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Design >> Mixed-Signal Design >> Reference Voltages in a Sigma delta Modulator
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Message started by filipe on Sep 26^th, 2007, 2:06pm

Title: Reference Voltages in a Sigma delta Modulator
Post by filipe on Sep 26^th, 2007, 2:06pm

Hi,
I'm designing a SC sigma delta modulator (Fs=5MHz) with 5V supply, and I need 2 reference voltages (1.75 and 3.25V) generated into the chip. So, I think that the best way is to build a BGR (such as PTAT, razavi pg 391, fig 11.20) , and with it, generate the other voltages. Is this a good approach??? Which circuit should I use to derive the 2 required voltages??
And what kind of buffer is required after the references generated??
Please, enlight me!!
Thanks
Filipe

Title: Re: Reference Voltages in a Sigma delta Modulator
Post by tosei on Sep 28^th, 2007, 6:41pm

Hi filipe.

I guess those reference voltages you are referring to are for setting the actual dynamic range for your modulator. If that's the case, then those reference should be as stable as possible over process and temp. Certainly a BGR is a very good option. May be I would use opamps with the required closed loop gains for scaling the BG voltage up to the desired values. If these opamps have large enough GBW you would not need to buffer the reference since the output impedance of the opamp would be low enogh (this might be important considering you are dealing with SC circuits, so you might want to avoid switching noise affecting your reference voltage)

Regards
tosei

Title: Re: Reference Voltages in a Sigma delta Modulator
Post by filipe on Oct 5^th, 2007, 12:37pm

and what circuit may I use to have 3.25 and 1.75V from the BGR (1.25V)???
Thanks
Filipe

Title: Re: Reference Voltages in a Sigma delta Modulator
Post by tosei on Oct 6^th, 2007, 9:45am

Hi filipe,

A simple non-inverting opamp topology would make it. It´s input would be the BGR reference and the gain would be 1.75/1.25 for the first reference and 3.25/1.25 for the second one.
In this way you are isolating the BGR from the rest of the circuit and providing the corresponding voltage references with low impedance.

Regards
Tosei

Title: Re: Reference Voltages in a Sigma delta Modulator
Post by Berti on Oct 7^th, 2007, 10:40pm

Hey Filipe,

the design of a reference buffer is quite challenging because if not done in a smart way, the reference buffer might consume
a big portion of the overall power consumption of your modulator.

I recommend to first design an efficient reference buffer (without worrying about the BGR). Usually this buffer provides
enough isolation that you can derive the reference voltage just from a bandgap current and a resistor (without need for
additional amplifiers).

Regards

Title: Re: Reference Voltages in a Sigma delta Modulator
Post by filipe on Oct 15^th, 2007, 6:22am

Thanks a lot!!!
And do you know a reference or a book concerning about reference buffer???
It seems to me they are difficult to find!!!
Thanks again.
Filipe