Is it good to equalize input impedances in cascade active opamp filters? - Page 1
Forum section: electronics / projects

author: electronx
author_rank: Regular Contributor
timestamp: November 28, 2023, 03:38:08 pm
content: In my design, I designed a first 2nd order high pass filter with a Sallen key cut-off frequency of 0.2Hz and an input impedance of 250kohm. Then, I designed a second-order Multiple-Feedback low pass filter with a cut-off frequency of 100 khz and its input impedance is 15kohm. Thus, I have a bandpass filter. I wonder if it is good to equalize the input impedance of both filters? What effects can different filter input impedances have?

author: coppercone2
author_rank: Super Contributor
timestamp: November 30, 2023, 09:56:30 pm
content: they are isolated by the amplifier. the input impedance of the amplifier effects the noise. Lower impedance means less noise but obviously more power consumption. It is also manifested by bias current. For filters obviously it effects the other component sizes etc.The op amp driving the other one has a low output impedance probobly in the low ohms, so its basically irrelevant so long your load is like at least 100 ohms, for minimum functional level.I would say having the high impedance for the input and the much lower impedance for the interstage is the standard way to do it.It sounds reasonable to me, so long the noise is acceptable. You could generally try to lower impedance if your input is not effected to decrease noise, based on resistor noise, if its significant compared to the op amp noise, so long you can tolerate the power loss. Unless its a micro battery powered device then that is usually not an issue and performance is more important (usually these things have lights and MCU that take 1000% more power then even a heavily driven op amp)I would say generally most op amp not going to have problem driving even a 600 ohm load (10k lower). Once you start going lower then that you need to check out the op amp to make sure it can handle it without being over driven (means error, possibly overload). 600 ohms is the standard audio load.for a filter though, if its working, and you like the size of the C, I would not bother messing with the 10Kohm too much unless its a problem. Its usually the input amplifier that is the problem. Messing around with a working designs capacitors to lower impedance a little is not worth it, if that is how the math works out). Changing the caps out to make it work is a burden.... be happy if you can get the caps that meet the transfer function unless you end up with something ridiculous like 10 meg at first attempt).For the input, 300kOhm means it can basically take any signal, from anything, including dividers/attenuators. If its specific and you know you got a 600 ohm output driver or something, then obviously you could lower the impedance to 10k, since you are not really getting any more accuracy when its more like 10 times the impedance of the source. then it just lowers noise... but again that means redesign of the C possibly and also loss of interoperability.

author: MarkT
author_rank: Frequent Contributor
timestamp: November 30, 2023, 10:22:43 pm
content: Sallen Key filters are designed to be driven by low impedance (ie the first stage is always a buffer opamp). Otherwise the response will be different from expected.Each Sallen Key stage acts as the buffer for the next, so only the input needs an extra stage.

author: Benta
author_rank: Super Contributor
timestamp: November 30, 2023, 10:36:53 pm
content: That's true for MFB and other topologies as well. No difference there.

author: mawyatt
author_rank: Super Contributor
timestamp: December 02, 2023, 02:49:20 am
content: The op-amp you choose can have a significant effect on the 100KHz response. Most GP op-amps have a rising inductive looking output impedance at moderately higher frequencies, and with limited GBW product insufficient gain to force the op amp output back close to ideal, which results in degraded stop band performance.Best,