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Old 22nd March 2004, 12:33 AM   #1
roibm is offline roibm  Romania
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Question What value for the coupling caps?

Having the schema below, what values should I use for C1 and C2?
2002tii(thank you!) gave me the following formula:
f = 1/(2*PI*R*C) and having f @ 10hz
C2 should be at least 160nF(R = 100K + 1K), so 1uF would be ~ 2Hz

C1 should be at least 1.29uF(see those 4K7+4K7+4K7 in paralell with the 100k)

Is this right?
Any recommandations?
Do you know where I could find more details about calculating the values for the coupling caps?

Thank you
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Old 23rd March 2004, 10:18 AM   #2
roibm is offline roibm  Romania
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so, from all the GURUs around here, there isn't any that could comment on this...?
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Old 23rd March 2004, 10:26 AM   #3
Zombie is offline Zombie  Sweden
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Hello!
If you have control over the DC in and out of the circuit, why use any at all?
That doesn't answer your question, but it is a recommendation...
Cheers,
Tom
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Old 23rd March 2004, 11:19 AM   #4
roibm is offline roibm  Romania
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I tried just that, and the DC increases with at least 10mV

And beside, the total DC is pretty high with the caps in place(~15mV), so I am trying to get rid of it not to add some more. How can I control the DC if not by using caps?
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Old 23rd March 2004, 11:34 AM   #5
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15mV is still a pretty save value...I wouldn't fret too much over it...
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Old 23rd March 2004, 11:36 AM   #6
mcp is offline mcp  United States
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Hi

Common values are C1=10uF; C2=33uF when opamps are cascaded.

With just a single opamp, you can even do with a 1uF for C1 and C2.
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Old 23rd March 2004, 11:50 AM   #7
roibm is offline roibm  Romania
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li_gangyi, I am trying not to, but you don't know me


mcp, what do you mean by cascaded opamps? the signal is going through multiple opamps?
if that's it, well, I think the signal(in my case) is going through multiple opamps as well, not on the DAC board, but on the signal path before the preamp stage, the signal goes through 3 more opamps, so in total 4 opamps from digital to preamp. each opamp has it's own set of coupling caps.
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Old 23rd March 2004, 12:10 PM   #8
mcp is offline mcp  United States
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It is quite safe to use 10uF for both C1 and C2. That will save you the trouble of sourcing for a 33uF.

BTW, it is prudent to have your filter inputs and outputs de-coupled. You wouldn't want 10mVdc injected into the next stage. Neither do you want DC injected into your input.
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Old 23rd March 2004, 01:08 PM   #9
roibm is offline roibm  Romania
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mcp, I think you already understood I am a beginner...
What do you mean by "have your filter inputs and outputs de-coupled"?

thanks
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Old 23rd March 2004, 02:31 PM   #10
mcp is offline mcp  United States
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mcp, I think you already understood I am a beginner...
No, i am not aware.....

Quote:
What do you mean by "have your filter inputs and outputs de-coupled"?
If you do not use any capacitors to separate one network from the other, you will be "direct-coupling" them. Any DC on the input will be amplified at the output. But you are not interested in amplifying DC. You are working on AC signals, like an active crossover to separate music signals. By having a capacitor at the input, it will prevent the input from been injected with DC from the source. A capacitor on the output also prevents any DC emitting from the output of the opamp from being injected into the next stage. Hence, the stages are "de-coupled".
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