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Help with mixed signal circuit power supply

meleghengersor

Apr 22, 2020
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I'm in the process of designing a mixed signal circuit powered by USB. Power supply requirements: 0-5V and 0-3.3V for a microcontroller and an audio codec (ADC/DAC), -5V to +11.5V (ground at +5V)to an analog IC. A step-up converter boosts +5V to 11.5V and an LM137 converts 5V to 3.3V. I understand that between the digital and analog supply a ferrite bead filters high frequency noise. The analog and digital ground are connected to their own ground planes. Would this circuit work? Is there a way to improve it?mxd_sgnl_pwr_sppl.png
 

Harald Kapp

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AV +5V is not, as you seem to think, a +5 V supply suitable for analog components. When you study the functional block diagram in the datasheet you will find that this node (input SW to U4) is switched to gnd periodically to achieve the step-up effect.
You'll need to draw 5 V from the node labeled DV +5V, best by an additional LC decoupling filter.
There is also no -5 V output recognizable in your circuit. Using + 5 V as 0 V reference for the analog part will not work. You need to have the same 0 V (gnd) reference for both analog and digital circuit. Otherwise the digital interface to ADC and DAC will not work.
You may be able to use a simple charge pump inverter ic to create -5 V from + 5 V, depending on the current you need on the -5 V line.
 

meleghengersor

Apr 22, 2020
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AV +5V is not, as you seem to think, a +5 V supply suitable for analog components. When you study the functional block diagram in the datasheet you will find that this node (input SW to U4) is switched to gnd periodically to achieve the step-up effect.
You'll need to draw 5 V from the node labeled DV +5V, best by an additional LC decoupling filter.
There is also no -5 V output recognizable in your circuit. Using + 5 V as 0 V reference for the analog part will not work. You need to have the same 0 V (gnd) reference for both analog and digital circuit. Otherwise the digital interface to ADC and DAC will not work.
You may be able to use a simple charge pump inverter ic to create -5 V from + 5 V, depending on the current you need on the -5 V line.

Harald, that's a very concise reply. Thank you! I have been trying to get my head around this problem and you explained it very well. According to the datasheet "The AS3394E was designed to operate from +5V and -6.5V supplies. The non-standard negative supply was necessary not to compromise the VCO frequency resolution, which ranges from -4 to +4 volts, in favor of a -5V supply (this is because there needs to be 4 diode drops for the current mirrors). Any one of the readily available 3-terminal regulators may be used to supply the -6.5V negative supply. Since the stability and jitter of the VCO are directly affected by noise on the positive supply, a supply as stable and clean as possible should be use (not the +5Vdigital supply) Maximum supply allowable across the device is 25 volts."

Inded, a charge pump inverter could generate the -6.5V supply.
 

bertus

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Nov 8, 2019
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Hello,

You could have a look at the LT3265.
That chip can mage a positive and negative voltage from a positive voltage.
The voltages can be adjusted by a couple of resistors.

LTC3265_typical application.png

Bertus
 

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