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Where is the -3db point of this low pass filter?

post #1 of 10
Thread Starter 
I'm building a dac based on a pre-exising circuit. The Iouts of the dac are followed by opa627s for the I/V conversion. This is followed by an RC low pass filter (series resistor followed by a decoupling capacitor).

The resistor is 1kohm, and the capacitor is 330pf and I need to know what the -3dB point is. Can someone please help me with the math here and also post the formula for working this out? Thanks.
post #2 of 10
The transfer function for a RC lowpass filter is (if I remember right)
Code:
1/(1+sCR)
. Taking the magnitude of the TF you get
Code:
1/(sqrt(1^2+(wCR)^2)
. From the magnitude you can see that the 3dB frequency will be when w=1/RC since the 3dB frequence is when the magnitude is 1/sqrt(2). So, in your case, w will be 1/(1k * 330p) which is 3030.30 ish. Since w=2*pi*f divide 3030.30 by 2*pi and you'll get 482.28 Hz which is the 3dB frequency.
post #3 of 10
Thread Starter 
U sure about that? This was copied verbatim from schematics of a dac output. I can't imagine it chopping off most of the audio band!

http://diyhifi.230v.dk/udgvmin.html <----- what i'm reading.
post #4 of 10
isnt it simply 1/(2*pi*R*C) ?
giving ~482k by my calc
post #5 of 10
Daki][er is right. That assumes, though, that the output impedance of the stage prior to the 1K resistor is 0. You'd have to take the actual impedance into consideration for a more accurate calculation.
post #6 of 10
Thread Starter 
482k is equally as absurd as 482. Does the filter do anything at all in the above circuit? Or is it indeterminate?

How else does one calculate the filter for the output of a dac?
post #7 of 10
C13 is also a low pass filter with a corner frequency of 48KHz. I think C14 is just there to cut out the really out-of-band stuff.
post #8 of 10
Quote:
Originally Posted by DaKi][er
isnt it simply 1/(2*pi*R*C) ?
giving ~482k by my calc
Oops, you're right. I forgot pico was -12 instead of -9
post #9 of 10
Thread Starter 
Ahhh thanks amb, I compltetly glossed over that cap.

What's the forumla for figuring out the low-pass of caps in the feedback path?
post #10 of 10
Quote:
Originally Posted by Garbz
What's the forumla for figuring out the low-pass of caps in the feedback path?
Same formula: f = 1 / (2*pi*R*C)
R is that of the feedback resistor.
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