AD8129ARMZ datasheet(34/40 Pages) AD

34 / 40 page

AD8129/AD8130

Rev. C | Page 34 of 40

0.1

μF

–V

VOUT

0.1

μF

VIN

100

499

–VS

+VS

200pF

AD8130

100

Figure 134. An Equalizer Circuit for Composite Video Transmissions

over 300 Meters of Category-5 Cable

–10

–20

–30

–40

–50

–60

–70

–80

FREQUENCY (Hz)

10k

100k

10M

100M

Figure 135. Transmission Response of 300 Meters of Category-5 Cable

The feedback network is between Pin 6 and Pin 5 and from

Pin 5 to ground. C1 and RF create a corner frequency of about

800 kHz. The gain increases to provide about 15 dB of boost

at 8 MHz. The response of this circuit is shown in Figure 136.

–10

–20

–30

–40

–50

–60

–70

–80

FREQUENCY (Hz)

10k

100k

10M

100M

Figure 136. Frequency Response of Equalizer Circuit

It is difficult to calculate the exact component values via strictly

mathematical means, because the equations for the cable

attenuation are approximate and have functions that are not

simply related to the responses of RC networks. The method

used in this design was to approximate the required response

via graphical means from the frequency response and then

select components that would approximate this response. The

circuit was then built, measured, and finally adjusted to obtain

an acceptable response—in this case, flat to 9 MHz to within

approximately 1 dB (see Figure 137).

–10

–20

–30

–40

–50

–60

–70

–80

FREQUENCY (Hz)

10k

100k

10M

100M

Figure 137. Combined Response of Cable Plus Equalizer

OUTPUT OFFSET/LEVEL TRANSLATOR

The circuit in Figure 133 has the reference input (Pin 4) tied to

ground, which produces a ground-referenced output signal. If it

is desired to offset the output voltage from ground, the REF

input can be used (see Figure 138). The level VOFFSET appears at

the output with unity gain.

0.1

μF

–V

VOUT = VIN +VOFFSET

0.1

μF

VIN

VOFFSET

–VS

+VS

AD8130

μF

Figure 138. The Voltage Applied to Pin 4 to the Unity-Gain Output Voltage

Produced by VIN

If the circuit has a gain higher than unity, the gain must be

factored in. If RG is connected to ground, the voltage applied to

REF is multiplied by the gain of the circuit and appears at the

output—just like a noninverting conventional op amp. This

situation is not always desirable; the user may want VOFFSET to

appear at the output with unity gain.

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