Resistor layout matching

[M.Sansen]

Can anyone pls tell me why matching resistors need metal1 as shielding and why matching capacitance do not need metal1 as shielding? thanks:-(

 

[erikl]

The purpose of shielding is just to reduce mismatching effects from (asymmetric) structures above or below the devices to be matched, so I think it doesn't depend on the type of device.

 

[dabing]

Can anyone pls tell me why matching resistors need metal1 as shielding and why matching capacitance do not need metal1 as shielding? thanks:-(

hi
I think you should refer to the capter 4&7 of <<The art of analog layout>>.

 

[protonixs]

Can anyone pls tell me why matching resistors need metal1 as shielding and why matching capacitance do not need metal1 as shielding? thanks:-(

where did you get this idea?

 

[cool_ic]

The resistor matching is done for better resistor poly arrangement and spacing(it is process dependent ),but the sheilding is used to protect critical nets from crossing noisy signals.

 

[kapilsn]

Resistors are noisy devices hence other signals and devices need to be protected form it.

 

[erikl]

Resistors are noisy devices hence other signals and devices need to be protected form it.

And what about the capacitors' noise :?: :-?

 

[kapilsn]

Capacitors being reactive elements do not produce electrical noise.It is true that the ESR (Equivalent Series Resistance) of a capacitor will produce thermal, also known as
Johnson noise but this is almost always negligible.

Do correct me if I am wrong

 

[erikl]

Do correct me if I am wrong

kTC noise. Just study the reference given above!

 

[kapilsn]

Hi Erik,
I am not able to find the link for studying ......
Kapil

 

[erikl]

Hi Erik,
I am not able to find the link for studying ......
Kapil

Oh dear, my answer from 09-08-2010 12:05 PM (above) contains a link "capacitors' noise". If you put the cursor above it, it gets underlined. If you click on it, you'll be led directly to its URL.
HTH! erikl

 

[dick_freebird]

Every diffused resistor is a MOSFET in disguise, and high
sheet resistivity ones are able to be modulated by any
"gate electrode" above them. By "shielding" them all you
enforce an identical gate potential. But this may not be
an equal Vgs at each segment if a common slab is used.
Field plating each segment to (say) the most negative
segment contact individually would be best.

 

[ira2008]

ING top

 

[stromer]

Quote: "Every diffused resistor is a MOSFET in disguise"
"Diffused" in this case includes polysilicon resistors.
In real life, metal above a resistor has two effects:
first, it acts as a local gate so the surface field on the resistor is well characterised.
second, metal is ductile, and relaxes under temperature cycling. The resultant strain will modify the resistor value. If you want resistors to match the physical environment (including the metal cover) should also match.

In practice, I don't allow metal over polysilicon resistors if I want optimum stability of matching. Instead, I use multiple interdigitated resistors arranged with the minimum of local metallisation, so that the field that one resistor "sees" is as closely related to that of its companions as practical.

 

[Teddy]

Metal over matched devices (same is for transistors etc. In ideal case you don't do it. It is the same as MIM caps should not have anything except substrate underneath. In real world you pay for silicon so you have to utilize as much as possible. Then you use area above resistors for routing, MIMcaps. Then you will not have perfect matching so to mimic that all devices "see the same" you put metal above resistors, or under MIMs.
In fact all the device models are extracted in those ideal conditions. You are also not allowed routing above matched MOS pairs....