Figure 9-2 shows an amplifier circuit that is used to convert single-ended signals to differential, and provides gain and level shifting. This circuit can be used for converting signals to differential in applications like line drivers for Cat5 cabling or driving differential-input SAR and ΔΣ ADCs.

With V_IN = V_REF + V_SIG , the output of the amplifier may be calculated according to Equation 4.

Equation 4.

The differential-signal gain of the circuit is 2 × G, and V_REF provides a reference around which the output signal swings. The differential output signal is in-phase with the single-ended input signal.

Figure 9-2 Single Ended to Differential Amplifier

Line termination on the output can be accomplished with resistors R_O. The differential impedance seen from the line will be 2 × R_O. For example, if 100-Ω Cat5 cable is used with double termination, the amplifier is typically set for a differential gain of 2 V/V (6 dB) with R_F = 0 Ω (short), R_G = open, 2R = 1 kΩ, R1 = 0 Ω, R = 499 Ω to balance the input bias currents, and R_O = 49.9 Ω for output line termination. This configuration is shown in Figure 9-3.

For driving a differential-input ADC the situation is similar, but the output resistors, R_O are selected with a capacitor across the ADC input for optimum filtering and settling-time performance.

Figure 9-3 Cat5 Line Driver With Gain = 2 V/V (6 dB)