ZHCSH86B December 2017 – February 2019 LMH5401-SP
PRODUCTION DATA.
The designs so far have included a source impedance, RS, that must be matched by RT and RG1. The total impedance with respect to the input at RG1 for the circuit of Figure 58 is the parallel combination of RT to ground and ZA (active impedance) presented by the amplifier input at RG1. That expression, assuming RG2 is set to obtain a differential divider balance, is given by Equation 7:
For designs that do not need impedance matching (but instead come from the low-impedance output of another amplifier, for instance), RG1 = RG2 is the single-to-differential design used without RT to ground. Setting RG1 = RG2 = RG in Equation 7 gives the input impedance of a simple input FDA driving from a low-impedance, single-ended source to a differential output.