[SOLVED] Trim caps for continuous time Sigma delta converter

[jgk2004]

Hello all,

I usually make my trim caps unit sizes then put them in parallel to add up into my bits required for trimming my integrator gains in a continuous time sigma delta converter. Question is.. is it really required to have them unit sizes? or is the integrator gain trim not required to be that fine.... Anyone have experience in this..... I am just sick of doing all this crazy wiring to make the unit elements connect in layout....!
Jgk

 

[erikl]

Not necessary to use unit size caps! Take square caps with the min. cap value you need. For high accuracy, use dummies around (not necessarily with the same width), and try to keep the routing as equal and symmetrical as possible, of course!

 

[jgk2004]

So you are saying I do NOT have to have unit size caps. I am little confused here.... When I mean unit size cap I mean the LSB I need in my trim....Example LSB is 10uX10u. the next bit can be 10uX20u. Is this ok??? I just don't think the trimming needs to be that accurate when trimming a continous time sigma delta....am I correct with this one?
Jgk

 

[erikl]

When I mean unit size cap I mean the LSB I need in my trim....Example LSB is 10uX10u. the next bit can be 10uX20u. Is this ok???

No, this was my misunderstanding, sorry! I thought your unit size cap was smaller than (necessary for) the LSB (usually there exists also a unit size cap in the library, that's why). If your unit size cap just stands for the LSB value, you better keep with its size, and better use multiples of them.

I just don't think the trimming needs to be that accurate when trimming a continous time sigma delta....am I correct with this one?

I think this just depends on the precision which you expect from your cap array (incl. routing), considering possible redundancy and comparator (in)accuracy.

 

[jgk2004]

Thanks Erikl, I will just use the good old unit size caps and multiples.... I just hate that routing...
Jgk