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OP-AMP choice for active PLL loop filters

Question asked by aharney Employee on Aug 12, 2009

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.

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