what is the purpose of the additional resistors and capacitors in the fully LPF? - Page 1
Forum section: electronics / projects

author: electronx
author_rank: Regular Contributor
timestamp: November 26, 2023, 10:01:29 pm
content: What is the purpose of the additional resistors and capacitors added C14 R12 R14 R13 R15 C15 in the reference design of the .second order multiple feedback fully differential low pass filter?When I enter the same parameters in Filterpro, I see these values,and then when I enter the normalization value with 1k, this classic filter appears.However, in the reference design, additionally C14 R12 R14 R13 R15 C15 was used.What exactly is their purpose? I want to design a second order multiple feedback fully differential low pass filter with a gain of 1. What difference does it make if I add these values externally or not?The differential filter output goes to an ADC input with a differential input.

author: coppercone2
author_rank: Super Contributor
timestamp: November 26, 2023, 10:15:24 pm
content: in almost every data sheet ever they call that a filter.its a RF filter differential mode.it filter out garbage in the high frequency region from your ADC. Leave that at datasheet values. It prevents BS from going into the converter that confuses it. Related to EMI immunity. Similar to the cylinder on a USB cable. It might have been determined experimentally. That one is set to around 700 KHz.if you let high frequency noise into your converter it can make a confusing error. sometimes they just draw a box that says "EMI" and it means basically that.

author: Benta
author_rank: Super Contributor
timestamp: November 26, 2023, 10:20:20 pm
content: C14 is ??.The four resistors and the cap on the output are most likely there to attenuate high-frequency feedthrough. This is "built-in" in Sallen Key filters, but MFB filters also suffer from this, though to a far lesser degree. A simple RC filter fixes that.

author: electronx
author_rank: Regular Contributor
timestamp: November 26, 2023, 10:32:25 pm
content: C14 cap is (beetween non inverting input and inverting input) in the last pic highlighted with yellow ,

author: Benta
author_rank: Super Contributor
timestamp: November 26, 2023, 10:37:43 pm
content: Sorry, my "??" meant that I don't know what it's for. Ambiguous, I apologise.

author: electronx
author_rank: Regular Contributor
timestamp: November 26, 2023, 10:42:56 pm
content: No problem and thanks for your answer that helped me , ım still wondering purpose of c14

author: coppercone2
author_rank: Super Contributor
timestamp: November 26, 2023, 11:57:57 pm
content: I think its the same thing? that is common with differential and instrumentation amplifiers to put a capacitor across the input with resistors.This one is more complicated because its within the feedback loop. But I think its just another RC filter with a lower frequency cutoff then the first one.

author: coppercone2
author_rank: Super Contributor
timestamp: November 27, 2023, 12:05:18 am
content: https://www.edn.com/fully-differential-amplifiers-remove-noise-from-common-mode-signals/Figure 7 A third-order, lowpass filter drives an ADC’s differential inputs.so the thing you got might be a 4th order ?We know for sure its a LPF. And a ADC driver. That capacitor must be there to increase the filtering. Its pretty big, similar to the other capacitors in the network.The two resistors on the output of the amp might be to like dampen it or something. You usually see those resistors to increase the output impedance of stuff. I see 25 and 25 , adds up to 50. might have something to do with 50 ohms.It would help to know the ADC and the purpose. Look at the ADC data sheet to see if they give filter recommendations. They are often there.

author: moffy
author_rank: Super Contributor
timestamp: November 27, 2023, 01:09:45 am
content: For moderately high speed and high speed ADCs the input impedance has switching glitches and to maintain full accuracy a minimum input impedance is needed, R13,R15 and C15, it also usually works as an anti aliasing filter for the ADC. The data sheet will normally specify these input impedances. R12 and R14 are probably to help stabilise the output of the differential amp while driving C15, to stop it oscillating and also produce a damped response which settles well.

author: Someone
author_rank: Super Contributor
timestamp: November 27, 2023, 01:12:55 am
content: Quote from: coppercone2 on November 27, 2023, 12:05:18 amSimulate it, see what happens. For sure not a 4th order design.

author: coppercone2
author_rank: Super Contributor
timestamp: November 27, 2023, 01:21:30 am
content: i mean it has a extra cap idk what to call it

author: electronx
author_rank: Regular Contributor
timestamp: November 27, 2023, 09:57:06 am
content: https://www.beis.de/Elektronik/Filter/ActiveLPFilter.htmljust change the filter order than you will see how to affect the order of the filterthe filter order is second orderand thanks for your comment !

