Do I really need 2 transformers?

[Rudi_Ratlos]

I am currently assembling a small pre-amplifier:



consisting of 2 sections:

1) the digital section
Heart of the digital section is a µC PIC18F4550 that controls a relais-board (source selection, not shown), accepts remote-control, drives a 2x16 LCD, ...

2) the analog section
Consisting of a OPA2132 attenuator and an AD823AN OpAmp.

Power consumption is approx. +5 V / 250 mA for the digital section and about +5 V /150 mA and -5 V / 50 mA for the analog section.

The PCB has provisions for 2 bridge-rectifiers.

Do I really need 2 transformers or can I take one transformer having 2 x 8 VAC / 750 mA secondary windings, one secondary winding to produce the -5 V and the other winding for the digital and analog +5 V rail?

Best regards - Rudi Ratlos

[nigelwright7557]

I dont see why one transformer wont do the job.
Just keep analog and digital grounds seperate and make sure the supplies are well decoupled.

[yeti]

Moin Moin Rudi;

One tranformer will probably work, but it's not a good Idea because the analogue and digital grounds should be strictly separated to avoid appearance of clock pulse residuals on the analogue section.('Übersprechen')

Regards

Arne

[Rudi_Ratlos]

Moin Arne,

so I will use 2 transformers indeed.

Best regards - Rudi_Ratlos

[SoNic_real_one]

Originally Posted by yeti

Moin Moin Rudi;
...because the analogue and digital grounds should be strictly separated to avoid appearance of clock pulse residuals on the analogue section

Really? So you will crack open the DAC and separate the two grounds? Because this is how the electronics engineers know how to make them - the ground MUST be tied toghether between the two sections, you don't have optical couplers inside.
There are many docs regarding grounding and two TRANSFORMERS are never mentioned. Just star ground. That means that the power can come from same transformer with 2 windings or even from the same winding if you use RF separation (ferrite beads, coils, capacitors...).
One exemple: http://www.analog.com/static/importe...0Grounding.pdf