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gaom9 Junior Member Offline Posts: 21 CHINA	Question about Vdd/2 reference voltage Apr 04^th, 2008, 7:19am Hi, I have a basic question about the Vdd/2 reference voltage. In many design, the Vdd/2 voltage will be used as the common voltage. But how we can get the voltage? Can we just use the same resistors in series or the same MOSFET in series to divide the Vdd to two parts? Or we should get it in other ways. Thank you. Best regards.
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adesign Community Member Offline Posts: 73	Re: Question about Vdd/2 reference voltage Reply #1 - Apr 7^th, 2008, 9:27pm It depends on the application requirements. Sometime, two resistors in series also solves the purpose. You could also use Unity gain buffer followed by these resistors output.
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loose-electron Senior Fellow Offline Best Design Tool = Capable Designers Posts: 1638 San Diego California	Re: Question about Vdd/2 reference voltage Reply #2 - Apr 8^th, 2008, 3:03pm resistors will be more accurate, less process variance and bias sensitivity, with better matching
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Berti

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Re: Question about Vdd/2 reference voltage
Reply #3 - Apr 9^th, 2008, 12:27am

Hi all,

About this kind of question I was also thinking about several times.
As the common-mode usually doesn't draw static/differential-current, there is no need for a buffer.
But is it better to use resistors or transistors for the voltage divider?

Resistors show better matching, but usually the common-mode voltage doesn't need to
be very accurate. On the other hand, I think that two diode connected transistors will provide
more current when the common-mode voltage leaves the middle point. Therefore providing a more
stable voltage with lower quiesent current compared to two resistors where the current is always constant.

What do you think?

Regards

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loose-electron

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Re: Question about Vdd/2 reference voltage
Reply #4 - Apr 10^th, 2008, 1:32pm

if accuracy is important - 2 polysilicon resistors
if area is important - 2 triode driven FET's as a voltage divider

I would never use two diode connected devices stacked power to groud because the current changes so much over PVT variables. Diode connected devices driven by a current source can make for useful Vthreshold proportional references however.

As part of a larger chip, use a bandgap driving a V to I converter and use the I generated to create your reference voltages (across a resistance )to anything you want.

Its a really basic question but the "right answer" depends a lot on the specifics of what you are trying to do.

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Jerry Twomey
www.effectiveelectrons.com
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Contract IC-PCB-System Design - Analog, Mixed Signal, RF & Medical

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gaom9 Junior Member Offline Posts: 21 CHINA	Re: Question about Vdd/2 reference voltage Reply #5 - Apr 11^th, 2008, 6:30am Thank you, everyone. Yes, it is a basic question. I think the resistors will word better than the diode connected mosfet, and easy to come out. Best regards!
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