[sdiy] Mixed-signal problems (dsPIC digital delay project)

[jays at aracnet.com]

Thanks, makes more sense now.

Jay S.


On 2017-01-26 14:02, Tom Wiltshire wrote:
> To clarify:
> 
> The op-amps provide the input/output mixers and filtering. This is all
> run at 9V with a 4.5V Vref virtual ground. Well within spec for the
> TL072.
> The analog 3.3V supply is used by the dsPIC's ADC and to feed the
> control pots that it reads. The rest of the chip is run from an
> independent 3.3V digital supply.
> The analog supply and the 9V circuits share the same analog ground.
> The digital part uses a separate ground.
> 
> Tom
> 
> 
> 
> On 26 Jan 2017, at 20:03, jays at aracnet.com wrote:
> 
>> Maybe there is something I don't get but I don't see how the analog 
>> section of this works at all. Don't under stand why the analog section 
>> is ran at 3.3V. Couldn't find any minimum voltage for the TL07X chips 
>> in the data sheets but the web site says 7 volts. Check the min volts 
>> in the parametric section.
>> 
>> http://www.ti.com/product/TL072
>> 
>> Next issue with the analog section is Vref at 4.5 volts and running 
>> the opamps at 3.3V. If low voltage, rail to rail, 
>> non-latching/non-inverting opamps are used on the input I might see it 
>> working but not the output. Just doesn't seem right having a 3.3V 
>> opamp with Vref 4.5V and 3.3V signals. Output doesn't seem to be AC 
>> coupled either. Don't know if that is a problem or not.
>> 
>> Grab the Microchip AN682 app note and take a look at Fig 7.
>> 
>> Personally would not recommend doing a ground plane fill. This should 
>> be left to people that understand ground planes and current flow over 
>> a circuit board.
>> 
>> Other things mentioned that are a good idea are more bypass caps, 
>> ferrites and make sure the digital and analog traces don't run 
>> parallel to each other.
>> 
>> Jay S.