Matched dual 4-order active low-pass filter with two dual version opamp

Question

I want to make two "matched" 4-order active filters, the -3db frequency will be below 100Hz. I have several choices to choose:

1. Choose four single version opamps. Each filter uses two of them.
2. Choose two dual version opamps. Each filter uses one of them.
3. Choose two dual version opamps. Each filter take two half-of-one-opamp.
4. Go directly to the quad version.
5. If there are more better one, please tell me :).

Is it worth to consider this when design an active filter? If so, then which choice are better? The quad version may derate in their performance, but may be routed easier. The single version may can't matched well, etc..

Any suggestions?

Answer 1

Low pass active filters are great but up to a certain point. For instance you may have the 3dB point set at 100 Hz and, for a 10 kHz input you'd expect the output to be 80 dB down. well, don't be surprised if this isn't as clear-cut as you anticipated. Maybe 10 kHz will be OK but at 100 kHz the op-amp may not be such a good active component and the signal at the output starts rising up as frequency increases.

I have no idea what the highest input frequency could be for this application but this could play a vital role in deciding what op-amp to select and, you will find that the better op-amps (i.e. ones that are more suitable to extreme applications) are not available as a quad package.

For "my" type of work the best quad op-amp is the OP4177 - it generally has pretty good specifications all round but it wouldn't suit being the active op-amp in a 100 Hz Sallen Key LPF where the input frequency might be as high as 1 MHz and input signals at 1 MHz are expected to be attenuated by 160 dB (4 decades at 40 dB/decade) - that just won't happen.

So maybe you can find a quad op-amp that compromises DC performance (i.e. produces several millivolts of DC error) but has a much wider gain-bandwidth-product (GBP) so that it adequately deals with high out-of-band frequencies.

My bottom line is this - look at what the circuit is meant to do, simulate it at worst-case high input frequencies and see if it performs. Chances are that it won't do what you want without changing to a device that compromises DC performance. So then you end up using a dual op-amp that has both decent DC performance and high GBP.

Answer 2

At low frequencies it all doesnt matter .By low frequencies i mean say 70Hz where I did a Biquad in a previous life .At high frequencies coupling between opamp sections may muck things up .Capacitive coupling gets noticably worse at 20KHz so you know when to use seperate chips.