
ADVANCED COMMUNICATIONS & SENSING
V1.8 2009 Semtech Corp.
www.semtech.com
35
SX8723
ZoomingADC for Pressure and Temperature 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 fs and to stage gain as given in
Equation 21.
gain
f
Hz
Z
s
in
≥
9
10
768
(Ω)
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 fs = 500 kHz (see ZoomingADC Specifications). Larger input impedance can be
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, Zin > 6.1MΩ.
The input impedance on channels that are not selected is very high (>100MΩ).