AD8145

Rev. A | Page 16 of 24

internal feedback loop. Figure 35 illustrates this approach.

GAIN

REF

R

R

0.01µF

0.01µF

+5V

–5V

C

The gain equation for the circuit in Figure 35 is

(3)

Another configuration that provides the same gain equation as

Equation 3 is shown in Figure 36. In this configuration, it is

than 350 Ω to avoid gain errors.

GAIN

REF

R

R

0.01µF

0.01µF

+5V

–5V

C

For stability reasons, the inductance of the trace connected to

the REF_x pin must be kept to less than 10 nH. The typical

inductance of 50 Ω traces on the outer layers of the FR-4 boards

is 7 nH/in, and on the inner layers, it is typically 9 nH/in. Vias

must be accounted for as well. The inductance of a typical via in

a 0.062-inch board is 1.5 nH. If longer traces are required, a 200 Ω

resistor should be placed in series with the trace to reduce the

Q-factor of the inductance.

ground right at the device.

TERMINATING THE INPUT

One of the key benefits of the active feedback architecture is the

separation that exists between the differential input signal and

the feedback network. Because of this separation, the differential

input maintains its high CMRR and provides high differential

and common-mode input impedances, making line termination

a simple task.

Most applications that use the AD8145 involve transmitting broad-

band video signals over 100 Ω UTP cables and use dc-coupled

terminations. The two most common types of dc-coupled

terminations are differential and common-mode. Differential

termination of 100 Ω UTP cables is implemented by simply

connecting a 100 Ω resistor across the amplifier input, as shown

in Figure 37.

GAIN

REF

R

R

0.01µF

0.01µF

+5V

–5V

C

OUT

100

Ω

UTP

100

Ω

Figure 37. Differential-Mode Termination with G = 1

Some applications require common-mode terminations for

common-mode currents generated at the transmitter. In these

cases, the 100 Ω termination resistor is split into two 50 Ω

resistors. The required common-mode termination voltage is

applied at the tap between the two resistors. In many of these

applications, the common-mode tap is connected to ground

GAIN

REF

R

R

0.01µF

0.01µF

+5V

–5V

C

OUT

100

Ω

UTP

50

Ω

50

Ω

Figure 38. Common-Mode Termination with G = 1