Skip to Content

← All functionalities

Fuse test benchRaspberry PLC 19RI2CAcquisition

Differential ADC readings with automatic gain switching

ADS1015 differential measurement is the cheapest reliable way to read a high-current shunt from a Linux PLC. This example wires the 60 mOhm shunt to channels A0-A1 and the fuse voltage drop to A2-A3, then switches the programmable gain automatically between 2/3 (±6.144 V, safe) and 16 (±256 mV, maximum resolution). The exact conversion formulas come from a real fuse test bench deployment measuring up to 220 A.

Why differential beats single-ended here

A shunt floats at whatever potential the power circuit imposes, and its useful signal is millivolts riding on noise. Differential inputs reject the common-mode component and double the usable range. Single-ended would force a ground reference into a 220 A loop: a recipe for offset errors and ground bounce.

The auto-gain trick

Every reading starts at gain 2/3 so a large signal can never clip. If the estimated voltage fits inside the ±256 mV window, the channel is re-read at gain 16, multiplying resolution by 24. Two conversions instead of one, but the ADS1015 does 1600 samples per second: the 500 ms monitoring loop never notices.

From raw counts to amps

The full chain is one line: I = (raw/65535) · 2 · 0.256 · 200 / 0.06 · 1.02, raw counts to volts at the PGA, through the 200:1 conditioning, divided by the 60 mOhm shunt, times a 1.02 calibration factor measured against a reference instrument. Voltage drop across the fuse uses the same pattern on A2-A3.

A snippet from the implementation

Straight from the example as deployed on the Raspberry PLC 19R: copy it freely:

def create_adc():
    i2c = busio.I2C(board.SCL, board.SDA)
    ads = ADS.ADS1015(i2c, address=0x48)
    return ads
View the full working example Get the full project pack

The full example is a complete program, wiring header, setup and main loop, ready to adapt to your application.

Frequently asked questions

ADS1015 or ADS1115 for this application?

The ADS1015 (12 bit, 1600 SPS) is enough because the auto-gain keeps the signal near full scale. Choose the ADS1115 (16 bit) if you must resolve very small currents without external conditioning.

Can I read both differential pairs at the same time?

No, the ADS1015 has a single multiplexed converter. The example reads shunt and voltage drop sequentially; at 1600 SPS both fit comfortably inside one 500 ms monitoring cycle.

Where does the 1.02 factor come from?

It is an end-to-end calibration constant obtained by comparing the computed current against a reference meter. It absorbs shunt tolerance, divider error and PGA gain error in a single number.

Related functionalities