LMH1983 Evaluation Kit Users Guide
Page 13 of 25
Figure 1: Loop Filter and LMP7711 Buffer Schematic
Op Amp Buffer
The LMP7711 Precision, Low-Noise Op Amp (U4) is used as a buffer to isolate the
relatively low input impedance of the VCXO, which would otherwise be the dominant
source of leakage current for loop filter circuit. The op amp offers very high input
impedance to minimize this leakage current and high slew rate to ensure proper loop
operation. The VCXO input control voltage input can be measured at TP29 (VC_BUF).
Note that when the LMH1983 is operating in free run mode, the VC_LPF output set by
the user-defined free run voltage control register may be limited by the output voltage
range of the LMP7711, as it is not a rail-to-rail op amp.
