Figure 9-4 shows a differential amplifier that is used to convert differential signals to single-ended and provides gain (or attenuation) and level shifting. This circuit can be used in applications like a line receiver for converting a differential signal from a Cat5 cable to a single-ended signal.

If V_IN+ = V_CM + V_SIG+ and V_IN– = V_CM + V_SIG– , then the output of the amplifier may be calculated according to Equation 5.

Equation 5.

The signal gain of the circuit is , V_CM is rejected, and V_REF provides a level shift around which the output signal swings. The single ended output signal is in-phase with the differential input signal.

Figure 9-4 Differential to Single-Ended Amplifier

Line termination can be accomplished by adding a shunt resistor across the VIN+ and VIN- 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 100-Ω Cat5 cable is used with a gain of 1 amplifier and R_F = R_G = 1 kΩ, adding a 100-Ω shunt across the input will give a differential impedance of 98 Ω, which is adequate for most applications.

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