sx8725c Semtech Corporation, sx8725c Datasheet - Page 48

• Manufacturer Part Number: sx8725c
• Description: Zoomingadc For Sensing Data Acquisition
• Manufacturer: Semtech Corporation

Revision 1.01
© Semtech
11.1.1 Switched Capacitor Principle
Basically, a switched capacitor is a way to emulate a resistor by using a capacitor. The capacitors are much easier to
realize on CMOS technologies and they show a very good matching precision.
A resistor is characterized by the current that flows through it (positive current leaves node V
One can verify that the mean current leaving node V
Therefore as a mean value, the switched capacitor 1/(f x C) is equivalent to a resistor.
It is important to consider that this is only a mean value. If the current is not integrated (low impedance source), the
impedance is infinite during the whole time but the transition.
What does it mean for the ZoomingADC?
If the fs clock is reduced, the mean impedance is increased. By dividing the fs clock by a factor 10, the impedance is
increased by a factor 10.
One can reduce the capacitor that is switched by using an amplifier set to its minimal gain. In particular if PGA1 is used
with gain 1, its mean impedance is 10x bigger than when it is used with gain 10.
One can increase the effective impedance by increasing the electrical bandwidth of the sensor node so that the
switching current is absorbed through the sensor before the switching period is over. Measuring the sensor node will
ADVANCED COMMUNICATIONS & SENSING
January 2011
Sensor
Figure 26. The Switched Capacitor Principle
Figure 27. The Switched Capacitor Principle
V
1
Capacitance
impedence
R
Sensor
I
Node
=
V
2
(
integration
I
V
1
Current
Equation 25
Equation 26
Page 48
=
1
with a capacitor switched at frequency f is:
−
V
1
V
−
R
2
V
V
1
)
V
2
⋅
1
ZoomingADC (model)
f
f
[
⋅
f
A
C
ZoomingADC for sensing data acquisition
]
[
C
A
]
f
f
V
2
V
2
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