Re: What kind of active filter I need?
- From: don@xxxxxxxxxxxxxx (Don Klipstein)
- Date: Mon, 16 Jun 2008 21:48:28 +0000 (UTC)
In article <g36aev$pav$1@xxxxxxxxxxxx>, Marco Trapanese wrote:
Hello,
my DAC outputs an audio signal between 0 and +1.25V. The finale stage is
a power amplifier that needs a signal below 500 mV p-p.
So I need to reduce the amplitude and filter the DAC output.
I thought a simple 2nd order MFB active filter but it's inverting and I
have a single supply. Of course I can biasing it at Vcc/2 but I'm
looking for a simple and straight way.
What type of active filter I should use which has:
* 0 < gain < 1
* cutoff frequency about 20 kHz
* 2nd order
I think unity gain Sallen Key lowpass to start. The filtering
components are two resistors and two capacitors. You also need a unity
gain buffer amplifier.
Replace the input resistor with a voltage divider to get the gain you
need. Know what the output impedance of your DAC is, and consider that to
be part of the top resistance of the voltage divider. The voltage
divider's output imedance (resistance value for purposes of the filter
circuit) is the top resistance and the bottom resistor in parallel with
each other.
Make the two capacitors unequal for the desired damping factor
(reciprocal of Q) (a common desired value is 1.414) - the positive
feedback one maybe twice the other one. I can't remember the formula now,
but a web search should turn it up easily enough.
The corner frequency is 1/(2*pi* SQR(R1*R2*C1*C2)). The -3 dB point of
a second order lowpass (or highpass) is the same as its corner frequency
if the damping factor is 1.414 (Q is .707).
This "traditionally" uses an op-amp but does work with a transistor
emitter follower. I would make the feedback capacitor a bit larger and
the other capacitor a bit smaller still if you use an emitter follower.
Make sure Xc of the bigger capacitor at the corner frequency is higher
than the value of the emitter resistor.
If you are less experienced with this sort of thing, use an op-amp.
Make sure the input signal (including bias voltage) stays within the
op-amp's common mode input range.
Analog audio circuits to be used on a single supply will need biasing
and typically need input and output capacitors. I would web search for a
circuit for a single supply unity gain Sallen Key lowpass filter.
Also, I would make the corner frequency more like 16 KHz,
although that depends on needs to preserve the upper treble and on how
much ultrasonic rubbish gets through your filter and how well your
tweeters withstand it.
Good luck!!!
- Don Klipstein (don@xxxxxxxxx)
.
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