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

David G Dixon dixon at mail.ubc.ca
Tue Dec 4 07:22:27 CET 2018


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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