FAQ: OP-AMP choice for active PLL loop filters

Document created by DLedger on Aug 12, 2009Last modified by AndyR on Jan 31, 2012
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What are some recommended op-amps for active loop filters?




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 recommend 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:





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 favored by customers.


This FAQ was generated from the following thread: OP-AMP choice for active PLL loop filters