AD8129ARMZ datasheet(33/40 Pages) AD

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AD8129/AD8130

Rev. C | Page 33 of 40

APPLICATIONS

BASIC GAIN CIRCUITS

The gain of the AD8129/AD8130 can be set with a pair of

feedback resistors. The basic configuration is shown in Figure 132.

The gain equation is the same as that of a conventional op amp:

G = 1 + RF/RG. For unity-gain applications using the AD8130,

RF can be set to 0 (short circuit), and RG can be removed (see

Figure 133). The AD8129 is compensated to operate at gains of

10 and higher; therefore, shorting the feedback path to obtain

unity gain causes oscillation.

–V

VOUT

VIN

–VS

+VS

AD8129/

AD8130

0.1

μF

0.1

μF

Figure 132. Basic Gain Circuit: VOUT = VIN (1 + RF/RG)

0.1

μF

–V

VOUT

0.1

μF

VIN

AD8130

–VS

+VS

μF

Figure 133. An AD8130 with Unity Gain

The input signal can be applied either differentially or in a

single-ended fashion—all that matters is the magnitude of the

differential signal between the two inputs. For single-ended

input applications, applying the signal to the +IN with −IN

grounded creates a noninverting gain, while reversing these

connections creates an inverting gain. Because the two inputs

are high impedance and matched, both of these conditions

provide the same high input impedance. Thus, an advantage of

the active feedback architecture is the ability to make a high

input impedance inverting op amp. If conventional op amps are

used, a high impedance buffer followed by an inverting stage is

needed. This requires two op amps.

TWISTED-PAIR CABLE, COMPOSITE VIDEO

RECEIVER WITH EQUALIZATION USING AN AD8130

The AD8130 has excellent common-mode rejection at its

inputs. This makes it an ideal candidate for a receiver for signals

that are transmitted over long distances on twisted-pair cables.

Category 5 cables are very common in office settings and are

extensively used for data transmission. These cables can also be

used for the analog transmission of signals such as video.

These long cables pick up noise from the environment they pass

through. This noise does not favor one conductor over another

and therefore is a common-mode signal. A receiver that rejects

the common-mode signal on the cable can greatly enhance the

signal-to-noise ratio performance of the link.

The AD8130 is also very easy to use as a differential receiver,

because the differential inputs and the feedback inputs are

entirely separate. This means that there is no interaction

between the feedback network and the termination network,

as there would be in conventional op amp types of receivers.

Another issue with long cables is that there is more attenuation

of the signal at longer distances. Attenuation is also a function

of frequency; it increases to roughly the square root of frequency.

For good fidelity of video circuits, the overall frequency

response of the transmission channel should be flat vs.

frequency. Because the cable attenuates the high frequencies, a

frequency-selective boost circuit can be used to undo this effect.

These circuits are called equalizers.

An equalizer uses frequency-dependent elements (Ls and Cs) to

create a frequency response that is the opposite of the rest of the

channel’s response to create an overall flat response. There are

many ways to create such circuits, but a common technique is to

put the frequency-selective elements in the feedback path of an

op amp circuit. The AD8130 in particular makes this easier

than other circuits, because, once again, the feedback path is

completely independent of the input path and there is no

interaction.

The circuit in Figure 134 was developed as a receiver/equalizer

for transmitting composite video over 300 meters of Category 5

cable. This cable has an attenuation of approximately 20 dB at

10 MHz for 300 meters. At 100 MHz, the attenuation is

approximately 60 dB (see Figure 135).

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