I created a fast and a slow mode for data acquisition. In slow mode, I get to see the processed data plotted in real time, as it is being taken. I can take measurements on four channels up to 10 Samples/sec and plot them in real time in the spreadsheet. Way cool! Just remember to disconnect the PLX-DAQ v2.9 controller before trying to upload another sketch. It ties up the serial port.

As an example, I grounded all the inputs to the channels, recorded 1,000 data points from each channel as fast as I could, read them back into Excel, and plotted up two channels. In addition, I calculated the average value and standard deviation for each channel (see Figure 6). This shows the noise floor of the ADC. I measure a DC value of less than 0.3 mV with an RMS value or standard deviation of about 0.33 mV. This is basically ± 2 LSB levels.

If I measure the same channel multiple times and average consecutive values, I can reduce this noise. It decreases with the square root of the number of samples. For example, I measured the RMS value of channel 0 with no averages as 0.3 mV. When I measured 25 consecutive samples, I got an RMS value of the noise as 0.057 mV.

This is almost exactly 1/5 of the single value noise. It takes 85 µsec to measure four sequential channels. This is an effective data rate of 12 kSamples per second. If I

measure just one channel and no averages, I get a sample rate of 43 kSamples per second. This is the limit for this system.

Modes of Operation

What we can do with this instrument is almost unlimited. I Zero the arrays.

Loop npts2meas.

Measure the time data is taken.

Loop npts2ave.

Loop nptsChan.

Analog.read(each chan).

Add new measurement to previous values of that channel. Store the sums in the array.

After all the measurements are made, go through each index and divide by npts2ave.

Print the time and value for each channel for each index.

20 June 2017

■ FIGURE 7. Styrofoam box with the temperature sensor and small fan to circulate the air. Four AA batteries were mounted to a small board; a bag of ice was included, and then dry ice was added to cool the air.