# Sallen-key filter reducing mid-supply bias **URL:** \ **Category:** General Electronics\ **Created:** [November 17, 2025, 12:51pm UTC](https://forum.arduino.cc/t/sallen-key-filter-reducing-mid-supply-bias/1414805 "2025-11-17T12:51:07Z")\ **Posts on this page:** 20\ **Page:** 1
**Author:** ![bblackledge](https://dub1.discourse-cdn.com/arduino/user_avatar/forum.arduino.cc/bblackledge/32/934922_2.png) [@bblackledge](https://forum.arduino.cc/u/bblackledge)\ **Post date:** [November 17, 2025, 12:51pm UTC](https://forum.arduino.cc/t/sallen-key-filter-reducing-mid-supply-bias/1414805/1 "2025-11-17T12:51:07Z")
Hi all, I'm hoping someone can help spot what is up with my schematic. I am taking a geophone differential signal, passing it through an in-amp AD623 (±3.3V) and then applying a low-pass Sallen-Key Butterworth filter (Vdd = 3.3V, Vss = GND, Fs = 357Hz, Q = 0.74). I apply a mid supply bias to the Vref pin on the in-amp, this comes from a REF2033AIDDCR. If I measure the signal immediately after the in-amp, the signal has the correct bias (1.65V) but after the filter, it has dropped to around 1V. This is my first foray beyond breakout boards so I am sure I have missed something quite simple! Thanks for your help! ![Screenshot 2025-11-17 125001](https://europe1.discourse-cdn.com/arduino/original/4X/5/9/e/59ea679899d9ec2e6f9b130576566da051f31d53.png) [Schematic.pdf](https://forum.arduino.cc/uploads/short-url/xqvsxne71f51J9vYc4zxzasekyY.pdf) (41.4 KB) ---
**Author:** ![PaulRB](https://dub1.discourse-cdn.com/arduino/user_avatar/forum.arduino.cc/paulrb/32/216330_2.png) [@PaulRB](https://forum.arduino.cc/u/PaulRB)\ **Post date:** [November 17, 2025, 1:05pm UTC](https://forum.arduino.cc/t/sallen-key-filter-reducing-mid-supply-bias/1414805/2 "2025-11-17T13:05:44Z")
5 unconnected wires are shown at the top of your schematic. Another 5 unconnected wires are shown on the right side. These wires need to be connected to something for the circuit to work correctly. ---
**Author:** ![bblackledge](https://dub1.discourse-cdn.com/arduino/user_avatar/forum.arduino.cc/bblackledge/32/934922_2.png) [@bblackledge](https://forum.arduino.cc/u/bblackledge)\ **Post date:** [November 17, 2025, 1:59pm UTC](https://forum.arduino.cc/t/sallen-key-filter-reducing-mid-supply-bias/1414805/3 "2025-11-17T13:59:14Z")
@PaulRB it is the relevant snapshot of the schematic, those wires are all connected. ---
**Author:** ![sonofcy](https://dub1.discourse-cdn.com/arduino/user_avatar/forum.arduino.cc/sonofcy/32/1283617_2.png) [@sonofcy](https://forum.arduino.cc/u/sonofcy)\ **Post date:** [November 17, 2025, 2:51pm UTC](https://forum.arduino.cc/t/sallen-key-filter-reducing-mid-supply-bias/1414805/4 "2025-11-17T14:51:00Z")
Am I understanding you correctly, you have a problem you can't solve, but at the same time have decided it can only be in a particular subset of the circuit? ---
