I've been working on a very simple DAC design. Nothing original, but it's my first. I've gotten it to the point where I need to figure out how to generate the various voltages (3.3, 5, +/-12 V). I would prefer not to deal directly with mains voltage (fear of electrocution), so I'm thinking of using an external supply. I'm just not sure whether it would be best to use AC, DC, single or multiple outputs? Any suggestions/advice would be much appreciated.
Not sure what level you are coming from but after reviewing your schematic, bottom one with differntial op-amp connections.
Some Tips
Digital to analog split is very important to get the most performance. Use a spilt ground plans thru DAC IC correct.
1) 3V on digital side only, and seperate 5V, +/- 12V on analog side only.
As phn noted can use 3 terminal regs but with seperate grounds.
Look for lower noise and ripple 3 terminal regs for analog side.
2) Connection between analog and digital supplies ground at a single point ground system.
Some Tips
Digital to analog split is very important to get the most performance. Use a spilt ground plans thru DAC IC correct.
1) 3V on digital side only, and seperate 5V, +/- 12V on analog side only.
As phn noted can use 3 terminal regs but with seperate grounds.
Look for lower noise and ripple 3 terminal regs for analog side.
2) Connection between analog and digital supplies ground at a single point ground system.
Looks good
Only the negative regulator data sheet LM337 specs a small tantalum cap on the output for stability. For all outputs, think about adding a larger electro for improved transient response. Probably around 100uF or so should be adequate. The protection diodes (1N4004) from Vreg in/out could be eleminated unless the Vu supply is shorted or drops very fast in relation to output.
Vadj pots could be eliminated for 1% resistors on most regulators if cost is a factor.
Double check pinouts on LM337 it is different than the LM317.
Only the negative regulator data sheet LM337 specs a small tantalum cap on the output for stability. For all outputs, think about adding a larger electro for improved transient response. Probably around 100uF or so should be adequate. The protection diodes (1N4004) from Vreg in/out could be eleminated unless the Vu supply is shorted or drops very fast in relation to output.
Vadj pots could be eliminated for 1% resistors on most regulators if cost is a factor.
Double check pinouts on LM337 it is different than the LM317.
Those look good ez,
You can enhance the temperature stability and spike rejection of these by placing a resitor in series with the input and paralleling an electrolytic or tant with the input cap. The value of the resistor depends on supply voltage. You calculate it to leave a minmum of 4 Volts drop across the reg at max current draw. Line spikes tend to ride right through these things.
And regardless of what any looneys say, tantalums would be a fine choice for output caps.
You can enhance the temperature stability and spike rejection of these by placing a resitor in series with the input and paralleling an electrolytic or tant with the input cap. The value of the resistor depends on supply voltage. You calculate it to leave a minmum of 4 Volts drop across the reg at max current draw. Line spikes tend to ride right through these things.
And regardless of what any looneys say, tantalums would be a fine choice for output caps.
ez,
Infinia makes a good point; you could lose the tants and just go with 50 - 100 uF on the outs... and save some $$ to boot.
No schems... but it is real simple. Hook one end of a resitor to the input terminal of the reg; the other end to your raw DC supply (I can help you with the resistor calc once you know your max load current and raw DC supply voltage). Then just put a lytic in parallel with the ceramic cap on the input; maybe about 20 - 100 uF. Don't lose the ceramic on the input... you need it there regardless of the lytic in parallel. This is minor point... but line spikes are nasty.
Yes, a 16 VAC tranny will work just fine... should give you about 18-20 Volts once rectified and smoothed.
Infinia makes a good point; you could lose the tants and just go with 50 - 100 uF on the outs... and save some $$ to boot.
No schems... but it is real simple. Hook one end of a resitor to the input terminal of the reg; the other end to your raw DC supply (I can help you with the resistor calc once you know your max load current and raw DC supply voltage). Then just put a lytic in parallel with the ceramic cap on the input; maybe about 20 - 100 uF. Don't lose the ceramic on the input... you need it there regardless of the lytic in parallel. This is minor point... but line spikes are nasty.
Yes, a 16 VAC tranny will work just fine... should give you about 18-20 Volts once rectified and smoothed.
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