|Alex_M wrote on Sun, 01 October 2006 06:51|
I look for schema of Apogee AD-500 A/D converter.
What chipset is used?
Also interesting input LPF and buffer with good CMRR at high freq.
AD-500 common mode rejection is:
better than 90dB at 100Hz
better than 70dB at 10kHz
New delta-sigma hardware become worse in this parameter.
One needs to know what to design for. In my view, one needs a lot more rejection at line frequency then it does at say 10KHz. Why? Because one can have an audio cable leading to an AD input run in parallel (and not too far) to an AC line, carrying a lot of power (relatively speaking) thus pick up a lot of common mode signal. As a rule, I can not think of "everyday case" where your AD input cable will be running in parallel to a cable with 110V (or 220V) with say some .1A (or 5A) at 10KHz.
I am not against good common mode at higher frequencies, but I pay most attention to common mode at 50-60Hz and at 100-120Hz.
Each LavryBlue AD channel is adjusted by hand to better then 100dB for up to 1KHz which includes line frequencies, and is certainly better then 70dB at 10KHz... We adjust it at the max voltage level that the Audio Precision is capable of providing for common mode!
As a rule, I would not blame the sigma delta technology for common mode, because most of the issue belongs to the analog front end, way before the signal gets to the AD.
Say you have some balanced line with some common mode signal, and your front end does not reject it. Then your AD will treat the un rejected signal as if it were intended to be there.
Of course, the specification for common mode rejection on an AD chip may be important as well to insure proper design. Most modern sigma delta receive balanced input and relay on good balance. But this is a different issue. If we are to be talking about common mode, then the pre sigma delta AD's had 0dB rejection, they were single ended, and the rejection of the PRODUCT was all in the AD analog front end...