sx8725 Semtech Corporation, sx8725 Datasheet - Page 35

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

ADVANCED COMMUNICATIONS & SENSING
Input Impedance
The PGAs of the ZoomingADC are a switched capacitor based blocks (see Switched Capacitor Principle
chapter). 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 f
Equation 21.
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.
PGA1 (with a gain of 10), PGA2 (with a gain of 10) and PGA3 (with a gain of 10) each have a minimum input
impedance of 150 kΩ at f
obtained by reducing the gain and/or by reducing the sampling frequency. Therefore, with a gain of 1 and a
sampling frequency of 125 kHz, Z
The input impedance on channels that are not selected is very high (>100MΩ).
V1.5 © 2007 Semtech Corp.
s
= 500 kHz (see ZoomingADC Specifications). Larger input impedance can be
in
> 6.1MΩ.
ZoomingADC™ for Pressure and Temperature Sensing
Z
in
768
Equation 21
f
s
10
35
gain
9
Hz
(Ω)
s
and to stage gain as given in
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SX8725