[sdiy] DG408 issues

David G Dixon dixon at mail.ubc.ca
Fri Aug 11 08:58:31 CEST 2017

Whenever I use DG409 or other DGXXX chips, I always power the logic inputs
(and the other chips supplying the logic inputs, such as counters, etc) with
5V from a 78L05.

I've never had a problem.

> -----Original Message-----
> From: Synth-diy [mailto:synth-diy-bounces at synth-diy.org] On 
> Behalf Of David Moylan
> Sent: Thursday, August 10, 2017 8:47 PM
> To: SDIY List
> Subject: [sdiy] DG408 issues
> 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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