DG408 glitch

Thread Starter

fieryfire

Joined Feb 14, 2017
150
Hello All,

Could someone please explain what is the significance of capacitor 35pF(the Cl capacitance at the output of the DG408 analg multiplexer. My device was working fine while i was switching for 8 channels. But when i desoldered the 35pF capacitance i now get only 1 signal duplicated output for all 8 inputs.
Could someone please me what has gone wrong. I recall that for ADG408 i desoldered the Cl capacitance and the device worked just fine. But for DG408 it doesnt seem to be the case. Please help

upload_2019-9-25_9-39-58.png
 

Thread Starter

fieryfire

Joined Feb 14, 2017
150
hi ff,
Looking at the DG408 d/s schematic, it appears that the 35pF is the Hold time Cap for the D output.
E
Thank you for the response. But is there any reason that removing this capacitance would cause all the outputs for each input to be duplicated?
 

bertus

Joined Apr 5, 2008
22,270
Hello,

The 35 pF capacitor is only for test purposes.
Have a look at fig 5 and 6 of the attached PDF where only a load resistor is attached to the output.

Bertus
 

Attachments

ericgibbs

Joined Jan 29, 2010
18,766
hi,
The TS says the DG408 will not work correctly without the 35pF, perhaps he has another error elsewhere on his project.?

@fieryfire Please post a circuit of the project, so that we can double check.

E
 

crutschow

Joined Mar 14, 2008
34,283
I agree that the 35pF is just a test capacitance to simulate the stray output capacitance in a typical circuit layout.
If it doesn't work without that, then the TS has oher problems in the circuit.
 

Thread Starter

fieryfire

Joined Feb 14, 2017
150
Hmmm interesting. it would seem maybe there is some kind of short happening in the PCB. I do not know why is it so. I tried a lot to understand it but i cant seem to find it
 

Thread Starter

fieryfire

Joined Feb 14, 2017
150
Hello,

I have posted the faces of the PCB. Its a 4 layer board. with 10 DG408 multiplexers. For some reason this PCB has all the VCC and GND layer shorted for all the multiplexer components. i have never had this problem before.I dont see any visual shorts on the surface of the cards
 

Attachments

Thread Starter

fieryfire

Joined Feb 14, 2017
150
I have ordered a new PCB. I suspect that there must be some kind of damage to the PCB. Removal of the CL capacitance should not affecting the functioning of the multiplexer except for adjusting the Hold time. Which is not the problem here.
 

ericgibbs

Joined Jan 29, 2010
18,766
hi ff,
Checking the PCB's there are lots of solder splashes, its possible that could be a problem.
If you have more than one set of PCB's of this type, unassembled, you could measure the Vcc to Gnd path.
E
072 Sep. 30 09.52.gif
 

Thread Starter

fieryfire

Joined Feb 14, 2017
150
On closer inspection it could be the solder splashes. But i had soldered them with flux and cleaned them with Ethanol and Dry Air duster. Seems strange why that appeared.
 
Top