**Author:** ![anon94801460](https://avatars.discourse-cdn.com/v4/letter/a/278dde/32.png) [@anon94801460](https://forum.arduino.cc/u/anon94801460)\ **Post date:** [November 17, 2025, 3:00pm UTC](https://forum.arduino.cc/t/sallen-key-filter-reducing-mid-supply-bias/1414805/5 "2025-11-17T15:00:53Z")
This input circuit makes no sense. The ADC inputs are essentially floating, except for stray impedances to ground. ![Capture](https://europe1.discourse-cdn.com/arduino/original/4X/2/9/6/29669c55c503fb051e9f406256b548e00546f159.jpeg) Take a look at some established geophone amplifier circuits and revise your circuit accordingly. Here is one: ![](https://europe1.discourse-cdn.com/arduino/original/4X/7/e/e/7ee01f052d875c9b38cb3448922e6db113a72e07.jpeg) > **[ADC for seismic data acquisition system - Simulation, hardware & system...](https://e2e.ti.com/support/tools/simulation-hardware-system-design-tools-group/sim-hw-system-design/f/simulation-hardware-system-design-tools-forum/565858/adc-for-seismic-data-acquisition-system)** > > Other Parts Discussed in Thread: ADS1258 , ADS1281 , ADS1282 , ADS1257 , ADS1262 , ADS127L01 , ADS1278 , THS4521 I want to design seismic data acquisition system Another, with selectable gain: ![](https://europe1.discourse-cdn.com/arduino/original/4X/d/9/3/d931793c156d74fbcfe61a020a896f90b5ab53df.png) ---
**Author:** ![bblackledge](https://dub1.discourse-cdn.com/arduino/user_avatar/forum.arduino.cc/bblackledge/32/934922_2.png) [@bblackledge](https://forum.arduino.cc/u/bblackledge)\ **Post date:** [November 17, 2025, 4:41pm UTC](https://forum.arduino.cc/t/sallen-key-filter-reducing-mid-supply-bias/1414805/6 "2025-11-17T16:41:11Z")
That’s correct, established through debugging individual stages as mentioned. ---
**Author:** ![bblackledge](https://dub1.discourse-cdn.com/arduino/user_avatar/forum.arduino.cc/bblackledge/32/934922_2.png) [@bblackledge](https://forum.arduino.cc/u/bblackledge)\ **Post date:** [November 17, 2025, 4:45pm UTC](https://forum.arduino.cc/t/sallen-key-filter-reducing-mid-supply-bias/1414805/8 "2025-11-17T16:45:55Z")
Thank you for this but the geophone inputs aren’t floating, the input symbol isn’t clear but I’m using a two wire connector block which will be connected to the relevant ± outputs from geophone. I’ll take a look at those schematics for some inspiration though. ---
**Author:** ![anon94801460](https://avatars.discourse-cdn.com/v4/letter/a/278dde/32.png) [@anon94801460](https://forum.arduino.cc/u/anon94801460)\ **Post date:** [November 17, 2025, 4:48pm UTC](https://forum.arduino.cc/t/sallen-key-filter-reducing-mid-supply-bias/1414805/10 "2025-11-17T16:48:21Z")
Have fun! ---
**Author:** ![PerryBebbington](https://dub1.discourse-cdn.com/arduino/user_avatar/forum.arduino.cc/perrybebbington/32/238766_2.png) [@PerryBebbington](https://forum.arduino.cc/u/PerryBebbington)\ **Post date:** [November 17, 2025, 5:13pm UTC](https://forum.arduino.cc/t/sallen-key-filter-reducing-mid-supply-bias/1414805/11 "2025-11-17T17:13:05Z")
> [@bblackledge](#): > > Thank you for this but the geophone inputs aren’t floating Why do you think that? There is nothing on the inputs to define their DC reference levels, therefore they are floating. Look at the examples @jremington provided. You have now told 2 people who are (were) trying to help you that they are wrong. Both those people have the experience and knowledge you came here for, please don't tell me I'm wrong too. ---
**Author:** ![bblackledge](https://dub1.discourse-cdn.com/arduino/user_avatar/forum.arduino.cc/bblackledge/32/934922_2.png) [@bblackledge](https://forum.arduino.cc/u/bblackledge)\ **Post date:** [November 17, 2025, 5:22pm UTC](https://forum.arduino.cc/t/sallen-key-filter-reducing-mid-supply-bias/1414805/12 "2025-11-17T17:22:52Z")
I haven’t told anyone they are wrong, I have tried to clarify whether it was a misunderstanding over the symbol used. Equally happy to post the full schematic but I’m not going to tolerate rudeness, I’d rather work it out on my own. ---
