AD8145

Rev. A | Page 15 of 24

APPLICATIONS INFORMATION

OVERVIEW

The AD8145 contains three independent active feedback amplifiers

that can be effectively applied as differential line receivers for

red-green-blue (RGB) signals or component video signals, such

as YPbPr, transmitted over UTP cable. The AD8145 also contains

two general-purpose comparators with hysteresis that can be

used to receive digital signals or to extract video synchronization

pulses from received common-mode signals that contain

encoded synchronization signals.

The comparators, which receive power from the positive supply, are

referenced to GND and require greater than 4.5 V on the positive

supply for proper operation. If the comparators are not used,

then a split ±2.5 V can be used with the amplifiers operating

normally.

The AD8145 includes a power-down feature that can be asserted to

reduce the supply current when a particular device is not in use.

BASIC CLOSED-LOOP GAIN CONFIGURATIONS

Each amplifier in the AD8145 comprises two transconductance

amplifiers—one for the input signal and one for negative feedback.

Note that the closed-loop gain of the amplifier used in the signal

path is defined as the single-ended output voltage of the amplifier

divided by its differential input voltage. Therefore, each amplifier

in the AD8145 provides differential-to-single-ended gain.

Additionally, the amplifier used for feedback has two high

impedance inputs—the feedback input, where the negative

feedback is applied, and the REF input, that can be used as

an independent single-ended input to apply a dc offset to the

output signal.

The AD8145 contains on-chip feedback networks between each

amplifier output and its respective feedback input. The closed-

loop gain of the amplifier is set to 1 by connecting the amplifier

output directly to its respective GAIN_x pin. Doing this places

the on-chip resistors and capacitor in parallel across the amplifier

output and feedback pin. The small feedback capacitor mitigates

the effects of the summing-node capacitance, which is most

problematic in the unity gain case. Closed-loop gain of an

amplifier is set to 2 by connecting the respective GAIN_x pin

to a reference voltage, often directly to ground. In Figure 1,

R = 350 Ω and C = 2 pF.

Some basic gain configurations implemented with an AD8145

amplifier are shown in Figure 33 through Figure 36.

GAIN

REF

R

R

0.01µF

0.01µF

+5V

–5V

C

OUT

The gain equation for the circuit in Figure 33 is

(1)

In this configuration, the voltage applied to the REF pin appears

at the output with a gain of 1.

Figure 34 illustrates one way to operate an AD8145 amplifier

with a gain of 2.

GAIN

REF

R

R

0.01µF

0.01µF

+5V

–5V

C

The gain equation for the circuit in Figure 34 is

(2)