參數(shù)資料
型號: LTC6421IUDC-20#PBF
廠商: Linear Technology
文件頁數(shù): 15/16頁
文件大?。?/td> 0K
描述: IC ADC DRIVER DIFF 20-QFN
標(biāo)準(zhǔn)包裝: 91
類型: ADC 驅(qū)動器
應(yīng)用: 數(shù)據(jù)采集
安裝類型: 表面貼裝
封裝/外殼: 20-WFQFN 裸露焊盤
供應(yīng)商設(shè)備封裝: 20-QFN(3x4)
包裝: 管件
產(chǎn)品目錄頁面: 1323 (CN2011-ZH PDF)
LTC6421-20
8
642120fb
Circuit Operation
Each of the two channels of the LTC6421-20 is composed
of a fully differential amplier with on chip feedback and
output common mode voltage control circuitry. Differential
gain and input impedance are set by 100Ω/1000Ω
resistors in the feedback network. Small output resistors
of 12.5Ω improve the circuit stability over various load
conditions.
The LTC6421-20 is very exible in terms of I/O coupling.
It can be AC- or DC-coupled at the inputs, the outputs or
both. If the inputs are AC-coupled, the input common mode
voltage is automatically biased close to VOCM and thus
no external circuitry is needed for bias. The LTC6421-20
provides an output common mode voltage set by VOCM,
which allows driving an ADC directly without external
componentssuchasatransformerorACcouplingcapacitors.
The input signal can be either single-ended or differential
with only minor differences in distortion performance.
Figure 1. Input Termination for Differential 50Ω Input Impedance
Using Shunt Resistor
Figure 2. Input Termination for Differential 50Ω Input Impedance
Using a 1:4 Balun
APPLICATIONS INFORMATION
642120 F01
+IN
IN+
OUT
IN
OUT+
–IN
100Ω
66.5Ω
1000Ω
1/2 LTC6421-20
100Ω
25Ω
VIN
1000Ω
+
642120 F02
+IN
IN+
OUT
IN
OUT+
–IN
100Ω
1000Ω
1/2 LTC6421-20
100Ω
25Ω
TCM4-19
25Ω
VIN
1000Ω
+
1:4
Input Impedance and Matching
The differential input impedance of the LTC6421-20 is
200Ω. If a 200Ω source impedance is unavailable, then
the differential inputs may need to be terminated to a
lower value impedance, e.g. 50Ω, in order to provide
an impedance match for the source. Several choices
are available. One approach is to use a differential shunt
resistor (Figure 1). Another approach is to employ a wide
band transformer (Figure 2). Both methods provide a
wide band impedance match. The termination resistor or
the transformer must be placed close to the input pins in
order to minimize the reection due to input mismatch.
Alternatively, one could apply a narrowband impedance
match at the inputs of the LTC6421-20 for frequency
selection and/or noise reduction.
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