The RF Apps team often gets asked for recommendations on op-amps for active loop filters.
Active loop filters are typically required to drive wide tuning range VCOs where the tuning range
is greater than the PLL on-chip charge pump output voltage.
ADIsimPLL allows you to simulate many different topologies and models the noise and spur contribution
of adding an active loop filter. This is a great starting point.
We typically recommned a non-inverting topology with pre-filtering as this allows you to use a fixed bias
on one of the op-amp input terminals.
Low noise is usually desirable - a number of less than 10nV/root(Hz) is a good target although this depends on your
exact PLL configuration - again check in ADIsimPLL.
Low input bias current and input offset current will result in lower spurious. Most modern op-amps easily achieve
sub-uA numbers. This is a more important spec in integer-N PLLs than fractional-N due to the lower PFD frequency at which spurious occur.
Another important spec.which is often overlooked is the ability of the op-amp to operate with rail-to-rail input voltages.
This is important even in fixed bias topologies as the input voltage connected to the PLL charge pump output will start at the lower rail
Some good op-amps that meet all three requirements above are:
The OP27 is a popular choice but care needs to be taken in using this part due to its non rail-to rail input voltage operation
(OP27 Input Voltage Range (IVR) = = +2.7V). Using a negative supply in your system greatly increases the number of
suitable op-amps but this is usually not an option favoured by customers.
Look out for an upcoming article from myself and Peadar on this topic.