參數(shù)資料
型號: XE8000EV120
廠商: Semtech
文件頁數(shù): 77/108頁
文件大?。?/td> 0K
描述: BOARD EVAL FOR SX8722I070TRLF
標(biāo)準(zhǔn)包裝: 1
系列: ZoomingADC™
ADC 的數(shù)量: 1
位數(shù): 16
數(shù)據(jù)接口: I²C
工作溫度: -40°C ~ 125°C
已用 IC / 零件: SX8722
已供物品: 板,CD
產(chǎn)品目錄頁面: 585 (CN2011-ZH PDF)
相關(guān)產(chǎn)品: SX8722I070LFDKR-ND - IC ZOOMING ADC 10BIT 16CH 44MLPQ
SX8722I070LFCT-ND - IC ZOOMING ADC 10BIT 16CH 44MLPQ
SX8722I070LFTR-ND - IC ZOOMING ADC 10BIT 16CH 44MLPQ
其它名稱: XM8000EV120
XM8000EV120-ND
ACS - Revision 4.2
October 2008
2008 Semtech Corp.
www.semtech.com
Page 70
ADVANCED COMMUNICATIONS & SENSING
DATASHEET
SX8722
High gain acquisition for sensor interface
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.
Figure 35. The Switched Capacitor Principle
A resistor is characterized by the current that flows through it (positive current leaves node V1):
One can verify that the mean current leaving node V1 with a capacitor switched at frequency f is:
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.
Figure 36. 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 show short
voltage spikes at the frequency fs, but these will not influence the measurement. Whereas if the bandwidth of the node is
(Eq. 22)
(Eq. 23)
V
1
V
2
R
V
1
f
fV
2
R
V
I
2
1
=
A
[]
() C
f
V
I
=
2
1
A
[]
Sensor
C
ZoomingADC (model)
V
1
f
fV
2
Sensor
impedence
Node
Capacitance
Current
integration
Not
Recommended
for
New
Designs
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