How to make a 5M resistor on chip?

[gsheng]

I want to use such a large resistor for ac coupling common mode clamp. Any idea is welcome. Thanks.

 

[strennor]

some process has resistor of sheet resistance about 3k

why you need such big resistor?

 

[gsheng]

Because I want to ac coupling a high speed signal, the cap must be small(around tens fF). Just a trade off between common mode settling time and input signal speed.

 

[pfd001]

It is not easy for you to get this resistor. For your spec, the parasitic capacitance of the resistor must be small. If not, a transistor resistor can be used

 

[gsheng]

The resistor's parasitic cap need not very small because the signal is very large. I also want cap divider.

 

[diemilio]

For such high resistive values you should use switched capacitors!!!

Check section 9.1 of "CMOS Analog Circuit Design" by Allen and Holberg. There you can find an example of how to simulate a 1 MOhm resistor using these kind of circuits.

I hope this helps,

diemilio

 

[gsheng]

But this need accurate clock.

 

[pfd001]

The resistor's parasitic cap need not very small because the signal is very large. I also want cap divider.

A long channel transistor can be used for 5M res

 

[gsheng]

The long channel mos shows different resistor value under different bias conditions. Its value is signal dependant.

 

[melc]

I don't know how can be done in a chip (I like just to use chips not to manufacture them) but two small signal diodes connected in opposite series have around 5 to 20M...

 

[xwcwc1234]

May be can use N-well resistor for 5M for above .

 

[paley]

I also don't kown how to create a high impedance, low parasitic capacitance ,accurate and signal independence RES

 

[gamma1]

Read you technology handbook, you will find what kind of material is good for your design, eg, poly, substrate, etc ..

 

[leohart]

Hey,5m is too big for normal poly/diffusion res,it will cost u far too much silicon...But as mos res is signal dependent and apt to parasitics,you can only the above res...
What is your application?maybe you can ignore the signal dependent things if the signal is small enough