**Author:** ![anon94801460](https://avatars.discourse-cdn.com/v4/letter/a/278dde/32.png) [@anon94801460](https://forum.arduino.cc/u/anon94801460)\ **Post date:** [November 17, 2025, 5:25pm UTC](https://forum.arduino.cc/t/sallen-key-filter-reducing-mid-supply-bias/1414805/13 "2025-11-17T17:25:05Z")
C1 and C2 in the original schematic together create a serious problem and both need to be removed. You should brush up on basic circuit theory. ---
**Author:** ![PaulRB](https://dub1.discourse-cdn.com/arduino/user_avatar/forum.arduino.cc/paulrb/32/216330_2.png) [@PaulRB](https://forum.arduino.cc/u/PaulRB)\ **Post date:** [November 17, 2025, 5:27pm UTC](https://forum.arduino.cc/t/sallen-key-filter-reducing-mid-supply-bias/1414805/14 "2025-11-17T17:27:16Z")
> [@bblackledge](#): > > those wires are all connected But are they connected to the correct places? We get people posting only the "relavent" parts of their code on a daily basis. Around half the time, the problem turns out to be in a part of the code they didn't think was relavent, which is why they didn't spot it themselves. Same can be true about circuits. 🙂 ---
**Author:** ![bblackledge](https://dub1.discourse-cdn.com/arduino/user_avatar/forum.arduino.cc/bblackledge/32/934922_2.png) [@bblackledge](https://forum.arduino.cc/u/bblackledge)\ **Post date:** [November 17, 2025, 5:32pm UTC](https://forum.arduino.cc/t/sallen-key-filter-reducing-mid-supply-bias/1414805/15 "2025-11-17T17:32:45Z")
I have many things to brush up on, basic circuit theory is definitely on that list 🙂 Thanks for that, I’ll take a look at adjusting those. I think my intention was that they would be AC coupling caps to remove DC prior to the in-amp. ---
**Author:** ![bblackledge](https://dub1.discourse-cdn.com/arduino/user_avatar/forum.arduino.cc/bblackledge/32/934922_2.png) [@bblackledge](https://forum.arduino.cc/u/bblackledge)\ **Post date:** [November 17, 2025, 5:34pm UTC](https://forum.arduino.cc/t/sallen-key-filter-reducing-mid-supply-bias/1414805/16 "2025-11-17T17:34:50Z")
Very fair, thanks for the respectful response. I’ve attached the full schematic below 🙂 [Schematic.pdf](https://forum.arduino.cc/uploads/short-url/xqvsxne71f51J9vYc4zxzasekyY.pdf) (41.4 KB) ---
**Author:** ![anon94801460](https://avatars.discourse-cdn.com/v4/letter/a/278dde/32.png) [@anon94801460](https://forum.arduino.cc/u/anon94801460)\ **Post date:** [November 17, 2025, 5:41pm UTC](https://forum.arduino.cc/t/sallen-key-filter-reducing-mid-supply-bias/1414805/17 "2025-11-17T17:41:22Z")
> [@bblackledge](#): > > I think my intention was that they would be AC coupling caps to remove DC prior to the in-amp. A DC bias and path to ground for the amplifier input bias current is required. Study the device data sheet. ---
