Optimizing a 2-stage active low-pass filter - Page 1
Forum section: electronics / projects

author: bonelli
author_rank: Contributor
timestamp: September 16, 2022, 12:10:41 pm
content: Hello,Before entering my ADC, I need an analog front end to offset -350mV, gain x2 and low-pass filter (-3dB @ 100kHz, -60dB@1MHz). I have 10 signals to digitize, meanings 10 time these analog blocks on each PCB. And some tenth of PCBs. Meanings cost and time to validate each channel on each PCB.I have already a working solution using an LTC6262 (dual opamp). I would like to "optimize" it:- The signal enters the first opamp without filtering. It would be better to pre-filter it at least with a basic RC-filter.- The stages order (offset+gain and Sallen-Key) could eventually be swapped.- The LTC6262 costs a little bit and may be replaced by a lower cost op-ampI've tried to use a single op-amp to do the Sallen-Key filter + offset + gain but it doesn't look easy to get a frequency response at least as good as the actual design.Below is 3 LT spices simulations using different typologies of the sames low-level analog blocks. The first schematic is the actual design.I'm also looking for something like a "analog FPGA", giving the ability to enter at least 10 analog signals in a chip and filter/pre-process (and ideally digitalize them) them but I didn't find any interesting product.Any ideas and suggestions will be well appreciatedThanks a lot

author: srb1954
author_rank: Super Contributor
timestamp: September 16, 2022, 12:58:22 pm
content: You will have trouble getting the same results from real H/W as opposed to your simulations. The main problem is that the capacitors are too small and your circuitry will be subject to variations due to stray capacitance. I would scale the capacitors up by at least a factor of 4-5 and scale the resistors down by the same factor.In circuit configurations A and B the Sallen & Key filter stage interacts with the preceding RC filter stage degrading the frequency response, particularly around the initial roll-off above 100kHz. Configuration C is preferable in that respect as the op amps isolate the filter stages so that there is no unwanted interaction.Depending on how tight your requirements are you can get a sharper corner on the roll-off by designing the whole filter as a unified 3-pole filter rather than a cascade of the single-pole filter (the RC stage) and a 2-pole filter (the Sallen & Key stage). You can use the same basic circuit configuration but the circuit values will have to be tweaked a little bit to get the optimum response.I suggest you consult filter design textbooks to review the different types of filter responses e.g. Bessel, Butterworth, Chebyshev etc and also their respective transient responses - this may be important if you are to accurately digitise transient signals.

author: MasterT
author_rank: Frequent Contributor
timestamp: September 16, 2022, 03:43:40 pm
content: QuoteNot so easy with low voltage. To avoid clipping, order of the filter stages must go from low Q to high, having max peak-to-peak right at the output.QuoteAnalog Devices & TI seems always in the high price area. For alternative use digikey, mouser, arrow search engine in video/ high speed OPA category. For myself, I'd choose microchip MCP652/ 662

author: RoGeorge
author_rank: Super Contributor
timestamp: September 16, 2022, 04:11:46 pm
content: Quote from: bonelli on September 16, 2022, 12:10:41 pmI've read some years ago that there are so called FPAA (https://en.wikipedia.org/wiki/Field-programmable_analog_array) integrated circuits, but never used or seen any. You may want to search if any FPAAs are available to buy.

author: Benta
author_rank: Super Contributor
timestamp: September 16, 2022, 04:49:56 pm
content: The FPAA company is called Anadigm.And yes, your filter capacitors are ridiculously small, it won't work properly.

author: Benta
author_rank: Super Contributor
timestamp: September 16, 2022, 07:57:11 pm
content: If you choose to stay with the opamp solution, I'd suggest a 4th order Bessel filter, comprised from two second-order MFB sections.DC injection is easy for your offset, and even though Bessel filters are a bit "soft" you'll have no problems reaching -60 dB as specified. Additionally, you get phase linearity in the pass band, and excellent impulse response = no waveform distortion.