author: coppercone2
author_rank: Super Contributor
timestamp: November 27, 2023, 10:07:16 am
content: maybe I missed it but I guess the EDN article is wrong then? I did not write that caption. Its basically the same as the op amps post but without the extra capacitor

author: Benta
author_rank: Super Contributor
timestamp: November 27, 2023, 08:20:06 pm
content: Quote from: coppercone2 on November 27, 2023, 10:07:16 amNo, but you forgot your calculator. The 2nd order active section has a cut off point at around 20 kHz, whereas the RC network at the output is around 800 kHz.A 2nd order section followed by a 1st order one could be called 3rd order, but in this case it's tenuous as the poles are not even in the vicinity of each other.

author: coppercone2
author_rank: Super Contributor
timestamp: November 27, 2023, 08:58:39 pm
content: I think the order stays the same because the topology is the same but the response changes (its not standard response)if the amp is fast enough, that 800KHz should still get the higher attenuation. its not like its a 50 MHz signal that is just flying through the first amp, I suspect that a FDA (usually fast) with low gain can still attenuate the 800KHz signal OK. at least for a little bit.but I see your point that the db/decade attenuation in the area of interest is wrong for the order.

author: moffy
author_rank: Super Contributor
timestamp: November 27, 2023, 09:40:00 pm
content: You could call it what it is, a LPF followed by an anti aliasing filter, both important but serving different functions, and the anti aliasing filter serves the important purpose of presenting a suitable low impedance into the input of the ADC.

author: Benta
author_rank: Super Contributor
timestamp: November 27, 2023, 10:01:05 pm
content: Quote from: coppercone2 on November 27, 2023, 08:58:39 pmYou're mathematically correct, as the transfer function will be a 3rd order function.But looking at the two responses using Bode-plots, it's clear that they are functionally separate in the circuit.

author: Benta
author_rank: Super Contributor
timestamp: November 27, 2023, 10:04:17 pm
content: Quote from: moffy on November 27, 2023, 09:40:00 pmIt's the other way around.The 2nd order LPF is the anti-aliasing filter, and the RC filter is for compensating deficiencies in that.I can highly recommend the TI application note sloa049d.pdf if you want detailed info.

author: coppercone2
author_rank: Super Contributor
timestamp: November 27, 2023, 10:12:42 pm
content: yes there would be little point to put a high frequency LPF as the alias when the main signal is clipped by another filter. There already should be nothing outside of the cutoff in front of the earliest frequency cutoff LPF, by definition.

author: moffy
author_rank: Super Contributor
timestamp: November 28, 2023, 12:27:09 am
content: Quote from: Benta on November 27, 2023, 10:04:17 pmRefering to the original OP and what TI calls the relevant filters in their documentation: ADS127L01.pdf (page 65) and tidud62.pdf (page 12)they call the first filter an LPF and the second an anti-aliasing filter which also conditions the input for the ADC.

author: coppercone2
author_rank: Super Contributor
timestamp: November 28, 2023, 12:43:37 am
content: I would say if its eliminating the images from the clock at 8MHz the LPF is the AA filter. I thought that was basically the definition of it.I think the ADC app note is not expecting the differential filter op amp infront of it. That op amp is 150MHz. It should be handling the image products I think. At least the strong ones. After that it seems more like RF noise that the passive filter is dealing with. I guess it depends on the precision.that filter has 6db gain. times 2. so the useful bandwidth is 75MHz. That covers the 9th harmonic of the fundamental (8*9)

author: moffy
author_rank: Super Contributor
timestamp: November 28, 2023, 12:49:26 am
content: Quote from: coppercone2 on November 28, 2023, 12:43:37 amThe clock frequency is not the same as the sampling rate. The sampling rate is what determines the aliasing frequency the clock rate refers to the digital switching noise which can interfere with the analog input signal.

author: coppercone2
author_rank: Super Contributor
timestamp: November 28, 2023, 12:54:46 am
content: if the frequency cutoff of the LPF is 20KHz and the bandwidth of opamp s 75MHz is the 800KHz really taking care of anything but really high frequency noise?? thats what im not getting. I can see the passive filter going to a really high frequency. I am just not expecting anything between something like 100KHz to some MHz to being there at all.