A trimming circuit for a resistor

[vadim888]

Hi all,

need help from experienced designers, could you please share schematic of trimming circuits which are you using.

I need to trim one resistor in a bandgap voltage reference in the ultra low technology node.

Best,
Vadim

 

[Warpspeed]

You mean like using a potentiometer ?

 

[vadim888]

I'm looking for some examples like this one:

 

[dick_freebird]

What you show (switch based trim) is what I've done many
times. You can put switches across series elements as well
and lower the impact of D-S leakage (as Vds on each, will be
small) and have a more code-linear trimming. Drive switches
with 2s-complement code (a small translation block) and you
can a make-to-center, trim up or down trim that makes an
untrimmed part come out roughly-right before any trim is
done (handy, at probe).

Beware the lumped capacitive loading of many parallel large
switches attached to a high-ish-Z node.

 

[Warpspeed]

My method of choice would still be a potentiometer.

A common method for fine trim would be to add a low value potentiometer to a voltage divider as in "A" below.

The problem with that, is that the potentiometer either has a very limited range of adjustment, or its very sensitive to adjust.
For production you usually need a wide range to cover stacking of tolerances around the rest of the circuit.

Much better is circuit "B" which gives incredibly fine adjustment around the centre range of the potentiometer.
But as you get nearer the ends the adjustment becomes more rapid.
You will never run out of adjustment with B !
And setting to millivolt accuracy near the middle is really easy.

 

[erikl]

Here are several trim schematics.

Here an array: View attachment A_0,35um_trimmed_CMOS_bandGap_reference.pdf

this is a serial trim chain:


and here a 5-bit R2R network: