sx8724s Semtech Corporation, sx8724s Datasheet - Page 50

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

Revision 1.0
© Semtech
Table 35
PGA1 (with a gain of 10) and PGA2 (with a gain of 10) have each a minimum input impedance of 300 kΩ at fs = 500 kHz.
PGA3 (with a gain of 10) have a minimum input impedance of 250 kΩ 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 MΩ for PGA1.
The input impedance on channels that are not selected is very high (>10MΩ).
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.
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
ADVANCED COMMUNICATIONS & SENSING
Table 36. Typical Input Impedances
gives typical impedance values for various gain configurations.
[kHz]
fs
Z
IN
[MΩ]
62.5
125
250
500
February 2011
10.26
5.14
2.57
1.29
PGA1gain
1
2.59
1.30
0.65
0.32
10
Figure 32. The Switched Capacitor Principle
V
1
7.95
3.99
1.98
0.99
1
R
I
V
5.05
2.54
1.26
0.63
=
PGA2 gain
2
2
I
1
(
Equation 25
Equation 26
Page 50
V
=
with a capacitor switched at frequency f is:
1
V
1
−
2.84
1.44
0.71
0.36
−
R
V
5
V
V
1
2
2
)
⋅
f
[
C
A
2.24
1.11
0.56
0.28
10
ZoomingADC for sensing data acquisition
]
[
A
]
6.25
3.13
1.56
0.78
1
f
V
2
4.32
2.16
1.08
0.54
2
PGA3 gain
2.86
1.43
0.72
0.36
4
1
):
1.87
0.94
0.47
0.24
SX8724S
8
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DATASHEET
1.63
0.82
0.41
0.21
10