# Design an active filter 2nd order

1. Sep 20, 2009

### funjoke88

• No ripple can exist in the output, but non-linear phase shift is allowed.
• The frequency response must be as close as possible to the ideal filter response.
• Signals of frequency 0 ~ 5 KHz are only allowed to pass through the filter with a desired gain of 4dB . Other frequencies should reduce at least 100 times of gain for every 10 times of frequency increase.
• Capacitor of 0.01uF must use in the active filter design due to its high availability in the stock.
• Minimize offset error of your design

how to determine it and i dunno how to start it ?

2. Sep 21, 2009

### Staff: Mentor

Start by listing the various choices for filter polynomial type. What are the characteristics of each? Which one(s) have ripple in the passband? Which one(s) don't?

Then look at the attenuation you need to have ("100 times for every 10 times frequency increase") -- what order filter are we talking about now? Is it a LPF, BPF or HPF?

Given all that information, start to sketch out ways to do the active filter... Show us your work.

3. Sep 21, 2009

### waht

4. Sep 22, 2009

### funjoke88

no ripple means butterworth, 4 db means this is HPF

fc=1/2pie RC=7957OHM
Apb =4
20logApb=4
Apb-1.585

Rf=Rapb=7957 x 1.585 =12.6kohm

Apb=1+Rf/Rs
1.585=1/(12.6k/Rs)
Rs=21.54kohm

i just know this few steps .if there any mistake