Mixed signal basic design - noise considerations

[mcdesca]

Dear everybody,

I did a small design with a pretty basic mixed signal modules: a microcontroller will generate a signal by means of a DAC (MCP4821) and then amplified and fed to the power mosfets. Some analog feedback signals through opamps are being then fed to the uC's ADC. The power supply will be done by means of switch mode dc/dc converter.

A first design works just fine but i guess I've payed too little attention to the pcb layout and hence, I've got some noise mainly, in the DAC output. Since all the software and hardware functionality have been proven, I'd like to redesign the layout in order to achieve good signal quality.

Attached is my first module organisation which in some way, is the way I'll first split the ground planes and interconnect them in one spot.

The Current Monitor (INA125) monitors a 0,2ohm shunt resistor and I just want to go to the milliamp resolution. Right now the dc/dc converter is producing some noise in this reading.

The DC/DC converter switches at 500KHz (both for the 5VDC and 12VDC channel).

I've readen that the opamp become less noisy when in inverting mode (which I haven't foreseen since I don't have a split supply, but can). Is this that bad (I'm using lm358 opamps)?.

I kindly ask you for feedback and what your experience on noisy DAC's is.

Thanks a lot.

[T3sl4co1l]

LM358 will read incorrectly in the presence of RF (input rectification effect), might be relevant?

Those cheap current sensors might have similar problems, and don't usually have fantastic CMRR either.

Tim