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Design >> Mixed-Signal Design >> Comparison of two layouts for resistor
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Message started by ywguo on May 28^th, 2004, 4:14am

Title: Comparison of two layouts for resistor
Post by ywguo on May 28^th, 2004, 4:14am

Hi,

I use a resistor string to get ref+, ref-, and CMO for a pipelined ADC. The resistor string is shown below.

R1 R2 R3
-----^^^-----^^^------^^^-----|
ref+ cmo ref- gnd

where R1=R2=2.25K, R3=5.5K.

Just now I argued with another guy about the layout of that resistor string. Here I have two questions.

The first, is it necessary to match R1 and R2?

The second, draw 4 parallel resistor strings shown below

R1=9K R2=9K R3=22K
-----^^^-----^^^------^^^-----|
ref+ cmo ref- gnd

the equivalent resistance is the same as the first structure. Which one has better matching? (Assuming that we use the equal width when drawing the resistors)

The sheet resistance is about 400 Ohm/sq. (n+ poly resistor with SAB, for SMIC 0.25um process.)

Best regards,

Yawei Guo

Title: Re: Comparison of two layouts for resistor
Post by Shiva on Aug 1^st, 2004, 12:15pm

Hello,

since the levels u want to generate are defined by a resistive division, the accuracy of the levels is defined by
a ratio of resistor values, which is generally considered to be a good thing as compared to a design depending
on absolute values.

The matching is still an issue, meaning that large, well-matched resistors will give less variation (sigma)
with the process (die-to-die) on the levels.

In that regard, placing 2 big resistors in parallel is better
than placing 1 smal resistor. U could also consider to place 'dummy' resistors around your resistor string.

Kind Regards

Shiva

Title: Re: Comparison of two layouts for resistor
Post by Shiva on Aug 1^st, 2004, 12:18pm

Hello,

I would like to add that for the ratio of resistors (same type) the variation due to non-matching can be considered to be a second-order effect.

So, you could just go on as you were before, since the division 'cancels' out the resistor variation (mostly).

Kind Regards