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Design >> Analog Design >> Question about Vdd/2 reference voltage
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Message started by gaom9 on Apr 4^th, 2008, 7:19am

Title: Question about Vdd/2 reference voltage
Post by gaom9 on Apr 4^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.

Title: Re: Question about Vdd/2 reference voltage
Post by adesign on 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.

Title: Re: Question about Vdd/2 reference voltage
Post by loose-electron on Apr 8^th, 2008, 3:03pm

resistors will be more accurate, less process variance and bias sensitivity, with better matching

Title: Re: Question about Vdd/2 reference voltage
Post by Berti on 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

Title: Re: Question about Vdd/2 reference voltage
Post by loose-electron on 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.

Title: Re: Question about Vdd/2 reference voltage
Post by gaom9 on 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!