I'm designing an analog synthesizer, which is controlled by an STM32 -microcontroller. In several parts of the circuit, I intend to use a GPIO pin to control switches in the circuit, and in two different parts as parts for generating audio. I'm worried about noise from the uC leaking in to the circuit, and my question is should I be worried, and if I should, what should I do to minimize the problems?
Some details: my PCB is laid out such that the right hand side is entirely analog, except for the said signals from the pins, and the left hand side contains the digital part, with DACs and an analog MUX (multiplexing the uC's built in DAC's) sitting on the dividing line. I have a ground plane on one of the inner layers of the 4 layer board.
The signals controlled from the GPIO's are:
- two analog SPST switches switching signal flow
- four on-off switches made from a single MOSFET, with the GPIO pin controlling the gate and switching unipolar analog signals on or off
- a leaky op-amp integrator integrating a narrow pulse generated by the PWM of the uC, which is used as an analog signal and processed further.
- and finally, a step transition in a GPIO is fed into the filter as a test signal for calibration.
The final output of the circuit has a one-pole lowpass filter around 70kHz to reduce spurious high frequencies.
It would be very difficult to try to place all the switches and such at the analog/digital boundary of the board, so they are placed rather deep inside the analog area of the board. On the other hand, being switch control signals, they carry very little current. Also the current into the integrator is quite small, some tens of microamperes during the pulses and otherwise essentially zero.
So, how much should I worry about inducing digital noise into the analog side here?