Hi everyone!

Im currently working on an analogue assignment where you take a square wave and convert it into a sine wave. So the method that is suggested and looks nice is to use a sallen-key low pass amplifier.

Now I have it done....if the world was ideal. My circuit works fine if I use resistors but because the second half of the assignemnt is to intergrate this thing into VLSI a 10Mega ohm resistor is not a good idea, unless you use a switched capacitor.

So I have designed and done the math!
R = 1/fC
R = 10 Meg
C = 10fF
Therefore f should be 10Mhz. Thats fine, I made a ring oscillator to do this. So I wire it all up (in S-Edit) and it just goes

It looks like that it has a considerbly lower resistance that it should. I checked the ring oscillator output, its fine. I replaced it with a function generator, nope, still same problem. If I remove the switch capacitors with resistors it works no question. What the darn is going on?

The program uses T-Spice to run (its the same as regular SPICE except it has thyme in it as well).

Here is the filter

Here is the switched capacitor

 

Btw i changed it to 6pf just to see the difference, still no working!

 

What are the MOSFET stray capacitances?

 

The only capacitances im aware of is the gate capacitance. Which should be around 1fF for an NMOSFET and 2fF for the P-type

 

The drain and source have some capacitance to the substrate.

Can you try it at a lower switching frequency so see if it works proportionally there?

 

I think I have an idea on what the problem is.
Assuming that SPICE is taking into account the stray capacitances, the sizes of this capacitance could be comparible to the capacitance of the switched capacitor. I will try a switched capacitor of 200fF and see if it follows the standard equation.

 

Figured it out >:|

Example:
Wanted R = 2Mohm
Used C = 500fF @ 1Mhz

Results:
R Actual = 1.63Mohm
Calculated C = 611fF
Stray C = 111fF

Now it works fine
I have done other tests taking the 111fF into account and it works fine ^_^

Thank you so much crutschow for your help, i would have never though about stray capacitance!

 

The upper frequency limit of digital ICs is generally determined by the circuit stray capacitances.

 

The upper frequency limit of digital ICs is generally determined by the circuit stray capacitances.

Learn something new everyday