**Author:** ![PerryBebbington](https://dub1.discourse-cdn.com/arduino/user_avatar/forum.arduino.cc/perrybebbington/32/238766_2.png) [@PerryBebbington](https://forum.arduino.cc/u/PerryBebbington)\ **Post date:** [November 17, 2025, 5:45pm UTC](https://forum.arduino.cc/t/sallen-key-filter-reducing-mid-supply-bias/1414805/18 "2025-11-17T17:45:41Z")
> [@bblackledge](#): > > AC coupling caps to remove DC prior to the in-amp. 3 things come to mind from that: - If you think there is DC to be removed where do you think it's coming from? - If there were DC to be removed then 1 capacitor would do the job because the components are all in series so it doesn't matter where the capacitor is, so one will do. However, this is only true if the wires from the geophone are short. If they were a long twisted pair then your 2 capacitors would be correct in order to maintain balance, but, are capacitors needed at all? See first comment. - Removing DC isn't your problem, (not) adding the correct DC bias is. ---
**Author:** ![bblackledge](https://dub1.discourse-cdn.com/arduino/user_avatar/forum.arduino.cc/bblackledge/32/934922_2.png) [@bblackledge](https://forum.arduino.cc/u/bblackledge)\ **Post date:** [November 17, 2025, 5:46pm UTC](https://forum.arduino.cc/t/sallen-key-filter-reducing-mid-supply-bias/1414805/19 "2025-11-17T17:46:38Z")
Okay gotcha, thanks, I’ll remove C1 and C2, and tie the geophone inputs to GND as per the first schematic you posted ---
**Author:** ![bblackledge](https://dub1.discourse-cdn.com/arduino/user_avatar/forum.arduino.cc/bblackledge/32/934922_2.png) [@bblackledge](https://forum.arduino.cc/u/bblackledge)\ **Post date:** [November 17, 2025, 5:49pm UTC](https://forum.arduino.cc/t/sallen-key-filter-reducing-mid-supply-bias/1414805/20 "2025-11-17T17:49:52Z")
Thanks Perry, I was thinking it would be prudent to clean the signal (?) but yeah, I guess that doesn’t make much sense if there’s no DC to be removed in the first place. I’ll remove those caps and add a ground source to the differential inputs, see if that helps. Thanks again 🙂 ---
**Author:** ![bblackledge](https://dub1.discourse-cdn.com/arduino/user_avatar/forum.arduino.cc/bblackledge/32/934922_2.png) [@bblackledge](https://forum.arduino.cc/u/bblackledge)\ **Post date:** [November 17, 2025, 7:06pm UTC](https://forum.arduino.cc/t/sallen-key-filter-reducing-mid-supply-bias/1414805/21 "2025-11-17T19:06:49Z")
I’ve added ground coupling via 1M resistors which I realise is a bit more than that first schematic but found a tech note recommending using higher values ([TI application note](https://www.ti.com/lit/an/sboa503/sboa503.pdf?ts=1707353719139&ref_url=https%253A%252F%252Fwww.ti.com%252Fproduct%252FINA326%253Futm_source%253Dsupplyframe%2526utm_medium%253DSEP%2526utm_campaign%253Dnot_alldatasheet%2526DCM%253Dyes%2526dclid%253DCIzRg_rDmoQDFZSz0QQdYW8GnA)). Also removed C1 and C2. Unfortunately seeing the same behaviour. Correct mid supply bias (1.65V) immediately after the in-amp but then dropping to around 1.2V after the RC filter. Here’s the new schematic: ![image](https://europe1.discourse-cdn.com/arduino/original/4X/7/e/5/7e58bc4e50980b427eab5968397d99df409aebb0.png) ---
**Author:** ![bblackledge](https://dub1.discourse-cdn.com/arduino/user_avatar/forum.arduino.cc/bblackledge/32/934922_2.png) [@bblackledge](https://forum.arduino.cc/u/bblackledge)\ **Post date:** [November 17, 2025, 7:27pm UTC](https://forum.arduino.cc/t/sallen-key-filter-reducing-mid-supply-bias/1414805/22 "2025-11-17T19:27:00Z")
Did you spot any issues with the wiring? Would appreciate another pair of eyes on it if you have a moment [Next page](https://forum.arduino.cc/t/sallen-key-filter-reducing-mid-supply-bias/1414805.md?page=2)