[sdiy] DG408 issues

[Donald Tillman]

The DG408 data sheet isn't really clear on this, but the logic inputs should have a voltage range from ground to about 4 volts above ground.  Any more and bad things happen.

  -- Don
--
Donald Tillman, Palo Alto, California
don at till.com
http://www.till.com


> On Aug 10, 2017, at 8:47 PM, David Moylan <dave at westphila.net> wrote:
> 
> 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