SX8725 - Semtech

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ADVANCED COMMUNICATIONS & SENSING SX8725 ZoomingADC for Pressure and Temperature Sensing Schematic SIGNAL MUX REF MUX Figure 18 - Recommended Operation Mode Schematic V1.8 © 2009 Semtech Corp. www.semtech.com 34
ADVANCED COMMUNICATIONS & SENSING SX8725 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 f s and to stage gain as given in Equation 21. Z in 9 768⋅10 ΩHz ≥ f ⋅ gain s 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 s = 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, Z in > 6.1MΩ. The input impedance on channels that are not selected is very high (>100MΩ). (Ω) V1.8 © 2009 Semtech Corp. www.semtech.com 35
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SX8725 - Semtech

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