[sdiy] Help, I'm Desperate! (Charge Injection with DG408)

Ullrich Peter Peter.Ullrich at kapsch.net
Tue Dec 4 08:17:12 CET 2018


Hi David!

There are special "slow" switchest hat alsoo have make before break characteristics so that they allow clickless/popless switching.
Some information can be found here and also a list for such parts:

https://www.maximintegrated.com/en/app-notes/index.mvp/id/5299  (search for "Audio Switches and the Break-Before-Make Capability")

Ciao
Peter

http://www.ullrich.at.tt (my webpage)
http://www.rc100-usb.at.tt (Roland Sampler Tool)
http://synpro.heimat.eu/datadial.htm (DataDial Synth Addon)



-----Ursprüngliche Nachricht-----
Von: Synth-diy <synth-diy-bounces at synth-diy.org> Im Auftrag von David G Dixon
Gesendet: Dienstag, 4. Dezember 2018 07:22
An: synth-diy at synth-diy.org
Betreff: [sdiy] Help, I'm Desperate! (Charge Injection with DG408)

Hello SDIY Team,

This is the next sorry chapter in my saga with this 9-channel scanner.

You may recall that I was shocked to learn that TL074 saturates to different voltages positive and negative, and that this was causing me grief with a 3-bit comparator ADC that I built to drive a couple of 8-channel analog muxes (DG408).

Well, I solved that problem pretty easily: 3.3k current-limiting resistors to back-to-back 5.1V zener diodes off the comparator outputs.  I hand-selected the zener pairs to give me exactly +5.52V and -5.52V (subsequently fed to the logic inputs of the DG408 through 30k resistors), and was able to size the divider resistors accordingly to get exactly the threshold voltages I needed for the ADC.  This all worked perfectly.

HOWEVER...

The scanner is creating wicked clicks when it switches the muxes.  After a bunch of thinking and reading, I have come to the conclusion that this is an issue of charge injection in the DG408s.

Here is what I have done to try to fix the problem:

1) Decoupled the rails at the TL074 hosting the ADC comparators with 100nF caps directly at the power pins (no effect).

2) Decoupled the rails at the DG408 power pins with 10uF electrolytic caps (no effect).

Obviously, the power rails are not the source of the clicks.  This was my big clue that it was charge injection and not dirty rails.

3) Added 1N4148 diodes at the DG408 logic inputs to limit the negative logic voltage excursions to about -0.5V (instead of the -5.5V that the zeners are providing)(no effect).  It turns out that the DG4XX switch family can take the full voltage range at the logic pins, all the way down to -22V.

4) Added 10nF caps off of the DG408 switch output pins, as shown in the datasheet (no effect).

5) Increased these caps to 2.2uF.  This didn't really eliminate the clicks, but it did attenuate the signals something fierce.

6) Ran the scanner output through a 6dB filter (the first output of my Doc Oc filter) -- this did eliminate the clicks, but only at a cutoff frequency which seriously filters the audio signals as well.  Hence, adding a simple RC filter to the output isn't really a viable option.

7) I tried a different pair of DG408s (no effect).

The one thing I haven't tried is decreasing the downstream impedance of the switch outputs.  They are going to the multiplier circuit through 100k resistors.  I could change all the resistors in that circuit to 30k, or even 10k, but something tells me that this is a fool's errand.

So, I'm stumped, and about to give up.  My customer has told me that the unit is useless to him with this clicking (I concur -- it would be useless to me too), and I basically see no alternative to offering him his money back and throwing the whole mess in the trash.  I'm very surprised at the extreme level of charge injection in these DG408 switches.  I didn't notice it to nearly the same extent (if at all) with the DG409 switch in my 5-channel scanner.  Earlier today, I realized that there is a dual 8-channel switch, the DG407 -- perhaps if I had used this instead of two DG408s, the clicking would be lesser.  However, I'm not prepared to order new chips and create a third layout for this stupid circuit.

Do any of you have any insight that might help me solve (or, at least,
minimize) this clicking problem that I haven't already thought of?

Anyone who can help me solve this problem gets a free scanner PCB, or any other PCB that I make by hand.

Thanks,
Dave Dixon

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