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AD8145 Datasheet(PDF) 19 Page - Analog Devices |
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AD8145 Datasheet(HTML) 19 Page - Analog Devices |
19 / 24 page ![]() AD8145 Rev. 0 | Page 19 of 24 DRIVING A CAPACITIVE LOAD The AD8145 typically drives either high impedance loads over short PCB traces, such as crosspoint switch inputs, or doubly terminated coaxial cables. A gain of 1 is commonly used in the high impedance case since the 6 dB transmission line termination loss is not incurred. A gain of 2 is required when driving cables to compensate for the 6 dB termination loss. In all cases, the output must drive the parasitic capacitance of the feedback loop, conservatively estimated to be 1 pF, in addition to the capacitance presented by the actual load. When driving a high impedance input, it is recommended that a small series resistor be used to buffer the input capacitance of the device being driven. Clearly, the resistor value must be small enough to preserve the required bandwidth. In the ideal doubly terminated cable case, the AD8145 output sees a purely resistive load. In reality, there is some residual capacitance, and this is buffered by the series termination resistor. Figure 42 illustrates the high impedance case, and Figure 43 illustrates the cable- driving case. POWER-DOWN The power-down feature is intended to be used to reduce power consumption when a particular device is not in use, and does not place the output in a high-Z state when asserted. The power-down feature is asserted when the voltage applied to the power-down pin drops to approximately 2 V below the positive supply. The AD8145 is enabled by pulling the power-down pin to the positive supply. RS CIN GAIN REF R R 0.01µF 0.01µF +5V –5V C OUT VIN VREF Figure 42. Buffering the Input Capacitance of a High-Z Load with G = 1 GAIN REF R R 0.01µF 0.01µF +5V –5V C VREF VIN RS CS RL OUT Figure 43. Driving a Doubly Terminated Cable with G = 2 |
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