WM8741 VMID voltage reference - diyAudio
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Old 14th December 2014, 03:27 PM   #1
wutzke is offline wutzke  Germany
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Lightbulb WM8741 VMID voltage reference

Hello,

let's say you have an WM8741 DAC in dual differential (mono) mode. The DAC has internal voltage divider resistors to generate its analogue midrail voltages (see datasheet page 2 and 11). The datasheet says to decouple the midrail volatages by connecting capacitors to the pins VMIDL and VMIDR.

What would speak against connecting a low noise, 2.5V voltage reference to the VMIDL and the VMIDR pins?
Than should improve the power supply rejection ratio.

In single supply applications, the reference voltage could also serve as a virtual ground in the output stage.

Thanks!
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Old 14th December 2014, 11:42 PM   #2
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Yes, you can follow John W. approach: Capcitor type for Vref
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Old 17th December 2014, 04:59 PM   #3
Zoran is offline Zoran  Serbia
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It is not input rather output pin. That value can be used like having a zero offset after the OP amps. Or feed the next low noise Vref modul doing the same. Like Yung Vref circuit with AD797. But if You have very good filtered Avcc then You can count that will be clear enough and have 1/2 value of Avcc power at the Vmid pins. You can use values like in datasheets i think 10uF? I did with 22uF tantal...
cheers
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Old 17th December 2014, 06:15 PM   #4
phase is offline phase  United States
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I'm using some spare, smaller-case 100uf Panasonic Pureism in that position, would think that a 47uf would be perfect, without going overboard. I haven't monitored the voltage here however.
My concern with a separate power configuration would be that possibly the timing of the inrush at startup may creat a problem, but the only way to find that out would be to try it...
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Old 17th December 2014, 09:12 PM   #5
Zoran is offline Zoran  Serbia
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This Vref or Vmid Voltage value is for use optionally to prevnt DC flow from Voutputs
Because @ Voutputs is the same potential 1/2 Vanalog supply typ. 5v/2=2.5V
or in 3.3V WM8741 vesion is 1.65V. That is useful with interstage transformer at the output
(what I do not recommend because dac chip have not driving "force" for that)
C.T. conn NOt to gnd BUT to Vmid.
Or to have Vmid reference when OP amps are used like output.
.
I think That is the idea fro the Vmid pin. So Value of the C vhen not in use is not important much. I would put there 10-100uF maybe with 0.1uF parallel decoupling small cap
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