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.

GAIN

REF

R

R

0.01µF

0.01µF

+5V

–5V

C

OUT

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

OUT

Figure 43. Driving a Doubly Terminated Cable with G = 2