Re: Whether to terminate IC shield to ground with a Cap or not?

Posted by Jon Risch (B) on October 21, 2004 at 21:21:36

In Reply to: Re: Whether to terminate IC shield to ground with a Cap or not? posted by Tony Montana on October 21, 2004 at 20:03:10:

[ One probably could even get by not using a shield since comon-mode rejection property of balance systems eliminate noise (via cable) very effectively :) ]

Nope, the common mode rejection of typical circuitry is not that great, and it does NOT e3xtend up into the RF region worth a hoot. Only a fully transformer based system has any chance of being able to do this, and even then, it is as much a bandwidth limiting aspect as anything else.

Typical electronic balanced input circuits do not have very good CMMR at audio band high frequencies, and the amount they have at low frequencies is usually dependant on the match of the output impedance between the two polarities of the source. If these are not both the same, AND very low, then the CMMR will suffer greatly, and often goes down to some nominal value around 20 or 30 dB, instead of the spec sheet number around 40-50 dB.

Most electronically based balanced input circuits do not have ANY CMMR at RF frequencies, in fact, it is not uncommon for them to actually have gain. That is why you often see these kinds of inputs with shunt caps to ground, ferrite beads, etc., in an attempt to reduce their susceptibility to RFI.

Jon Risch

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Follow Ups Full Thread

Topic - Whether to terminate IC shield to ground with a Cap or not? - Tony Montana 15:30:13 10/20/04 (14)

Re: Whether to terminate IC shield to ground with a Cap or not? - Jon Risch 18:00:05 10/21/04 (9)

Where is Jneutron when we need him :) - Tony Montana 19:54:25 10/21/04 (8)

Re: Where is Jneutron when we need him :) - jneutron 11:20:54 10/25/04 (2)

Speaking of the devil :) - Tony Montana 21:56:00 10/25/04 (1)

Re: Speaking of the devil :) - jneutron 07:11:16 10/26/04 (0)

Re: Where is Jneutron when we need him :) - Jon Risch 21:14:08 10/21/04 (4)

PS - Jon Risch 21:29:09 10/21/04 (3)

Maybe you should have e-mailed Kurt. - jneutron 11:13:27 10/25/04 (2)

Re: Maybe you should have e-mailed Kurt. - Tony Montana 22:06:27 10/25/04 (1)

Re: Maybe you should have e-mailed Kurt. - jneutron 07:29:41 10/26/04 (0)

Re: Whether to terminate IC shield to ground with a Cap or not? - blackrock john 09:04:52 10/21/04 (0)

Re: Whether to terminate IC shield to ground with a Cap or not? - Top Jimmy 23:49:45 10/20/04 (2)

Re: Whether to terminate IC shield to ground with a Cap or not? - Tony Montana 20:03:10 10/21/04 (1)

Re: Whether to terminate IC shield to ground with a Cap or not? - Jon Risch 21:21:36 10/21/04 (0)

Follow Ups

Re: Whether to terminate IC shield to ground with a Cap or not? - Jon Risch 21:21:36 10/21/04 (0)

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