LT6236 datasheet(18/50 Pages) LINER

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LTC2372-16

237216f

For more information www.linear.com/LTC2372-16

applicaTions inForMaTion

Figure 4. Equivalent Circuit for the Differential Analog Inputs of the LTC2372-16

ANALOG INPUTS

The LTC2372-16 can be configured to accept one of

three voltage ranges: fully differential (±4.096V), pseudo-

differentialunipolar(0Vto4.096V),andpseudo-differential

bipolar(±2.048V).Inallthreeranges,theADCsamplesand

digitizes the voltage difference between the two ADC core

analog input pins (ADCIN+ − ADCIN−), and any unwanted

signal that is common to both inputs is reduced by the

common mode rejection ratio (CMRR) of the ADC. The

MUXoutputsthevoltagesoftheselectedMUXanaloginput

channels to MUXOUT+/–, according to the MUX configu-

ration. MUXOUT+/– may be wired directly to ADCIN+/– or

connected through a buffer. Refer to the Configuring the

LTC2372-16sectionfordetailsonhowtoselecttheanalog

input range and MUX channel configuration.

Independentoftheselectedrangeorchannelconfiguration,

the MUX analog inputs can be modeled by the equivalent

circuit shown in Figure 4. CHx and CHy are distinct input

pins selected from the CH0 to CH7 MUX analog inputs,

depending on the MUX configuration. Each pin has ESD

protection diodes. The ADC core analog inputs, ADCIN+/–,

each see a sampling network consisting of approximately

50pF (CIN) from the sampling CDAC in series with 40Ω

(RON) from the on-resistance of the sampling switch.

The MUX is modeled by a 40Ω resistor representing the

MUX switch on-resistance (RSW) and a capacitance to

ground, CPAR, at the output summing node of the MUX.

CPAR is a lumped capacitance on the order of 20pF formed

primarily by pin parasitics and diode junctions. Parasitic

capacitances from the PCB will also contribute to CPAR.

This capacitance is discharged through a switch to ground

every conversion cycle or when a first new configuration is

programmed to minimize crosstalk due to charge sharing

between channels.

During acquisition, each active MUX analog input sees a

cascade of two first order lowpass filters formed by RSW,

CPAR and the ADC sampling network when MUXOUT+/– is

wired directly to ADCIN+/–. If a buffer is inserted between

MUXOUT+/– and ADCIN+/–, then each active MUX analog

input only sees a first order lowpass filter formed by RSW

and CPAR that is loaded with the input impedance of the

buffer.

BothCINandCPARdrawcurrentspikeswhilebeingcharged

during acquisition. If MUXOUT+/– is wired directly to

ADCIN+/–, the current spikes from the charging of both

capacitors are drawn from the active MUX analog inputs.

A buffer inserted between MUXOUT+/– and ADCIN+/– will

absorbthecurrentspikefromCIN,leavingthecurrentspike

from CPAR to be drawn from the active MUX analog inputs.

During conversion and sleep, the MUX analog inputs and

ADC core analog inputs draw only a small leakage current.

237216 F04

VDD

RON

CIN

50pF

ADCIN+

VDD

RON

CIN

50pF

BIAS

VOLTAGE

ADCIN–

MUXOUT+

MUXOUT–

MUX

ADC CORE

VDD

RSW

CPAR

20pF

CHX

VDD

RSW

CHY, COM

CPAR

20pF

VDD

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