Texas Instruments UCD3138 Technical Reference Manual page 298

Unbuffered multiplexed input is a popular option in CMOS switched capacitor ADCs. Compared with
buffered inputs, the power consumption of unbuffered option is much lower. But there exists several
problems: First, in general, the input impedance of this kind of ADC is very high at low frequency range
and rolls off when frequency gets higher. Second, in unbuffered multiplexed ADC, the charge injection
from the internal sampling capacitors and network reflects a small amount of signal, which is packed with
high frequency content back onto the front-end circuitry and incoming signal. This may cause settling
errors for the elements connected to the analog inputs of the converter. Another problem is that all the
sampling channels use the same S/H capacitor in a sequence, if the sampling speed is very high, the
charge remain on the S/H capacitor of one conversion may affect the result of the next conversion
channel.
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Topic
8.1 ADC12 Input Impedance Model
8.2 ADC12 Impedance vs. Sampling Frequency Data
8.3 Effect of External Capacitance
8.4 Channel to Channel Crosstalk
8.5 Impedance Roll-Off Due to Crosstalk
8.6 ADC12 Control FSM
8.7 Conversion
8.8 Sequencing
8.9 Digital Comparators
8.10 ADC Averaging
8.11 Temperature Sensor
8.12 Temp Sensor Control Register (TEMPSENCTRL)
8.13 PMBus Addressing
8.14 Dual Sample and Hold
8.15 Usage of Sample and Hold Circuitry for High Impedance Measurement
8.16 ADC Configuration Examples
8.17 Useful C Language Statement Examples
8.18 ADC Registers
298
ADC12 Overview
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Copyright © 2016, Texas Instruments Incorporated
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SNIU028A - February 2016 - Revised April 2016
www.ti.com
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