Datasheet LTC6421-20 (Analog Devices) - 8
| Manufacturer | Analog Devices |
| Description | Dual Matched 1.3GHz Differential Amplifiers/ADC Drivers |
| Pages / Page | 16 / 8 — APPLICATIONS INFORMATION. Circuit Operation. Input Impedance and … |
| File Format / Size | PDF / 251 Kb |
| Document Language | English |
APPLICATIONS INFORMATION. Circuit Operation. Input Impedance and Matching. Figure 1. Input Termination for Differential 50
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LTC6421-20
APPLICATIONS INFORMATION Circuit Operation Input Impedance and Matching
Each of the two channels of the LTC6421-20 is composed The differential input impedance of the LTC6421-20 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 resistors in the feedback network. Small output resistors an impedance match for the source. Several choices of 12.5Ω improve the circuit stability over various load are available. One approach is to use a differential shunt conditions. resistor (Figure 1). Another approach is to employ a wide The LTC6421-20 is very fl exible in terms of I/O coupling. band transformer (Figure 2). Both methods provide a It can be AC- or DC-coupled at the inputs, the outputs or wide band impedance match. The termination resistor or both. If the inputs are AC-coupled, the input common mode the transformer must be placed close to the input pins in voltage is automatically biased close to V order to minimize the refl ection due to input mismatch. OCM and thus no external circuitry is needed for bias. The LTC6421-20 Alternatively, one could apply a narrowband impedance provides an output common mode voltage set by V match at the inputs of the LTC6421-20 for frequency OCM , which allows driving an ADC directly without external selection and/or noise reduction. components such as a transformer or AC coupling capacitors. The input signal can be either single-ended or differential with only minor differences in distortion performance. 1/2 LTC6421-20 1/2 LTC6421-20 25Ω 100Ω 1000Ω 25Ω 100Ω 1000Ω +IN +IN IN+ OUT – 1:4 IN+ OUT – V • • IN V + IN 66.5Ω – + – TCM4-19 IN– OUT+ IN– OUT+ 25Ω 100Ω 1000Ω 25Ω 100Ω 1000Ω –IN –IN 642120 F01 642120 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
642120fb 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
APPLICATIONS INFORMATION Circuit Operation Input Impedance and Matching
Each of the two channels of the LTC6421-20 is composed The differential input impedance of the LTC6421-20 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 resistors in the feedback network. Small output resistors an impedance match for the source. Several choices of 12.5Ω improve the circuit stability over various load are available. One approach is to use a differential shunt conditions. resistor (Figure 1). Another approach is to employ a wide The LTC6421-20 is very fl exible in terms of I/O coupling. band transformer (Figure 2). Both methods provide a It can be AC- or DC-coupled at the inputs, the outputs or wide band impedance match. The termination resistor or both. If the inputs are AC-coupled, the input common mode the transformer must be placed close to the input pins in voltage is automatically biased close to V order to minimize the refl ection due to input mismatch. OCM and thus no external circuitry is needed for bias. The LTC6421-20 Alternatively, one could apply a narrowband impedance provides an output common mode voltage set by V match at the inputs of the LTC6421-20 for frequency OCM , which allows driving an ADC directly without external selection and/or noise reduction. components such as a transformer or AC coupling capacitors. The input signal can be either single-ended or differential with only minor differences in distortion performance. 1/2 LTC6421-20 1/2 LTC6421-20 25Ω 100Ω 1000Ω 25Ω 100Ω 1000Ω +IN +IN IN+ OUT – 1:4 IN+ OUT – V • • IN V + IN 66.5Ω – + – TCM4-19 IN– OUT+ IN– OUT+ 25Ω 100Ω 1000Ω 25Ω 100Ω 1000Ω –IN –IN 642120 F01 642120 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
642120fb 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
