Active High-Pass Filter Design - Different Gains, Identical Phases - Page 1
Forum section: electronics / projects

author: Evan.Cornell
author_rank: Regular Contributor
timestamp: November 29, 2017, 10:24:41 pm
content: I have a need to design two high-pass filters, one with a gain of ~42dB, one with a gain of ~12dB. The phase response needs to be identical (|phase difference| < 0.1deg) between the two.I have tried making the topologies of the two chains identical (high-pass single-pole stage followed by high-pass Multiple Feedback 2-pole), with only the gain of the first stage being different, but I'm still having phase difference between the two of around 1deg.Can anyone point me in the right direction to designing this filter properly?Thanks!

author: danadak
author_rank: Super Contributor
timestamp: November 30, 2017, 12:01:37 am
content: Is this a component tolerance issue ?How are you making the phase measurement ?Regards, Dana.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 30, 2017, 01:03:35 am
content: Stupid idea: make them both 42dB then add a 30dB resistor attenuator after one.Tim

author: Evan.Cornell
author_rank: Regular Contributor
timestamp: November 30, 2017, 02:17:19 am
content: This is all in simulation world.Resistor attenuator is undesirable from both signal ranging issue (would need attenuator before first stage amplifier to avoid clipping rails), and from noise performance (don't want to add Johnson noise).

author: mikerj
author_rank: Super Contributor
timestamp: November 30, 2017, 11:42:13 am
content: How close to the GBWP are you getting on the high gain stage?

author: Evan.Cornell
author_rank: Regular Contributor
timestamp: November 30, 2017, 12:41:43 pm
content: GBP is 45MHz (min - LT1037). Gain of high-gain stage is 50V/V, frequency range of interest only extends to 35kHz.Question: does input resistance of op-amp vary with closed-loop gain?

author: danadak
author_rank: Super Contributor
timestamp: November 30, 2017, 12:51:22 pm
content: Adding an attenuator after would not add significant noise, butgaining up the low G channel would.Regards, Dana.

author: danadak
author_rank: Super Contributor
timestamp: November 30, 2017, 12:54:58 pm
content: QuoteYes, but only significantly when OpAmp is running out of G at high frequency.Regards, Dana.

author: Evan.Cornell
author_rank: Regular Contributor
timestamp: November 30, 2017, 01:26:11 pm
content: So, theoretically, for ideal op-amp in high-pass gain non-inverting configuration (passive RC into non-inverting input, Rf/Ri into inverting input), the only factors that affect phase ought to be the passive RC network, not the Rf/Ri network?

author: danadak
author_rank: Super Contributor
timestamp: November 30, 2017, 02:37:35 pm
content: A sim of input Z. You can see 741 running out of gas at 100 Khz, for G = 10.741 is single pole compensated, GBW ~ 1 Mhz.Zin inductive at higher freq as you can see.Note vertical axis is actually K ohms, as current source is 1 mA.Regards, Dana.

author: Alex Nikitin
author_rank: Super Contributor
timestamp: November 30, 2017, 02:51:59 pm
content: I would make two identical filters with low gain followed by two amplifiers with different gain but matching phase responses.CheersAlex

author: Evan.Cornell
author_rank: Regular Contributor
timestamp: November 30, 2017, 05:51:17 pm
content: Alex,So the different gain / matching phase wouldn't be part of the filter?Thanks,Evan

author: Alex Nikitin
author_rank: Super Contributor
timestamp: November 30, 2017, 07:51:20 pm
content: Quote from: Evan.Cornell on November 30, 2017, 05:51:17 pmYes

author: Evan.Cornell
author_rank: Regular Contributor
timestamp: November 30, 2017, 08:15:41 pm
content: Is that most optimal configuration for low-noise performance as well? Or is it a tradeoff between phase alignment and noise performance?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 30, 2017, 11:18:28 pm
content: In principle, phase can be matched by a passive filter: no cost to SNR and dynamic range. You might not want to realize such a design, though.Have you considered if this is an X-Y problem -- precision phase matching between two different signal paths is not a typical problem, so you have perhaps strayed away from a better, more conventional solution?Tim

author: Evan.Cornell
author_rank: Regular Contributor
timestamp: December 01, 2017, 12:20:39 am
content: Okay, let's back up to the design goals:1. two signal conditioning paths, one of 42dB overall gain, one of 12dB overall gain2. Both paths have high-pass filtering with -0.5dB down at 18kHz, down 100dB from passband by 100Hz3. Precisely matched phase from 25k-35kHzWhat are possible implementations that can meet these goals?

author: Evan.Cornell
author_rank: Regular Contributor
timestamp: December 01, 2017, 01:16:50 am
content: And then, wrapping back to existing implementation, why is it that when I sweep R11 (see attached) from 10k to 42k, the phase changes?I would've expected that only the C1/R1 network would affect the phase response.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 01, 2017, 01:59:35 am
content: Finite GBW. Don't think of op-amps as huge gain blocks. Think of them as integrators.Quote from: Evan.Cornell on December 01, 2017, 12:20:39 amNo, I mean the motivation behind those specs, what is it?Tim

author: Evan.Cornell
author_rank: Regular Contributor
timestamp: December 01, 2017, 02:15:18 am
content: Quote from: T3sl4co1l on December 01, 2017, 01:59:35 amTim,Thanks, that was the root cause I was trying to determine.Motivation is to have two signal chains that sampled data can be selected from on a sample-by-sample basis, based on optimal signal amplitude. Frequency components coming thru both chains have to arrive at the same time, hence need to have identical phase behavior.-Evan

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 01, 2017, 08:59:18 am
content: Aha! Solved problem:- AGC or VGA- Selectable-gain amp/atten. (e.g. oscilloscope front end)- "Fix it in software" (how much CPU power do you have to do DSP, anyway?)- Don't even worry about it because it doesn't matter (what is the justification for this precision requirement?)Tim

author: bson
author_rank: Unknown
timestamp: December 02, 2017, 12:59:10 am
content: Since you only need to match phase response in such a narrow portion of the band - 25kHz to 35kHz - it should be fairly straightforward to match this as if it were any other impedance to match, using e.g. a Smith chart or a VNA.Not sure why R11 affects the phase response, but maybe the simulator models input capacitance which with R11 adds a pole, and changing R11 shifts it around? Actually, checking the LT1007/1037 datasheet I see: "As with all operational amplifiers when RF > 2k, a pole will be created with RF and the amplifier’s input capacitance, creating additional phase shift and reducing the phase margin. A small capacitor (20pF to 50pF) in parallel with RF will eliminate this problem." So, yeah, likely root cause there.

author: Evan.Cornell
author_rank: Regular Contributor
timestamp: December 02, 2017, 02:05:38 am
content: Thanks for the suggestion. Adding a 33pF cap in parallel to RF seemed to make things worse, actually. I double-checked simulation in LTSpice vs. Tina-TI too, just to verify my LT1037 model.I dropped in AD8021 in place of LT1037. Roughly same noise performance, but much higher GBW. Phases seem to be nailed tight together. At first glance, it seems lack of GBW was the main culprit.