[sdiy] DG408 issues

David Moylan dave at westphila.net
Fri Aug 11 05:47:26 CEST 2017


As mentioned in my other thread I'm using a DG408 to switch modes on a 
Roland IR3109 filter.  Can't quite figure out why the DG408 is so hot. 
The digital inputs are rather large as they're coming from the outputs 
of 3 opamps set up as a 3-bit ADC.  However, I'm sending them through 
large resistors (220k) and they don't appear to be drawing much current, 
approximately .13 mA.

Datasheet suggests that anything within the supply range is ok. 
However, I'm not sure if the negative supply should be considered ground 
for the digital inputs.  Even so, if I set the ADC to all positive 
outputs so there's no negative voltage on the digital pins the chip is 
still just as hot.

I had tied the enable pin directly to the +15 supply, but hacked in a 
resistor to tie it to a divider that was already in use, after seeing a 
comment online that the chip doesn't like large voltages on the digital 
pins.  There's about 8V on the pin with 120k to +15.

Voltages at all pins appear normal, but the chip is very hot.  Can't 
keep my finger on it for more than a few seconds.  Other than that 
symptom everything works correctly.

Anyone experienced problems with this chip?

Thanks.

Dave



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