This project follows the principle introduced in the TI application note:
Interfacing a Differential-Output (Isolated) Amp to a Single-Ended Input ADC (TI SBAA229B)
The AMC1301 is an isolated amplifier with a differential output. However, many microcontrollers only support single-ended ADC inputs. To resolve this, the differential output of the AMC1301 must be converted to a single-ended signal using a differential amplifier or a voltage divider network.
Main concepts:
- AMC1301 provides isolated differential voltage output
- A differential op-amp or resistor network converts it to single-ended signal
- Output voltage is within the 0–3.3V range (with ~1.5V DC offset for AC sensing)
- Ensures electrical isolation and safe interfacing with high voltage systems
This design includes a switch for selecting between AC and DC measurement modes.
- DC Mode: The signal tracks the measured voltage linearly
- AC Mode: The signal is biased with a 1.5V DC offset to ensure it stays within ADC range (0–3.3V). The ADC reads a sine wave centered around 1.5V.
Notes:
- The 1.5V offset can be generated using a voltage divider or a 1.5V reference
- Helps accurately measure bipolar signals using a unipolar ADC
Below is a sample Arduino Firmware using a Moving RMS algorithm to read AC signals with a DC offset:
#include <avr/interrupt.h> #include <math.h> #define OFFSET_VOL 1.504f #define OFFSET_ADC 308.00f #define FACTOR_ADC 1.42286217f #define MOVING_RMS_MAX_BUF 100 volatile float voltageValue; volatile float rms; typedef struct { uint16_t L; float invL; uint16_t count; float in_sq_L[MOVING_RMS_MAX_BUF]; float out_sq; float rms; } MovingRMS; MovingRMS mrms; void MovingRMS_Init(MovingRMS *mrms, uint16_t L) { mrms->L = L; mrms->invL = 1.0f / L; mrms->count = 0; for (uint16_t n = 0; n < L; n++) mrms->in_sq_L[n] = 0.0f; mrms->out_sq = 0.0f; } void MovingRMS_Update(MovingRMS *mrms, float in) { float in_sq = in * in; mrms->in_sq_L[mrms->count] = in_sq; if (mrms->count == (mrms->L - 1)) { mrms->count = 0; } else { mrms->count++; } mrms->out_sq += mrms->invL * (in_sq - mrms->in_sq_L[mrms->count]); } ISR(TIMER1_COMPA_vect) { voltageValue = (float(analogRead(A0)) - OFFSET_ADC) * FACTOR_ADC; MovingRMS_Update(&mrms, voltageValue); } void setup() { Serial.begin(115200); MovingRMS_Init(&mrms, MOVING_RMS_MAX_BUF); // Timer1 setup at 5kHz TCCR1A = 0; TCCR1B = (1 << WGM12) | (1 << CS10); OCR1A = 3199; // 16MHz / 3200 = 5kHz TIMSK1 |= (1 << OCIE1A); sei(); } void loop() { rms = sqrt(mrms.out_sq); Serial.println(rms); }