The part under consideration is UDA1334BTS - the DAC I use in my design.
Here's the typical connection diagram:
If the circuit is being powered off, and then quickly on, the Vref voltage does not get as low as 0.75 V to get part properly reset, and the consequence is DAC output powering up with some arbitrary voltage, causing click when external interface writes initial value of 0 to the DAC. I can not state if only analog part is guilty, or digital part is also involved.
Here's the related circuit:
We can see that charge to the reference 1.25 V (analog part out of reset) takes about 1.7 seconds, and discharge to 0.75 V (analog part complete reset) takes about 1 second. My tests show that time frame, causing clicks, is actually longer than one second, thus actual reset threshold during powering down must be lower than 0.75 V.
The good thing in the circuit is that there's a system reset signal at the start of power up, which I can use to do something to this Vref input. I drew the following circuit:
and here the simulation zoomed:
Green is reset signal, blue is Vref output, and red is current through transistor's emitter.
I am looking for the feedback on this circuit, in particular:
- is it correct in general, is there any better circuit?
- what would be the best choice for transistor (I picked 2N2222 out of blue);
- any advice on resistor values?
- will the chosen values be ok for accelerated capacitor discharge not causing audible click/pop (the DAC output follows the Vref input very well)?
Edit (info for jonk): timing can be extended to the whole reset pulse width, e.g. 200-400 ms, and slope must not be sharp not to cause click/pop as DAC output follows the Vref voltage; I think small residue discharge voltage voltage (let's say, 0.4-0.7 V) would be acceptable, but must be confirmed through testing.