www.semtech.com/products/ Page 47 ADVANCED COMMUNICATIONS & SENSING DATASHEET SX8725C ZoomingADC fo" />
參數資料
型號: SX8725CWLTDT
廠商: Semtech
文件頁數: 42/68頁
文件大小: 0K
描述: IC DAS ADC 16BIT I2C/SRL
標準包裝: 1
系列: ZoomingADC™
類型: 數據采集系統(DAS)
輸入類型: 差分
輸出類型: 數字
接口: I²C
安裝類型: 表面貼裝
封裝/外殼: 12-WDFN 裸露焊盤
供應商設備封裝: 12-MLPD-EP(4x4)
包裝: 標準包裝
其它名稱: SX8725CWLTDTDKR
www.semtech.com/products/
Page 47
ADVANCED COMMUNICATIONS & SENSING
DATASHEET
SX8725C
ZoomingADC for sensing data acquisition
Revision 1.01
January 2011
Semtech
11 Typical Performances
11.1 Input impedance
The PGAs of the ZoomingADC are a switched capacitor based blocks (see Switched Capacitor Principle section). This
means that it does not use resistors to fix gains, but capacitors and switches. This has important implications on the
nature of the input impedance of the block.
Using switched capacitors is the reason why, while a conversion is done, the input impedance on the selected channel
of the PGAs is inversely proportional to the sampling frequency fs and to stage gain as given in Equation 24.
The input impedance observed is the input impedance of the first PGA stage that is enabled or the input impedance of
the ADC if all three stages are disabled.
Cg multiplied by gain is the equivalent gain capacitor and Cp is the parasitic capacitor of the first enabled stage. The
values for each ZoomingADC bloc are provided in Table 37:
PGA1 (with a gain of 10) and PGA2 (with a gain of 10) have each a minimum input impedance of 300 kOhm at fs = 500
kHz. PGA3 (with a gain of 10) have a minimum input impedance of 250 kOhm at fs = 500 kHz. Larger input impedance
can be obtained by reducing the gain and/or by reducing the over-sampling frequency fs. Therefore, with a gain of 1
and a sampling frequency of 62.5 kHz, Zin > 10.2 MOhm for PGA1.
The input impedance on channels that are not selected is very high (>10MOhm).
Note
The graphs and tables provided following this note are statistical summary
based on limited number of samples and are provided for informational
purposes only. The performance characteristics listed herein are not tested or
guaranteed. In some graphs or tables, the data presented may be outside the
specified operating range and therefore outside the warranted range.
Equation 24
Table 37.
Capacitor values
Acquisition Chain Stage
Gain capacitor Cg
Parasitic capacitor Cp
Units
PGA1
0.45
1.04
pF
PGA2
0.54
1.2
pF
PGA3
0.735
1.53
pF
ADC
2.4
pF
()
]
[
1
Ω
+
Cp
gain
Cg
f
Z
s
in
Not
Recommended
for
New
Designs
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