current limiter after a voltage divider for op amp input
R5 provides DC bias of Vss/2=2.5V  to Vin+ used by Amp A.

Amp B does not use Vin+ and rather uses 2 stages of inverting gain thru Vin-.  Thus R5 does not affect the signal impedance on Amp B.  Mainly it has more sensitivity with the extra filtered gain stage with the dual Op Amp then finally a comparator to produce a pulse.

Basically Amp A is missing the comparator but Amp B looks better.

The filter bandwidths are also different but cannot say why but Amp B at least has a spec.  --- although an odd looking 999 Hz even with 1% tolerances.