What kind of active filter I need?

Looks like you will need to attenuate your DAC output BEFORE applying to the filter, otherwise you get an inversion.

What do you have against setting Q-point at Vcc/2?

...Jim Thompson

Jim Thompson ha scritto:

I get an inversion in any case. I need a non-inverting filter or I should set the q-point to vcc/2.

Marco / iw2nzm

Life is hard, and then you die ;-)

...Jim Thompson

Low frequency corner?

Source impedance?

...Jim Thompson

Jim Thompson ha scritto:

The final stage has a low cut-off frequency of about 50 Hz. The DAC output goes down to DC.

The DAC datasheet (AD5663) says:

Output DC impedance = 0.5 ohm Output short-circuit current = 30 mA Typical Rl = 2 kohm

I know, my friend, I know :)

Marco / iw2nzm

Use the Reference Pin as your "mid-point".

...Jim Thompson

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 ( snipped-for-privacy@misty.com)

Jim Thompson ha scritto:

This is a good idea, indeed!

Marco / iw2nzm

Don Klipstein ha scritto:

Ok. Using a dual op-amp package could lead to any advantages? I'm talking about using a follower after the divider.

I got it.

I'm going to use an op-amp :)

I know. I was looking for a non-inverting DC-coupled filter because my source signal is unipolar (0-1.25V) and only the final stage will be AC-coupled. Avoiding biasing and I/O capacitor seems to be nice. Anyway, I'll follow the standard way.

You're right again. I don't need such a frequency. 16 kHz will be enough.

Thanks! Marco / iw2nzm

This is fun to play with

martin

The MFBs come in both flavours and you can build 'loss' in at the same time. A la ...

|| ,----||--------------------, | ||1.3nF | | | | | ___ | ___ |\\ | IN o--|___|--o--|___|----o-------|+\\ | 15k | 15k | | >---o--o OUT | | ,--|-/ | | | | |/ | | | '---------' .-. | Gain X0.4 | | --- 20kHz LP | | --- 2nd order '-'10k | 560pF | | GND GND

(created by AACircuit v1.28 beta 10/06/04

snipped-for-privacy@jjdesigns.fsnet.co.uk ha scritto:

It's very nice! I'll give it a try!

Thanks you very much Marco / iw2nzm