SX8722I070LF SEMTECH [Semtech Corporation], SX8722I070LF Datasheet - Page 70

• Manufacturer Part Number: SX8722I070LF
• Description: High gain acquisition for sensor interface
• Manufacturer: SEMTECH [Semtech Corporation]

12.11. 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 show short
voltage spikes at the frequency fs, but these will not influence the measurement. Whereas if the bandwidth of the node is
ACS - Revision 4.2
©2008 Semtech Corp.
ADVANCED COMMUNICATIONS & SENSING
I
=
I
(
V
(Eq. 22)
(Eq. 23)
=
1
V
−
1
V
−
R
2
V
)
2
⋅
October 2008
f
⋅
C
Figure 35. The Switched Capacitor Principle
Figure 36. The Switched Capacitor Principle
Sensor
V
1
Capacitance
impedence
R
Sensor
Node
V
1
2
integration
with a capacitor switched at frequency f is:
Current
A [ ]
A [ ]
Page 70
V
1
V
1
ZoomingADC (model)
f
f
High gain acquisition for sensor interface
C
f
f
V
2
V
2
1
):
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