# Where & how to block DC?

Discussion in 'General Electronics Chat' started by atferrari, Feb 18, 2012.

1. ### atferrari Thread Starter AAC Fanatic!

Jan 6, 2004
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My design of a DDS finally works OK but I want AC at the output. Block diagram attached.

My questions:

What is the correct place to get rid of the DC level?

A cap alone, is it the right way? If so, thinking of an output of 5Hz to 16 KHz what would be the right value?

Right now I have available non-polarized caps of 2uF and little space to parallel some.

What if I resort to an electrolytic or tantalum? How to know where the "+" terminal goes to?

Gracias for any help.

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2. ### MrChips Moderator

Oct 2, 2009
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Remember that in any circuit there will be some R. So you have to take this into account and calculate the roll-off frequency:

$f = \frac{1}{2\pi RC}$

It is best to use non-polarized capacitors. If all you have are low value aluminum electrolytics, these are ok to use. At low voltages I would not worry too much about polarity but you can use a DC voltmeter across the cap in circuit and figure out which end is more positive.

3. ### atferrari Thread Starter AAC Fanatic!

Jan 6, 2004
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The pot already at point (C) is 10K. I think I could use it with a cap in series between the buffer and the amplitude control stage. Am I right?

What is the roll off frequency I have to look for? I am at lost here.

4. ### MrChips Moderator

Oct 2, 2009
16,737
5,132
If you had a total series resistance of 10K in series with 2uF that would give you a roll off at about 8Hz.

5. ### atferrari Thread Starter AAC Fanatic!

Jan 6, 2004
2,968
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After revising my options I came to the attached circuit.

1uF and 100K seem to work OK.

In way of recalculating the filter (with higher fc) to have the last KHz unattenuated.