danielpasti said:ok. I think that is the best solution for linearity reasons. But it needs a two stages opamp with a low output resistance. In that way you can use lower resistances to the common mode sense.
xshou said:I won't worry about noise at all since your output is differential. All noise contribution from CMFB loop is common mode -- it won't be visible at the differential output.
-- Well said! Got you!
Resistor sensing will cause lower differential mode open loop gain at the output - if the resistor too low, more distortion you may see. A unity gain buffer at each output (eg source follower) shall serve you well in this regard. Tap the common mode signal from the resistor divider at the output of those two buffers.
If you worry about the buffer input cap non-linearity loaded to your output, just put dummy mos cap cross coupled to cancel out. Or add series resistor to the gate.
danielpasti said:"I won't worry about noise at all since your output is differential. All noise contribution from CMFB loop is common mode -- it won't be visible at the differential output. "
Excuse me Xshou
Always there is a confusion between NOISE and INTERFERENCE source. Noise is a king of random frequency (not deterministic) coming from the components in the circuit. Interference is a external disturb which "strikes" the circuit in common mode and are attenuated by the CMRR capability of it. In fact, the noise has a attenuation of 10dB of power in differential circuits comparing with single ended circuits.
Added after 4 minutes:
danielpasti said:"Resistor sensing will cause lower differential mode open loop gain at the output - if the resistor too low, more distortion you may see. A unity gain buffer at each output (eg source follower) shall serve you well in this regard. Tap the common mode signal from the resistor divider at the output of those two buffers."
The use of a source follower in your circuit will decrease the common mode swing of your circuit because it has a Vgs of drop before the resistors. I
jnuhope said:xshou said:I won't worry about noise at all since your output is differential. All noise contribution from CMFB loop is common mode -- it won't be visible at the differential output.
-- Well said! Got you!
Resistor sensing will cause lower differential mode open loop gain at the output - if the resistor too low, more distortion you may see. A unity gain buffer at each output (eg source follower) shall serve you well in this regard. Tap the common mode signal from the resistor divider at the output of those two buffers.
If you worry about the buffer input cap non-linearity loaded to your output, just put dummy mos cap cross coupled to cancel out. Or add series resistor to the gate.
-- Can you explain more why the series resistor to the gate will cancel or help the non-linearity caused by the input cap?
Thanks!
danielpasti said:xshou
Im sure you can read with more details in Gray n Mayer book about the NOISE contribution on diff output stages. You will increase the SNR because the doubled output stage. This means 3dB of power or 6dB of voltage. I wrote 10dB but I was wrong.
danielpasti said:xshou
About the source follower as sensing the output vref, I can say to you that you need the same output swing for diff mode and for common mode to keep CMRR and reference over all the signal level. You can read it in the same book I mentioned above.
Sorry for my english dude, I am Brazilian
Regards
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