[SOLVED] Switched capacitance integrator

[JanniS]

Hello

I am studying integrators and just found the switched capacitor integrator. I saw, that the switched capacitor, C1, acts as a resistor with the value of 1/fC1.
The time constant is then fC1/C2, where C2 is the feedback capacitor.

Just to be sure: The time constant changes as a function of input frequency and this topology will not be suited for unknown input signals, since the output changes with the input?
If the above is correct, when is it the preferrable to use a switched capacitor integrator instead of for instace a RC integrator followed by S/H?

Best regards
janni

 

[LvW]

Hello
I am studying integrators and just found the switched capacitor integrator. I saw, that the switched capacitor, C1, acts as a resistor with the value of 1/fC1.
The time constant is then fC1/C2, where C2 is the feedback capacitor.
i

No - that´s not correct.
The frequency f is NOT the signal frequency but - of course - the CLOCK frequency Fcl.
The capacitor is switched according to this clock Fc.
Hence we have the equivalence R=1/Fcl*C1.

 

[JanniS]

Ah ok.
So why would one choose to use the SC integrator and not an RC integrator?

Janni

 

[LvW]

Ah ok.
So why would one choose to use the SC integrator and not an RC integrator?

The most important reason is: SC circuits can be realized - with good accuracy because time constants are defined by capacitor RATIOS only - as fully integrated circuits.
Moreover, varying the clock frequency changes the time constants and, thus, the frequency behaviour of the whole circuit.
Hence, SC filters for higher order are on the market, which are externally tunable.

 

[JanniS]

Okay.
Thank you very much