I'm working on a project for school (a discrete dual-slope ADC) that will combine digital and analog circuitry and operate off of a ±15V dual-rail supply. The "analog ground" will be at 0V, as expected. However, I'm not sure where I should ground my digital circuits.
- Should I put the digital ground on the same node as the analog ground? Or should it be connected to my -15V supply with VCC regulated to -10V?
- Since I'm dealing with "precision" analog circuitry, I want to minimize switching noise.
- My Vref regulators are referenced relative to ground, and control +Vref and -Vref via feedback loops (not shown).
- The digital output from the analog block will be either open collector or open drain, so I can adjust the output level as needed.
- The first option is more intuitive, but will the second offer significantly better performance?
- In the digital section, I plan on using a mixture of 74HC and 74AHC chips. Will using the AHC chips at HC speeds introduce more switching noise than using all HC chips?
- What degree of decoupling is necessary? Will 100nF caps on each digital chip plus something like 10uF between the supplies and ground be sufficient?