In many applications, it is required to drive a differential input ADC from a single ended source. Traditionally, transformers have been used to provide single to differential conversion, but these are inherently bandpass by nature and cannot be used for DC coupled applications. The LMH6554 provides excellent performance as a single-ended input to differential output converter down to DC. Figure 7-3 shows a typical application circuit where an LMH6554 is used to produce a balanced differential output signal from a single ended source.

Figure 7-3 Single-Ended Input with Differential Output

When using the LMH6554 in single-to-differential mode, the complimentary output is forced to a phase inverted replica of the driven output by the common mode feedback circuit as opposed to being driven by its own complimentary input. Consequently, as the driven input changes, the common mode feedback action results in a varying common mode voltage at the amplifier's inputs, proportional to the driving signal. Due to the non-ideal common mode rejection of the amplifier's input stage, a small common mode signal appears at the outputs which is superimposed on the differential output signal. The ratio of the change in output common mode voltage to output differential voltage is commonly referred to as output balance error. The output balance error response of the LMH6554 over frequency is shown in the Section 5.6.

To match the input impedance of the circuit in Figure 7-3 to a specified source resistance, R_S, requires that R_T || R_IN = R_S. The equations governing R_IN and A_V for single-to-differential operation are also provide in Figure 7-3. These equations, along with the source matching condition, must be solved iteratively to achieve the desired gain with the proper input termination. Component values for several common gain configuration in a 50Ω environment are given in Table 7-1.