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Layout of Differential Opamp

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Puppet123

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I am looking for resources including books, papers or even a screen capture from Cadence of the layout of a fully differential opamp.

Most books talk about single ended but not fully differential opamps.

Looking for either folded cascode or a two stage opamp.

If anyone has any papers, notes, or a screen capture from Cadence that would be very helpful.

What are the special considerations for fully differential as opposed to single ended layouts of opamps ?

Thanks.
 

Looking for either folded cascode or a two stage opamp.
... a screen capture from Cadence that would be very helpful.
Here's a 10 years old screen shot from a fully differential two stage S&H OTA, with its sample, feedback and compensation caps:

sh_layout_outn_outp.jpg

Don't know if this helps a lot - without schematic.

What are the special considerations for fully differential as opposed to single ended layouts of opamps ?
Stringent symmetry.
 

Thanks erikl.

Any way you can include the schematic - if not, what architecture of opamp is this (two stage opamp?) and what do each of C1 thru to C7 do - that would help...
 

Internal schematic of 741 op amp. Screen shot of Falstad's simulator. The circuit can be found in the program's pop-down menu. I added a 220 ohm load at the output terminal.

www.falstad.com/circuit

741 internals with load (falstad).png

It contains:
* differential amplifier,
* current mirrors,
* output stage consisting of a half-bridge.

Green wires are positive polarity. Red shows negative polarity.
 

Any way you can include the schematic
I think that's still company confidential - even after 10 years time, sorry!

- if not, what architecture of opamp is this (two stage opamp?)
Right, 2-stage fully diff switched OTA.

and what do each of C1 thru to C7 do - that would help...

Top left caps C3 & C4 are compensation caps.
Top right caps C6 & C7 are serial connection caps from OTA outputs to CMFB input

Bottom C1 & C2 are the serial input caps
Bottom C3 & C4 are the feedback caps - all together they define the OTA gain (here: -1)
- they have to match very exactly

Bottom C5 & C6 are caps from the OTA inputs to vref

D are two dummies.
 

Thanks.

Are C3, C4 and C6, C7 MIM caps ?

What are the other caps, also MIM ?
 

Just rechecked with schematic: yes, all of them were MIM caps (type: cmim_sq ).
 

Hello erikl,

Thank you for your response,

Any reason one uses MIM Cap as opposed to MOS cap or MOM cap for analog / mixed signal layout for these capacitors ?

Thank you.
 

In contrast to MOS caps, MIM caps have negligible voltage dependency.

Same is valid for MOM caps, but due to their much thicker isolation layer they need a lot more real estate for the same capacitance.
 

Thank you erikl.

In your attachment (attached here again) you show the connection of the capacitor in what I assume to be a common centroid arrangement (the capacitors in the lower part of the figure).

I have seen many old layout books and papers show common centroid layouts of poly capacitors (I attached one), but you do you know of a book, paper or other resource that shows me how to do the common centroid layout of MIM capacitors ? I would like to know how to route and interconnect them properly when they are in a common centroid arrangement and cannot visualize it.

Thanks.
 

Attachments

  • sh_layout_outn_outp.jpg
    sh_layout_outn_outp.jpg
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  • Picture1.png
    Picture1.png
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... how to do the common centroid layout of MIM capacitors ?
Why should there be any difference between the common centroid arrangement of MIM caps and that of any other device which needs high symmetry layout? - There is none, I think.

I would like to know how to route and interconnect them properly when they are in a common centroid arrangement ...
It is important not to disturb the high symmetry possible with common centroid layout by different lengths (capacitances) of the connection wires.
 

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