Single-supply op-amp vs. charge-pump

[ROb]

Hi,

Based on some previous posts, I need to build an op-amp subtractor
(Error = Vref - Vfeedback) feeding an op-amp control-loop.

I am pondering the trade-off of a single-supply subtractor / control-
loop vs. a charge-pump + standard op-amp control-loop. Op-amp based
control-loops are challenging for me already, the single-supply part is
additionally tough for me.

I have available a 200 kHz freely running 50% duty cycle signal, with a
PFET/NFET (buffer) + diodes + cap I can probably generate a negative
supply rail.

Question is, is the noise generated by the charge-pump (even with heavy
filtering, say a 100 uF capacitor) going to severely affect the op-amp
circuit? I expect 20 Hz 0 dB open-loop BW for control-loop.

Any thoughts on using charge-pumps and their effects on noise for op-amp
circuits would be appreciated.

Rob.

 

[Joerg]

Hi,

Based on some previous posts, I need to build an op-amp subtractor
(Error = Vref - Vfeedback) feeding an op-amp control-loop.

I am pondering the trade-off of a single-supply subtractor / control-
loop vs. a charge-pump + standard op-amp control-loop. Op-amp based
control-loops are challenging for me already, the single-supply part is
additionally tough for me.

I have available a 200 kHz freely running 50% duty cycle signal, with a
PFET/NFET (buffer) + diodes + cap I can probably generate a negative
supply rail.

Question is, is the noise generated by the charge-pump (even with heavy
filtering, say a 100 uF capacitor) going to severely affect the op-amp
circuit? I expect 20 Hz 0 dB open-loop BW for control-loop.

Any thoughts on using charge-pumps and their effects on noise for op-amp
circuits would be appreciated.

Personally I don't like charge pumps because of their current spikes.
Check out the little transformers for modems and LAN. I often use those
to transform one voltage to another or to another polarity. The rectify,
filter and regulate if needed (often it's not necessary). If you want to
be extra good provide a little choke between rectifier and cap.