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

[Tom Wiltshire]

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.