AD8145

Rev. 0 | Page 15 of 24

APPLICATIONS

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 unshielded twisted pair (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.

It is important to 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,

which 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. Closed-loop

gain of an amplifier is set to 1 by connecting the amplifier output

directly to its respective GAIN 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 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 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)