Switch capacitor = low pass filter ?

[Chengjun Li]

I understand that a switched-capacitor can be analog to a resistor with value R = 1/C*f, but I really don't see how can a switched-capacitor work as a low pass filter, could anyone give some explanation?

 

[chinathermostats01]

I understand that a switched-capacitor can be analog to a resistor with value R = 1/C*f, but I really don't see how can a switched-capacitor work as a low pass filter, could anyone give some explanation?

 

[Colin Mitchell]

I don't understand the question, but a filter is a circuit that only allows a certain frequency or a range of frequencies to emerge from the circuit.
A LOW PASS filter only allows the LOW FREQUENCIES to emerge.
The simplest filter consists of a capacitor and a resistor.
We know that if we place a capacitor with one lead on the 0v rail and connect a voltage to the top lead, the capacitor will take time to charge and discharge.
The capacitor is just like a battery and we are trying charge it and discharge it. This takes TIME.
It is a slow process.
So, if we connect a signal to the top of the capacitor, the low frequency signals will charge and discharge the capacitor and we will see the result by the voltage rising and falling.
But if we connect a higher frequency to the top of the capacitor, the signal will rise too fast and fall too fast and the capacitor will not be able to follow.
It's like expecting your flat car battery to get fully charged in 10 seconds. It is not going to happen.
We can now add a resistor as shown in the diagram and this resistor does not change the concept of what we are saying. It is just designed to prevent damage to the incoming signal.
The capacitor in the diagram will allow low frequency signals to charge and discharge the capacitor and thus the signals will emerge from the filter.But the high frequency signals will simply be lost in the process of trying to charge and discharge the capacitor AT THE WRONG TIMES.

I can't make it much simpler.

 

[Harald Kapp]

The switched capacitor alone doesn't work as a filter. The switched capacitor replaces the resistor in an RC filter. You therefore need at least 2 capacitors, one switched, one unswitched.
Here's an application note that expalins this in detail.

 

[LvW]

In addition to Haralds answer I like to point out that a switched capacitor can mimic an ohmic resistor (within certain frequency limits, of course) only if it is operated BETWEEN two fixed voltages (one of which my be ground or opamp output).
In many documents this fact is not mentioned.
Another capacitor (as mentioned by Harald) can do this job only if it is much larger than the switched caapacitor.
For this reason, it is common not to realize a first order lowpass by replacing the R in a passive RC lowpass but to use an active circuit as shown in the LTC application note referenced by Harald Kapp.

 

[Chengjun Li]

In addition to Haralds answer I like to point out that a switched capacitor can mimic an ohmic resistor (within certain frequency limits, of course) only if it is operated BETWEEN two fixed voltages (one of which my be ground or opamp output).
In many documents this fact is not mentioned.
Another capacitor (as mentioned by Harald) can do this job only if it is much larger than the switched caapacitor.
For this reason, it is common not to realize a first order lowpass by replacing the R in a passive RC lowpass but to use an active circuit as shown in the LTC application note referenced by Harald Kapp.

Thanks for your reply.

I brought up this question because a manual I am reading said"The circuit is a one pole, 32KHz, switched-capacitor LPF that functions as a sample/hold circuit." But it didn't show the circuit diagram in the manual.

Now I think the switched capacitor LPF should be as you said, like

I still have one further question, I wonder how could this circuit function as a sample/hold circuit? The charge accumulated on C1 while S1 is closed will flow away when S2 is closed, so how could it achieve the "hold" function?