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
on power-up.
Some good op-amps that meet all three requirements above are:
OP184
AD820
AD8661
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.
