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Old 26th February 2013, 11:50 PM   #1
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Default DAC chip

On a DAC chip. What does the digital power supply pin do compared to the analog power pin does? I have been looking around on the net but I can't seem to find any information to explain the basic functionality of each.

I know on higher end equipment some of the sales propaganda says that they have separate power supplies to reduce noice.


Any help would be appreciated?
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Old 27th February 2013, 12:11 AM   #2
tomchr is offline tomchr  United States
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Quote:
Originally Posted by steven2583 View Post
I know on higher end equipment some of the sales propaganda says that they have separate power supplies to reduce noice.
I can't comment on specific equipment, but one of the reasons to keep the analog and digital supplies separate is to avoid coupling digital switching noise into the analog circuits. Also, on newer CMOS processes, the digital logic can't handle the voltages needed for the analog circuits, so the digital might run on 1.2 or 1.8 V while the analog section supports operation at 5 V. That's another reason to separate the supplies.

~Tom
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Old 27th February 2013, 12:15 AM   #3
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You not only have to separate analogue and digital grounds but also different powered analogue grounds.

I designed a mixer with digital and audio signals and kept them separate yet still got a lot of noise. I found I had to separate the smoothing capacitors ground from the analogue ground to stop interference.

I also had a SMPS that wouldn't work right. Again I hadn't separated the 3 different grounds it had of different powers.

Good PCB design takes a lot of consideration of ground loops.
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Old 27th February 2013, 02:21 AM   #4
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What is the digital doing with the power? What is the analog doing with the power. I wanted an an explanation of the purpose of the digital in and the analog power input. What is it used for in the chip?
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Old 27th February 2013, 02:37 AM   #5
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read the datasheet, it will vary. analogue may be used for a low noise reference voltage for the output to be referenced to, so its important for a high quality regulator to be used to gain high performance. it may also be used for a bias voltage and power for internal transistors for the modulator. digital may be used for logic supplies. its impossible to answer generally, but this is pretty much universally recommended in the manufacturers datasheets if you expect to get the specified performance
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Old 27th February 2013, 03:26 AM   #6
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The chip has 2 distinct sections.

The digital section has a bunch of logic and memory to clock in the data. This has a lot of switching going on with associated ground bounce and noise rejected onto the power rail.

The analog section has a bunch of stuff to turn digitally represented numbers into voltages or currents as accurately as possible, sometimes bipolar. This needs as little disturbance on the power and ground as possible.

Google 'R2R ladder DAC' to see the inner workings of (one type of) number to current/voltage section.
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Old 27th February 2013, 04:26 AM   #7
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Thank you for the link. Now I can get a clue.
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Old 27th February 2013, 04:31 AM   #8
qusp is offline qusp  Australia
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its pretty unlikely your dac is R2R and even less likely to have bipolar supplies these days. it will give some idea, but the best way would be to look up the datasheet for the specific dac you are talking about
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