DAC + and -output

Discussion in 'General Electronics Chat' started by Slacker, Jun 20, 2012.

  1. Slacker

    Thread Starter New Member

    Jun 20, 2012
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    Hi, I am newbie in things electrical so don't give me too hard time.

    I know how DACs work in concept, but i don't get how single ended DAC with +V power supply and 0V ground can have -V output. I see datasheets stating e.g. DAC left or right channel outputs 2 pins one V+ and the other V-.

    Is it biased in the middle? (output from e.g.: 0 to 2 Volts is considered as minus and output from 2 - 4 Volts represents the positive side of audio wave)

    Or is it biased only up to I/V converter which somehow splits positive wave part to +V electrical and the lower part to -V?

    I think i am missing something deeper, can anyone help me please?
     
  2. ErnieM

    AAC Fanatic!

    Apr 24, 2011
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    Got a particular spec sheet in mind?
     
  3. Slacker

    Thread Starter New Member

    Jun 20, 2012
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  4. Slacker

    Thread Starter New Member

    Jun 20, 2012
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  5. MrChips

    Moderator

    Oct 2, 2009
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    PCM1798 and AD1995 are both differential CURRENT output, not voltage output.
     
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  6. Slacker

    Thread Starter New Member

    Jun 20, 2012
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    Ok, than how DAC makes + and - output from +V supply and and ground? I mean what happens inside?
     
  7. crutschow

    Expert

    Mar 14, 2008
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    Both outputs of the AD1995 have a current output referenced to the plus supply. + and - refers to the phase of the output. The + output increases with increasing digital input and the - output decreases with an increasing digital input. The outputs are biased at about 1/2 full-scale (see ANALOG OUTPUTS in the ANALOG PERFORMANCE table). The differential outputs are designed to drive a differential amplifier (see Figures 7 through 12).
     
  8. Slacker

    Thread Starter New Member

    Jun 20, 2012
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    16 bit DAC has 65536 values which typically are represented from 0 to 4 volts at analog output(or something like that). 0 to 2 volts represent 32768 of negative part of the waveform and 2 to 4 volts represent the positive part of the waveform. The first 0 - 2 volts are electrically positive, how do you make them into electrically negative output?

    Differential outputs would use phase splitter like this, here is an animation:

    http://www.falstad.com/circuit/e-phasesplit.html

    But there you have an input of 40Hz which goes from + to -V. Lets say this input would go from DAC outs. So how in the hell DAC can output negative voltage using +V supply and GND only? and how does it split 0 to 2 V as negative electrical output and 2 to 4 as positive?

    P.S. I find circuit sim app much better than spice for learning.

    http://www.falstad.com/circuit/
     
  9. crutschow

    Expert

    Mar 14, 2008
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    You seem to be hung up on this negative thing. The plus and minus indicators for the outputs don't mean the output is a negative voltage, it's referring to the fact that they have opposite phase (even though both are positive voltages).

    Your sim app example shows that very thing. The two outputs from the transistor have opposite phase, but they are both positive voltages.

    If you really want a negative output voltage then you will need to use an op amp level shifter that is powered by plus and minus supply voltages.

    Sim app may be fine for simple simulations but Spice is much better for simulating complex circuits.
     
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  10. Slacker

    Thread Starter New Member

    Jun 20, 2012
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    The outputs of phase splitter are both positive but opposite phase, thats fine. But the input isn't. The input is changing from + to -. I believe a phase splitter input would receive a varying + to - Voltage signal from DAC , the point is i don't know how the DAC can do varying output of + to - Voltage with +V supply and GND only. Am i missing something else?
     
    Last edited: Jun 20, 2012
  11. crutschow

    Expert

    Mar 14, 2008
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    The DAC doesn't do a varying + to - voltage, either internally or externally. The input you are talking about doesn't exist.

    The DAC differential output is generated by its own particular internal differential stages which are nothing like the phase splitter circuit you posted. Here's some general info on DAC differential outputs.
     
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  12. Slacker

    Thread Starter New Member

    Jun 20, 2012
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    Would you have any more specific example of DAC differential output generated by its own particular internal differential stages.

    BTW how would single ended DAC output look like before any I/V conversion, before any post conversion filtering internally (low pass filters etc.)?
     
    Last edited: Jun 21, 2012
  13. crutschow

    Expert

    Mar 14, 2008
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    Here's another link that discusses the output interface.

    Finding the actual internal circuit design for a specific DAC is difficult since that is usually considered proprietary information by the manufacturer.

    A current output DAC generates an output current whose magnitude is determined by the digital input word value. If you have this current go through a small resistor to ground (the maximum allowed resistor value is given in data sheet) then you will convert that varying current into a varying voltage.
     
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