Just finished my new small project.
This is simple stand-alone voltage logger with SD card! No computer or tablet is required, cheap and easy to use tool.
Features:
– 8 input channels 0V to 3.3V,
– sample time: 0.5 ms … 5000 ms (5 sec), 0.2 Hz … 2 kHz,
– configurable calibration coefficients (zero and gain),
– configurable moving average filter for oversampling and resolution enhancement,
– all the settings are stored in configuration text file on the SD card.
Internal ADC is used for measurements and SDIO mode for SD card (FAT32 and up to 32Gb supported). This is open source and open hardware project: GitHub repository
Here is some test results for illustration of operation and measurement accuracy.
The picture of first prototype:
DC measurements
Histogram of ADC codes for 0V measurement:
mean = 2.2471, stdev = 2.2171
Histogram of ADC codes for 3V measurement (from Li-ion cell):
mean = 3741.2, stdev = 3.9101
Then, the zero and gain calibration was performed to get measurement result in volts. Here is example of configuration file ADC.txt. The channel 1 is enabled, 0.5 ms sample time.
sample 0.5 ch1_en 1 ch1_zero 2.247096239442946 ch1_gain 0.0008018788553247611 timestamp 1
Input DC voltage histogram:
And recorded voltage plot:
Then it is possible to estimate SNR of DC measurement based on equation 
mean = 3.0000, stdev = 0.0030, SNR = 60.0160, n = 9.6771
To enhance accuracy and resolution, oversampling with moving average filter is implemented:
where N is configurable filter order.
Here is configuration ADC.txt file for N=2 moving average filter applied to channel 1:
sample 0.5 ch1_en 1 ch1_zero 2.247096239442946 ch1_gain 0.0008018788553247611 ch1_filt 2 timestamp 1
Input DC voltage histogram:
And recorded voltage plot:
Obtained accuracy results: mean = 2.9998, stdev = 0.0017, SNR = 65.0610, n = 10.5151
Same testing was performed for N=4 filter order:
Obtained accuracy results: mean = 2.9998, stdev = 0.0011, SNR = 68.7229, n = 11.1234
Thus, using three-sigma rule, the accuracy without filtering is 10 mV (0.3% of full scale), but with order 2 filter accuracy is 5 mV (0.15% of full scale), and with order 4 average filtering is 3.3 mV (0.1% of full scale).
AC measurements
For testing of dynamic performance and recording of AC signals, all channels were connected to the signal generator. The moving average filter of order N was assigned to channel #N. And following configuration file used:
sample 0.5 ch1_en 1 ch2_en 1 ch3_en 1 ch4_en 1 ch5_en 1 ch6_en 1 ch7_en 1 ch8_en 1 ch1_zero 2.247096239442946 ch1_gain 0.0008018788553247611 ch1_filt 1 ch2_zero 2.247096239442946 ch2_gain 0.0008018788553247611 ch2_filt 2 ch3_zero 2.247096239442946 ch3_gain 0.0008018788553247611 ch3_filt 3 ch4_zero 2.247096239442946 ch4_gain 0.0008018788553247611 ch4_filt 4 ch5_zero 2.247096239442946 ch5_gain 0.0008018788553247611 ch5_filt 5 ch6_zero 2.247096239442946 ch6_gain 0.0008018788553247611 ch6_filt 6 ch7_zero 2.247096239442946 ch7_gain 0.0008018788553247611 ch7_filt 7 ch8_zero 2.247096239442946 ch8_gain 0.0008018788553247611 ch8_filt 8 timestamp 1
50 Hz pulses.
ADC input:
Recorded with various filter orders:
50 Hz sin wave.
ADC input:
Recorded with various filter orders:
5 Hz pulses.
ADC input:
Recorded with various filter orders:
5 Hz sine wave.
ADC input:
Recorded with various filter orders:
