Samuel Groner
Well-known member
Hi
I wondered about the phenomenon "common-mode distortion" a long time and did some research now. Not everything clear yet, so let my ask a few things.
First to the mechanism:
* in noninverting mode, there is a common-mode voltage at the inputs present which is proportional to the input voltage and inverse proportional to the configured gain. Due to finite CMRR a little of this voltage adds up to the differential mode signal; as the CM path is highly nonlinear, a distorted version of the converted signal appears at the output. Is the way I put this correct?
Now:
* Web_Ch6_final_I.pdf, page 6.52/6.53, mentiones that balancing the source impedances for inverting/noninverting input reduces CM distortion. What is the reason for this? Does CMRR improve with balanced source impedances? (I do not have access to ref. 8 in the above PDF)
* www.dself.dsl.pipex.com/ampins/webbop/2134.htm, bottom of page, writes that choosing higher supply rails can reduce CM distortion. This is probably related to the fact that we are further away from exceeding the CM input range with more supply volts. However, with 10 dB gain and 5 Vrms out, I would have thought that we are far away from exceeding CM range anyway. Any ideas?
* 990.pdf, page 6, shows a way to cancel input bias currents; does this method affect CM distortion? I does surely reduce CM input impedance.
Thanks for you replies!
Samuel
I wondered about the phenomenon "common-mode distortion" a long time and did some research now. Not everything clear yet, so let my ask a few things.
First to the mechanism:
* in noninverting mode, there is a common-mode voltage at the inputs present which is proportional to the input voltage and inverse proportional to the configured gain. Due to finite CMRR a little of this voltage adds up to the differential mode signal; as the CM path is highly nonlinear, a distorted version of the converted signal appears at the output. Is the way I put this correct?
Now:
* Web_Ch6_final_I.pdf, page 6.52/6.53, mentiones that balancing the source impedances for inverting/noninverting input reduces CM distortion. What is the reason for this? Does CMRR improve with balanced source impedances? (I do not have access to ref. 8 in the above PDF)
* www.dself.dsl.pipex.com/ampins/webbop/2134.htm, bottom of page, writes that choosing higher supply rails can reduce CM distortion. This is probably related to the fact that we are further away from exceeding the CM input range with more supply volts. However, with 10 dB gain and 5 Vrms out, I would have thought that we are far away from exceeding CM range anyway. Any ideas?
* 990.pdf, page 6, shows a way to cancel input bias currents; does this method affect CM distortion? I does surely reduce CM input impedance.
Thanks for you replies!
Samuel