Hello,
I have designed a fully differential folded cascode OTA with SC-CMFB.
I have tested the circuit using a SC-CMFB network using Verilog-A macromodel switches. And the OTA seems to work very well.

When I replace the switches with transistors the clock injection is so bad that the CMFB voltage (which should be around 900mV) never is mantained at a fixed value and varies in a much wider range . The common mode open loop gain seems to be as large as the differential gain (~70dB) as well is the Unit gain Bandwidth (>125MHz).

I attached the images of the schematic and the simulations.

When using the MOS switches the CMFB voltage varies in a much wider range. As you can see in the simulation screenshots.

Schematic Screenshot





Transient simulation using Ideal Switches
Left: Diferential output, Rigth: VCMFB, Differential input: 300uV



Transient simulation using MOS Switches
Left: Diferential output, Rigth: VCMFB, Differential input: 300uV

hi,
  have you checked normal operation properly? because i don't think Charge Injection will give this much problem.... some times inadequate bandwidth in CMFB also troublesome..

Thanks,
Rajasekhar.

cdbular wrote on Aug 1^st, 2009, 12:10pm:


Hi Carlos,

One way to check the CMFB BW is to use Periodic STB analysis (Pstb). This tool allows you to analyize the stability of any loop that has two different periodic operating points, like the case of the SC CMFB you are showing.

Another simpler approach is just to apply a step to the input of your CMFB and see how it looks and what the settling time is. From there you can check what the BW is for such circuit.

Hope this helps.
Tosei

In case that I have inadequate CMF bandwitdh, How should I correct this?