sx8724s Semtech Corporation, sx8724s Datasheet - Page 51

• Manufacturer Part Number: sx8724s
• Description: Zoomingadc For Sensing Data Acquisition
• Manufacturer: Semtech Corporation

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© Semtech
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 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.
Notes:
(1)
(2)
(3)
ADVANCED COMMUNICATIONS & SENSING
One can increase the mean input impedance of the ZoomingADC by lowering the acquisition clock fs.
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 electri-
cal bandwidth much higher than fs).
February 2011
Sensor
Figure 33. The Switched Capacitor Principle
Capacitance
impedence
Sensor
Node
integration
Current
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V
1
ZoomingADC (model)
f
ZoomingADC for sensing data acquisition
C
f
V
2
SX8724S
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