FAQ: in a PLL active loop filter, what effect does the op amp bias current have?

Discussion created by rbrennan Employee on Mar 1, 2017
Latest reply on May 16, 2017 by TimW

The op amp bias current has to be sourced from the PLL phase detector/charge pump, and so, will cause the PLL to lock with a phase offset. This causes reference spurs. For integer-N loops where the loop bandwidth is as high as 0.1 × Fpfd, this is usually a problem, and so, low input bias currents are required.


For fractional-N loops, the situation is different. Loop bandwidths are a much lower fraction of Fpfd, and so reference spurs are much better suppressed. Also, typically, a deliberate offset current is introduced at the charge pump to make the loop lock with a phase offset to improve the linearity of the charge pump. So, here, bias currents are not important as long as they don't move the phase detector offset significantly.