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

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