Datasheet LTC6420-20 (Analog Devices) - 8

LTC6420-20
APPLICATIONS INFORMATION Circuit Operation Input Impedance and Matching
Each of the two channels of the LTC6420-20 is composed The differential input impedance of the LTC6420-20 is of a fully differential amplifi er with on chip feedback and 200Ω. If a 200Ω source impedance is unavailable, then output common mode voltage control circuitry. Differential the differential inputs may need to be terminated to a gain and input impedance are set by 100Ω/1000Ω lower value impedance, e.g. 50Ω, in order to provide an resistors in the feedback network. Small output resistors impedance match for the source. Several choices are avail- of 12.5Ω improve the circuit stability over various load able. One approach is to use a differential shunt resistor conditions. (Figure 1). Another approach is to employ a wide band transformer (Figure 2). Both methods provide a wide The LTC6420-20 is very fl exible in terms of I/O coupling. It band impedance match. The termination resistor or the can be AC- or DC-coupled at the inputs, the outputs or both. If transformer must be placed close to the input pins in the inputs are AC-coupled, the input common mode voltage order to minimize the refl ection due to input mismatch. is automatically biased close to VOCM and thus no external Alternatively, one could apply a narrowband impedance circuitry is needed for bias. The LTC6420-20 provides an match at the inputs of the LTC6420-20 for frequency output common mode voltage set by VOCM, which allows selection and/or noise reduction. driving an ADC directly without external components such as a transformer or AC coupling capacitors. The input signal Referring to Figure 3, LTC6420-20 can be easily confi gured can be either single-ended or differential with only minor for single-ended input and differential output without a differences in distortion performance. balun. The signal is fed to one of the inputs through a 1/2 LTC6420-20 1/2 LTC6420-20 25Ω 100Ω 1000Ω 25Ω 100Ω 1000Ω + IN + IN IN + OUT – 1:4 IN + OUT – • • VIN V + IN + 66.5Ω – – IN – OUT + IN– OUT + 25Ω 100Ω 1000Ω 25Ω 100Ω 1000Ω – IN – IN 640020 F01 640020 F02
Figure 1. Input Termination for Differential 50
Ω
Input Impedance Figure 2. Input Termination for Differential 50
Ω
Input Impedance Using Shunt Resistor Using a 1:4 Balun
1/2 LTC6420-20 RS 0.1μF 50Ω 100Ω 1000Ω + IN VIN + – RT IN+ OUT – 66.5Ω IN– OUT + RS//RT 0.1μF 28.7Ω 100Ω 1000Ω – IN 642020 F03
Figure 3. Input Termination for Single-Ended 50
Ω
Input Impedance
642020fb 8 Document Outline FEATURES DESCRIPTION APPLICATIONS TYPICAL APPLICATION ABSOLUTE MAXIMUM RATINGS PIN CONFIGURATION ORDER INFORMATION SELECTOR GUIDE DC ELECTRICAL CHARACTERISTICS AC ELECTRICAL CHARACTERISTICS TYPICAL PERFORMANCE CHARACTERISTICS PIN FUNCTIONS BLOCK DIAGRAM APPLICATIONS INFORMATION TYPICAL APPLICATIONS PACKAGE DESCRIPTION REVISION HISTORY TYPICAL APPLICATION RELATED PARTS