sx8725 Semtech Corporation, sx8725 Datasheet - Page 36

• Manufacturer Part Number: sx8725
• Description: Sx8725 Zoomingadc? For Pressure And Temperature Sensing
• Manufacturer: Semtech Corporation

ADVANCED COMMUNICATIONS & SENSING
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
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 f
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 f
if the bandwidth of the node is lower, no spikes will arise, but a small offset can be generated by the integration
of the charges generated by the switched capacitors, this corresponds to the mean impedance effect.
Note:
One can increase the mean input impedance of the ZoomingADC by lowering the acquisition clock f
One can increase the mean input impedance of the ZoomingADC by decreasing the gain of the first enabled amplifier.
One can increase the effective input impedance of the ZoomingADC by having a source with a high electrical bandwidth (sensor electrical
bandwidth much higher than f
V1.5 © 2007 Semtech Corp.
s
clock is reduced, the mean impedance is increased. By dividing the f
s
).
Figure 19 - The Switched Capacitor Principle
Figure 20 - The Switched Capacitor Principle
Sensor
V
1
Capacitance
impedence
ZoomingADC™ for Pressure and Temperature Sensing
R
Sensor
I
Node
V
I
2
integration
V
Current
Equation 22
Equation 23
V
1
1
1
f
with a capacitor switched at frequency f is:
1
36
R
V
s
C
, but these will not influence the measurement. Whereas
V
V
2
1
2
V
is equivalent to a resistor.
1
ZoomingADC (model)
f
f
f
(A)
C
(A)
C
f
f
V
2
V
2
s
.
s
clock by a factor 10, the
1
):
www.semtech.com
SX8725