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Poly Resistor Matching on an IC

Hi,

I am designing a bandgap reference on an IC and I am finding it hard to tune the resistors so that all the resistances can be broken into unit resistors AND the bandgap curve is as expected.

So my question is : If I place one resistor, say l=37u, w=5u and another one of l=38u, w=5u close to each other (ie. their lengths differ by only 1u) will their resistance per unit square (for a given IC) match? Will the only error be due to possible dimensional mismatch or will the resistance per unit square change too, due to the length difference?

Thanks QQ

Reply to
QQ
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That should match ok, but if you really wanted to make it out of unit resistors, then you could write a little program to exhaustively search through all series and parallel and series-parallel combinations of a small number of resistors, to find a couple of networks which give you very close to the right ratio. Your unit resistor is pretty big, it might be easier to find networks of unit resistors if your unit resistor were smaller and you were to use more of them.

Make sure you account for metal resistance as this can be bigger than the resistor mismatch.

If you want to have two different value resistors that match reasonably well then it may be preferable to keep the length of both resistors the same and get the different values by changing the width, trying to keep the number of contacts proportional to the width. This should keep the contact resistance roughly in proportion to the resistance of the middle section.

Chris

Reply to
Chris Jones

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