Datasheet ADA4312-1 (Analog Devices) - 9

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Data Sheet ADA4312-1 APPLICATIONS INFORMATION VCC 0.1µF 10µF CIN + RBT ADA4312-1 VCC – RF 1:1 10k R Ω BIAS RG RL 0.1µF 10k R Ω BIAS RF – ADA4312-1 RBT + C RIADJ IN
017 11044- Figure 17. Typical G.hn Application Circuit
FEEDBACK RESISTOR SELECTION HALF-DUPLEX OPERATION
The feedback resistor value has a direct impact on the closed- In systems such as G.hn PLC modems, half-duplex or time- loop bandwidth of the current feedback amplifiers used in the division duplex (TDD) systems require the line driver to be architecture of the ADA4312-1 differential line driver. Table 5 switched between transmit mode and high output impedance provides a guideline for the selection of feedback resistor values receive mode. The ADA4312-1 is equipped with a shutdown pin used in typical differential line driver circuits (refer to Figure 17). (SD, Pin 9) that stops the line driver from transmitting while switching the outputs to a high output impedance equivalent to
Table 5. Resistor Values and Frequency Performance
10 kΩ in paral el with 2RF + RG (see Figure 17). The shutdown
Gain R (Ω) R (Ω) −3 dB SS BW (MHz) F G
(SD) pin is compatible with standard 3.3 V CMOS logic. If the 16 V/V 732 97.6 195 SD pin is left floating, an internal pull-up resistor places the 12 V/V 750 137 200 output in a disabled, high output impedance state. SD logic is 8 V/V 768 221 209 referred to GND (Pin 4), which should be connected to 0 V. 4 V/V 806 536 222
ESTABLISHING VMID
Selecting a feedback resistor with a value that is lower than the In single-supply applications such as the one shown in values in Table 5 can create peaking in the frequency response; Figure 17, it is necessary to establish a midsupply operating in extreme cases, this peaking can lead to instability. Conversely, point (V a feedback resistor that exceeds the values in Table 5 can limit MID). To establish VMID, use two 10 kΩ resistors to form a resistor divider from V the closed-loop bandwidth. CC to ground and a 0.1 µF ceramic chip capacitor for decoupling. Place the VMID decoupling capacitor
GENERAL OPERATION
and the RBIAS resistors as close as possible to the ADA4312-1. The ADA4312-1 is a differential line driver designed for single-
BIAS CONTROL AND LINEARITY
supply operation in G.hn line driver applications. The core The ADA4312-1 is equipped with a biasing adjustment feature architecture comprises two high speed current feedback amplifiers. that lowers the quiescent operating current. A resistor (R The inputs of these amplifiers are arranged in a unique way that IADJ) must be placed between I facilitates extended differential bandwidth, linearity, and stability ADJ (Pin 5) and GND (Pin 4) for proper operation of the ADA4312-1. Using a resistor larger than 0 Ω while limiting common-mode bandwidth and enhancing reduces the quiescent current of the line driver and improves common-mode stability. efficiency in transmit mode. Figure 13 shows the quiescent The patented input stage of the core amplifiers is not conducive current vs. RIADJ. to operating either core amplifier independently. The ADA4312-1 input stage is designed to operate only in differential applications similar to the circuit shown in Figure 17. Rev. A | Page 9 of 12 Document Outline Features Applications General Description Functional Block Diagram Typical Application Circuit Revision History Specifications Absolute Maximum Ratings Thermal Resistance Maximum Power Dissipation ESD Caution Pin Configuration and Function Descriptions Typical Performance Characteristics Test Circuit Applications Information Feedback Resistor Selection General Operation Half-Duplex Operation Establishing VMID Bias Control and Linearity PCB Layout Thermal Management Power Supply Bypassing Evaluation Board Outline Dimensions Ordering Guide