Digitally programmable high pass filter - Page 1
Forum section: electronics / beginners

author: rfguy2020
author_rank: Contributor
timestamp: February 09, 2024, 12:44:43 pm
content: Hi,I need digitally programmable high pass filter. Cutoff frequency should be adjustable 10kHz-150kHz. Bandwidth should be couple megahertz.What is best way to design programmable high pass filter? I have been looking for digitally adjustable capacitors (DTC), but those are in pF level and I need something nF level.

author: MasterT
author_rank: Frequent Contributor
timestamp: February 09, 2024, 01:53:18 pm
content: Digital potentiometer may be? Microchip mcp4151 5 kOhm version has pass band up to 2 MHz

author: David Hess
author_rank: Super Contributor
timestamp: February 09, 2024, 01:55:47 pm
content: What filter order and distortion level were you thinking of?A low order low pass switched capacitor filter could be combined with a summing amplifier to generate a high pass response, and some switched capacitor filters are good to 200 kHz.Operational transconductance amplifiers can be used to make current variable filter elements for any response that you want:https://vahe.people.uic.edu/spring2016/ece412/OTA-structures2.pdfLarger value capacitors can be switched into and out of a filter circuit using transistors or multiplexers.

author: dobsonr741
author_rank: Frequent Contributor
timestamp: February 11, 2024, 03:08:31 pm
content: Switched capacitor single part universal analog filter block:https://www.anadigm.com/_doc/DS231000-U001.pdf

author: radiolistener
author_rank: Super Contributor
timestamp: February 12, 2024, 01:43:47 am
content: Quote from: rfguy2020 on February 09, 2024, 12:44:43 pmI think the best way to design digital filter is to use Matlab/Octave.I tried different tools, but the only way that allows me to get the best result is playing with filter parameters in Matlab/Octave.It has many instruments for filter design, testing and optimizations for your exact needs.

author: Ground_Loop
author_rank: Frequent Contributor
timestamp: February 12, 2024, 02:39:46 am
content: Maxim MAX260, 261, 262 family of digitally programmed filters may work for you. I’ve used them in audio range with PIC controllers. The 262 should be good to 1.5 MHz.

author: rfguy2020
author_rank: Contributor
timestamp: February 12, 2024, 07:11:41 am
content: Quote from: David Hess on February 09, 2024, 01:55:47 pmI have been looking for 1st order filter, but I'm not sure if I need higher order filter. Distortion level requirement is not clear yet.I have tried to simulate large capacitors + multiplexer, but I have not been too successful. I'm not sure what kind of switch/multiplexer I need to use. I was thinking to put four different capacitors (1x, 2x, 4x and 8x) in parallel with ADG601 switches. Then I could get 16 different cutoff frequencies. For some reason, steps are not expected due to some parasitic effect?

author: rfguy2020
author_rank: Contributor
timestamp: February 12, 2024, 07:16:14 am
content: Quote from: radiolistener on February 12, 2024, 01:43:47 amThe challenge is that I cannot get my signal to ADC without analog filter. My signal has strong DC component and I need to filter that out before I can digitize the signal. The signal I need to digitize is not always the same and for that reason I need programmable analog filter.

author: rfguy2020
author_rank: Contributor
timestamp: February 12, 2024, 07:21:30 am
content: Quote from: dobsonr741 on February 11, 2024, 03:08:31 pmInteresting product and I have not heard about this before. It might be overkill for my project and it seems to be difficult to get chips from common sources.

author: rfguy2020
author_rank: Contributor
timestamp: February 12, 2024, 07:29:19 am
content: Quote from: Ground_Loop on February 12, 2024, 02:39:46 amI have been looking MAX26X as well. I tested LTC6602, which seems to be similar than MAX26X. I don't like switched capacitor filters, since the clock signal is causing noise. I would prefer to find continuous time filter solution.

author: tszaboo
author_rank: Super Contributor
timestamp: February 12, 2024, 11:28:41 am
content: Quote from: rfguy2020 on February 12, 2024, 07:11:41 amWith analog switches, you want small capacitors and larger resistance values. Analog switch isn't exactly 0 Ohm, and it changes with temperature and voltage, so you want to minimize the susceptibility of the circuit on this error resistance.IMHO it's a lot simpler to just prototype the circuit, and use the right value filter rather than switching circuits.And if you already have an ADC in mind, place the lowest cutoff filter before it, and filter it digitally if necessary. Changing filter coefficients is easy, compared to verifying all that analog stuff.

author: jonpaul
author_rank: Super Contributor
timestamp: February 12, 2024, 11:39:14 am
content: no need for filter, use oversampling Delta Sigma ADC.https://www.analog.com/en/resources/technical-articles/sigmadelta-adcs-tutorial.htmlhttps://en.wikipedia.org/wiki/Delta-sigma_modulationhttps://inst.eecs.berkeley.edu/~ee247/fa05/lectures/L24_f05.pdfSharp digital filter built in, Fcut is proportional to sample rate.Bon chanceJon

author: tszaboo
author_rank: Super Contributor
timestamp: February 12, 2024, 12:54:28 pm
content: Quote from: jonpaul on February 12, 2024, 11:39:14 amOP wants a high pass filter. Delta sigma has a low pass filter.

author: rfguy2020
author_rank: Contributor
timestamp: February 12, 2024, 02:25:00 pm
content: Quote from: tszaboo on February 12, 2024, 11:28:41 amIt seems that ADG601 has very low resistance, which should minimize loss. I went for ADG612, which has higher resistance but it seems to work better in simulator. I'm not sure if the difference is caused by charge injection or Cs/Cd... Or something else.

author: kripton2035
author_rank: Super Contributor
timestamp: February 12, 2024, 03:52:54 pm
content: pretty sure you could achieve this filter by programming a cypres psoc.

author: radiolistener
author_rank: Super Contributor
timestamp: February 12, 2024, 09:31:48 pm
content: Quote from: rfguy2020 on February 12, 2024, 07:16:14 amyou can remove DC with a simple capacitor, non flat response can be compensated with FIR filter in digital domain.

author: rfguy2020
author_rank: Contributor
timestamp: February 13, 2024, 07:08:27 am
content: Quote from: radiolistener on February 12, 2024, 09:31:48 pmIt is not that simple, since my DC level is not constant (so it is not actually DC). That is the reason I need programmable HPF, which I can adapt for changing "DC" level.

author: RFDx
author_rank: Regular Contributor
timestamp: February 15, 2024, 02:56:16 am
content: Quote from: rfguy2020 on February 12, 2024, 07:11:41 amSwitch resistors instead of capacitors. See attached example, a single supply, adjustable 2nd order Sallen & Key HP. The filter has ~3dB gain in order to have same value resistors in the filter sections. With bipolar supply there is no need for the 2nd opamp (U2) and associated components.