AD8129ARM datasheet(21/28 Pages) AD

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REV. 0

AD8129/AD8130

–21–

APPLICATIONS

Basic Gain Circuits

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

back resistors. The basic configuration is shown in Figure 5.

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 zero (short circuit), and RG can be removed.

(See Figure 6.) The AD8129 is compensated to operate at gains

of 10 and higher, so shorting the feedback path to obtain unity

gain will cause oscillation.

0.1 F10 F

–V

VOUT

0.1 F

10 F

VIN

–VS

+VS

AD8129/

AD8130

Figure 5. Basic Gain Circuit: VOUT = VIN (1 + RF/RG)

0.1 F10 F

–V

VOUT

0.1 F

10 F

VIN

AD8130

–VS

+VS

Figure 6. An AD8130 with Unity Gain

The input signal can be applied either differentially or single-

endedly—all that matters is the magnitude of the differential

signal between the two inputs. For single-ended input applica-

tions, applying the signal to the +IN with –IN grounded will

create a noninverting gain, while reversing these connections

will create an inverting gain. Since the two inputs are high-

impedance and matched, both of these conditions will 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 Equal-

ization 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. Cat-

egory 5 type cables are now very common in office settings and

are extensively used for data transmission. These same cables

can also be used for the analog transmission of signals like video.

These long cables will pick up noise from the environment they

pass through. This noise will not favor one conductor over an-

other, and will therefore be 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 of the

feedback network and the termination network as there would

be in conventional op amp-type receivers.

Another issue to be dealt with on long cables is the attenuation

of the signal at longer distances. This attenuation is a function of

frequency and increases as roughly as the square root of frequency.

For good fidelity of video circuits, the overall frequency response

of the transmission channel should be flat versus frequency. Since

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) in

order to create a frequency response that is the opposite of the

rest of the channel’s response in order 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 particu-

lar makes this easier than other circuits, because, once again, the

feedback path is totally independent of the input path and there

is no interaction.

The circuit in Figure 7 was developed as a receiver/equalizer for

transmitting composite video over 300 m of Category 5 cable. This

cable has an attenuation of approximately 20 dB at 10 MHz

for 300 m. At 100 MHz, the attenuation is approximately

60 dB. (See Figure 8.)

0.1 F

10 F

–V

VOUT

0.1 F

10 F

VIN

100

499

–VS

+VS

200pF

AD8130

100

Figure 7. An Equalizer Circuit for Composite Video

Transmission over 300 m of Category 5 Cable

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