The previous differential output circuits were set up to receive a differential input as well as provide a differential output. Figure 8-11 illustrates one way to provide a single-to-differential conversion, with dc coupling, and independent output common-mode control using a quad op amp.

The circuit of Figure 8-11 provides several useful features for isolating the input signal from the final outputs. Using the first amplifier as a simple noninverting stage gives an independent adjustment on R_I (to set the source loading) while the gain can be easily adjusting in this stage using the R_G resistor. The next stage allows a separate output common-mode level to be set up. The desired output common-mode voltage, V_CM, is cut in half and applied to the noninverting input of the second stage. The signal path in this stage sees a gain of –1V/V while this (1/2 × V_CM) voltage sees a gain of +2V/V. The output of this second stage is then the original common-mode voltage plus the inverted signal from the output of the first stage. The 2nd stage output appears directly at the output of the noninverting final stage. The inverting node of the inverting output stage is also biased to the common-mode voltage, equal to the common-mode voltage appearing at the output of the second stage, creating no current flow and placing the desired V_CM at the output of this stage as well.

Figure 8-11 Wideband, DC-Coupled, Single-to-Differential Conversion