Figure 9-2 shows an amplifier circuit that converts single-ended signals to differential signals and provides gain and level shifting. This circuit can convert signals to differential in applications such as driving Cat5 cabling or driving differential-input SAR and ΔΣ ADCs.

By setting V_IN = V_REF + V_SIG, then 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 = ∞Ω (open), 2R = 2 kΩ, R1 = 0 Ω, R = 1 kΩ 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)