SX8724C Datasheet, PDF (48/67 Pages) Semtech Corporation – ZoomingADC for sensing data acquisition

SX8724C
ZoomingADC for sensing data acquisition
ADVANCED COMMUNICATIONS & SENSING
DATASHEET
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.
V₁
V₂
V₁ f
f V₂
R
Figure 26. The Switched Capacitor Principle
A resistor is characterized by the current that flows through it (positive current leaves node V1):
I = ^{V1 −V2} [A]
R
Equation 25
One can verify that the mean current leaving node V1 with a capacitor switched at frequency f is:
I = (V1−V 2)⋅ f ⋅ C [A]
Equation 26
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.
Current
integration
Sensor
impedence
ZoomingADC (model)
V₁ f
f V₂
Sensor
Node
C
Capacitance
Figure 27. The Switched Capacitor Principle
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
Revision 1.01
© Semtech
January 2011
Page 48
www.semtech.com/products/