Stereo aux input direct into PCM3168 - Page 1
Forum section: electronics / beginners

author: jvphotog
author_rank: Contributor
timestamp: August 25, 2025, 12:23:03 am
content: Hi all. I was wondering if you could check my work? I want a stereo aux input into a PCM3168. I don't want to use buffers, and I'm using single ended inputs. I have two other inputs that will be differential. This will be used with a computer audio out or an Echo, etc. The resistors are thin film and the 22nF cap is C0G. Thank you!!

author: BeBuLamar
author_rank: Super Contributor
timestamp: August 25, 2025, 03:09:45 pm
content: Perhaps I don't understand but it seems that you connect inputs to inputs.

author: jvphotog
author_rank: Contributor
timestamp: August 25, 2025, 07:02:18 pm
content: I have my inputs and codec on different pages. I redrew is on the same page.

author: Benta
author_rank: Super Contributor
timestamp: August 25, 2025, 07:45:42 pm
content: You'll have to do something about DC biasing,As it is right now, your reference inputs are gounded and the line inputs are floating.Why are C526 and C527 placed after the attenuators? And what's the purpose of R517 and R518?The device has plenty of reference voltage outputs for biasing.Pay attention to section 9.3.1 in the datasheet.What you have won't work.

author: jvphotog
author_rank: Contributor
timestamp: August 26, 2025, 12:05:21 am
content: I appreciate the response Benta! Section 9.3.1 says, "In single-ended mode, VINx+ pins are used and VINx– pins must be terminated withAGNDAD1/2 through a capacitor or terminated with VCOMAD." That's why I have the 2 VIN- pins tied to ground through those 22nF caps.The only other section I see about inputs are, "12.1.3 ...In case of direct interface to VINx±, 1-μF electrolytic capacitors are recommended because the ac-coupling capacitor (which gives a 2-Hz HPF corner frequency and 47-Ω and 0.1-μF to 470-Ω and 0.001-μF differential LPF) is recommended as the anti-aliasing filter that gives a 160-kHz LPF corner frequency... There are three ways to terminate any unused input pins. First, terminate these pins to AGNDAD with 0.001-μF to 1-μF capacitors. This termination is applied on unused pins whose channels are configured in single-endedmode..."The is only one single-ended input diagram, and one passive diagram, but it's differential. I'm confused on out how to properly input a singled-ended signal with a pad and a HPF and LPF! How do I bias just the + input to DC?

author: Benta
author_rank: Super Contributor
timestamp: August 26, 2025, 07:10:58 pm
content: Please disregard my post above, it's based on a very quick skim of the datasheet.You should be OK, the inputs seem to be self-biasing.I still don't understand R517 and R518 though.

author: jvphotog
author_rank: Contributor
timestamp: August 28, 2025, 01:20:42 am
content: I think they're half of the anti-aliasing LPF

author: Benta
author_rank: Super Contributor
timestamp: August 28, 2025, 08:42:54 pm
content: Quote from: jvphotog on August 28, 2025, 01:20:42 amI doubt it.Did you ever do the maths?Right now you have a RC low-pass filter with R=5.047 Kohms and 22 nF. That gives a cutoff frequency of around 1.5 kHz. The 47 ohm resistors have negligible influence on that. (This presumes the input source has very low output impedance).If you remove the two 10 kohm resistors, cutoff frequency is around 150 kHz.I don't know which of those values are realistic, you'll have to decide.

author: jvphotog
author_rank: Contributor
timestamp: August 29, 2025, 12:26:09 am
content: Haha, well it was. You're right - silly me calculated before the pad...Im just going to remove the two 10k resistors. As a friend pointed out, aux sources usually have volume controls!