8.1.7 Differential to Single-Ended Amplifier

Figure 78 shows a differential amplifier that converts differential signals to single-ended in a single stage and provides gain (or attenuation) and level shifting. This circuit can be used in applications such as a line receiver for converting a differential signal from a Cat5 cable to a single-ended output signal.

Figure 78. Differential to Single-Ended Amplifier

The output of the amplifier can be calculated according to Equation 9 if V_IN+ = V_CM + V_SIG+ and V_IN– = V_CM + V_SIG–.

Equation 9.

The signal gain of the circuit is shown in Equation 10, V_CM is rejected, and V_REF provides a level shift or reference voltage around which the output signal swings. The single-ended output signal is in-phase with the noninverting input signal. V_REF is often ground when split supplies are used on the op amp.

Equation 10.

Line termination can be accomplished by adding a shunt resistor across the V_IN+ and V_IN– inputs. The differential impedance is the shunt resistance in parallel with the input impedance of the amplifier circuit, which is usually much higher. For low gain and low line impedance, the resistor value to add is approximately the impedance of the line. For example, if a 100-Ω Cat5 cable is used with a gain of 1 V/V amplifier and R_F = R_G = 2 kΩ, adding a 100-Ω shunt across the input gives a differential impedance of 99 Ω, which is an adequate match for most applications.

For best CMRR performance, resistors must be matched. Assuming CMRR ≈ the resistor tolerance, a 0.1% tolerance provides approximately 60-dB CMRR.