[sdiy] DG408 issues

Ben Bradley ben.pi.bradley at gmail.com
Fri Aug 11 06:14:41 CEST 2017


"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."

The way I read the data sheet(s) for this DG series, there's a +V
input (+15v), -V (-15V) and GND, and these are for the analog switch
section. All digital inputs should be at least at GND pin and at most
+5V above GND pin. There's internal level translation for the
switches. 8V sounds way too high for a digital input, even if it's got
little current going into it through a high value resistor. I think
it's turning on something that shouldn't be turning on, and pulling
lots of current through the +15V and/or -15V power inputs, and that's
what's causing heating.

Make the resistive dividers so the digital inputs are at most +5V, and
at lowest, the potential of the DG's GND pin. Both Vishay and Intersil
say a digital input over 2.4V is a Logic 1, and below 0.8V is a logic
0. I might make the 1 voltage 3.3V

On Thu, Aug 10, 2017 at 11:55 PM, Tim Ressel <timr at circuitabbey.com> wrote:
> Check the datasheet for the exact DG408 you have. Some have narrower voltage
> maximums.
>
> --timbo
>
>
>
> On 8/10/2017 8:47 PM, David Moylan 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
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>>
>
> --
> --Tim Ressel
> Circuit Abbey
> timr at circuitabbey.com
>
>
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