Guitar Pedal Router: cross talk & noise from the LCD - Page 1
Forum section: electronics / general

author: Dan Moos
author_rank: Frequent Contributor
timestamp: March 29, 2016, 05:30:57 am
content: OK, I have a very simple version of my pedal router up and running. Just four loops, and I have the assignments temporarily hard coded for testing purposes.My temp, just test the concept, set-up:Arduino controlling a 16x16 Analog Devices AD75019JPZ crosspoint switch chip. I have four inputs and four outputs hooked up. The inputs are hooked in x0, x1, x2, x3. The outs to y0,y1,y2,y3.I have hard coded three configurations. x0-y0, x0-y1, x0-y2. I can select from these three possibilities from push buttons. I specifically clear all the unused switchs, ie, they are all off no matter what. I do the clear operation before writing the serial data with the desired on switches.I have my guitar hooked to x0, and no matter which of the y's I hook the amp to, it works well, EXCEPT:Say I have the amp hooked to y0, but select any of the other y's as the output. I should hear silence, but I hear my guitar, albiet greatly attenuated. Its much louder than I can accept though.Is it possible that this is internal cross-talk in the chip? I really hope not because this chip is otherwise perfect for my plans.Also, my I2C LCD screen is injecting an oscillation into the signal. Its being powered directly from the Arduino's 5v pin. Its the data lines, not the power lines. I pulled the two data lines, and it went away. Its faint but annoying if the amp/pedals are in a low gain configuration, but at higher gain levels its pretty loud. This oscillation is getting to the amp just as loud whether the amp's output on the switcher is selected, or if I have an unused output selected , ie, I'm listening to an open signal path.Also, if I have proper output selected so that I should hear the guitar, I can completely attenuate this oscillation with the guitar's volume pot. If I select another, unused output, the sound remains. This is strange to me, because the input that the guitar is hooked to is common to all the outputs in my test config, so you'd think if it was being inject at the input, it could be attenuated from the guitar even when other outputs were selected.Also, the whole shebang is cobbled together on two breadboards and a hacked together patch bay for the jacks. In otherwords, it looks like oscillation waiting to happen. Problem is, the LCD wires are well segregated from pretty much everything else.Both these problems must go!While I'm babbling, I should also ask about bypass caps for the switcher chip. I'm not completely clear what the guidelines are for bypassing on IC's. The switcher chip has a Vcc and a DGTL GRND for the logic, and a Vdd and Vss for the analog. Assuming that the rails are good (i'm running of a bench supply), what further bypassing should I do for this chip, and how do you spec this for any particular IC?Sorry I went long!

author: Dan Moos
author_rank: Frequent Contributor
timestamp: March 29, 2016, 05:41:22 am
content: ADDENDUM:I notice in the datasheet for the AD75019, it says at a source resistance of 600R, and a load of 10k, with a 2v PtoP signal, isolation between any two channels should be 92 dB. I feel like that should be sufficient.Ok, my signal level is less than that, but in that ball park. My amp's input Z is 1 meg. Not sure what the guitar's output Z is, but its more than 600R. In the realm of 10k or more. Does this higher source resistance imply that a buffer would help with crosstalk? I had planned to buffer every in and out in the final product, but didn't think that would be relevant to crosstalk. Is that my issue?

author: obiwanjacobi
author_rank: Super Contributor
timestamp: March 29, 2016, 09:58:59 am
content: Are you grounding unused inputs in the matrix?

author: Dan Moos
author_rank: Frequent Contributor
timestamp: March 29, 2016, 02:20:13 pm
content: Ok, I can ground all my x pins, since the x half of the matrix I have arbitrarily designated as inputs, but is it that simple?I presume each switch pair boils down to a MOSFET or a BJT. I'm not sure if the x's are all tied to drains, and the y's to sources, or vice-versa, but I feel it should matter which one I ground, regardless of whether I have deemed it an input in my design.Or is it enough that one half of each switch be grounded?

author: obiwanjacobi
author_rank: Super Contributor
timestamp: March 29, 2016, 04:30:11 pm
content: I don't know - it was just a guess.In the previous thread we talked about having on row (or column) connected to ground in order to short any unused signals...In my own design (not based on a cross-point IC but with individual switches) I also had the option to ground each cross-point - I forgot the exact reason, but I remember discussing it then...