Interfacing Bipolar AC Signals to Arduino ADC

 

Arduino's analog-to-digital converter (ADC) can only measure voltages in the range of 0V to its reference voltage (typically 5V or 3.3V, depending on the board). Bipolar AC signals, however, swing both above and below 0V, which presents a challenge: negative voltages can damage the Arduino and cannot be measured directly.

To safely and accurately interface a bipolar AC signal to the Arduino ADC, you need to shift (bias) and scale the signal so that it fits entirely within the ADC's input range.

One possible solution is described below.

 

Circuit Operation Overview

This circuit (fig.1) is a non-inverting amplifier based on the AD8541 operational amplifier. It is designed to convert a bipolar input signal (ranging from -0.1V to +0.1V) into a unipolar output signal referenced to a single supply voltage (+5V). The circuit achieves this by biasing the input and using AC coupling.

fig.1

Key Components and Their Functions

1. Input Coupling Capacitor (C1, 1µF):

• Purpose: Blocks any DC component from the input signal (Vin), allowing only the AC (bipolar) part to pass through to the op-amp.
• Effect: Ensures that the input signal is superimposed on a DC bias set by the resistor divider (R3 and R4), which is crucial for single-supply operation.

2. Biasing Network (R3, 1kΩ and R4, 9kΩ):

• Purpose: Creates a DC bias voltage at the non-inverting input of the op-amp.

·         Bias Voltage Calculation (R3-R4 Network)

The voltage divider formed by R3 and R4 sets the DC bias at the non-inverting input of the op-amp. The correct formula for the voltage at the junction (with respect to ground) is:

 

Vbias = R3/(R3+R4) × V+

 

Given:

• R3 = 1kΩ
• R4 = 9kΩ
• V+ = 5V

 

Vbias = 1k/(1k+9k) × 5V = 1/10 × 5V = 0.5V

 So, the bias at the non-inverting input is 0.5V, not 4.5V as previously calculated.

 3. Amplification of Both AC and DC Components

This is a non-inverting amplifier, so both the AC input (after C1) and the DC bias from R3-R4 are amplified by the same gain:

Gain = 1 + R2/R1 = 1+2k/1k = 3

The output voltage (Vout) is given by:

Vout = Gain × (V_inAC + V_bias)

• V_inAC is the AC signal after C1 (ranging from -0.1V to +0.1V).
• V_bias is 0.5V.

So,

Vout = 3 × (V_inAC + 0.5V)

 4. Output Signal Range

• When Vin,AC = −0.1V:

                         Vout,min = 3 × (−0.1V + 0.5V) = 3 × 0.4V = 1.2V

• When Vin,AC = +0.1V:

                          Vout,max = 3 × (0.1V + 0.5V) = 3 × 0.6V = 1.8V

Therefore, the output will swing from 1.2V to 1.8V as the input swings from -0.1V to +0.1V.

 

5. Purpose of Capacitor C1

• C1 is an AC coupling (blocking) capacitor. It blocks any DC component from the input, ensuring that only the AC signal is passed to the non-inverting input, which is then superimposed on the DC bias voltage provided by R3 and R4.
• This allows the circuit to convert a bipolar AC input (centered at 0V) into a unipolar output (centered at the amplified bias voltage).

Summary Table

Component	Function	Effect on Signal
C1	AC coupling	Passes AC, blocks DC from Vin
R3, R4	Voltage divider (bias)	Sets input bias at 0.5V
R1, R2	Feedback (gain)	Gain = 3 for both AC and DC
C2	Supply decoupling	Reduces power supply noise

 

 

 

 

 

 

 

In this circuit, we can use many popular single-supply IORR op-amps (MCP6021, OP37, LM358, etc.).

  Conclusion

• The circuit shifts the input AC signal (±0.1V) up to a DC level of 0.5V, then amplifies both the AC and DC components by 3.
• The output is a unipolar signal, swinging from 1.2V to 1.8V, suitable for single-supply systems.
• C1 ensures only the AC component of the input is processed, while the DC operating point is defined by the R3-R4 divider and amplified along with the signal.