AD8129ARM datasheet(23/28 Pages) AD

23 / 28 page

REV. 0

AD8129/AD8130

–23–

VOUT =

VIN

(1+ RF/RG) +VOFFSET

0.1 F10 F

–V

0.1 F

10 F

VIN

VOFFSET

–VS

+VS

AD8129/

AD8130

Figure 12. In this circuit, VOFFSET appears at the output

with unity gain. This circuit works well if the VOFFSET

Source Impedance is low.

A way around this is to apply the offset voltage to a voltage

divider whose attenuation factor matches the gain of the ampli-

fier, and then apply this voltage to the high-impedance REF

input. This circuit will first divide the desired offset voltage by

the gain, and the amplifier will multiply it back up to unity. (See

Figure 13.)

VOUT =

VIN

(1+RF/RG) + VOFFSET

0.1 F10 F

–V

0.1 F

10 F

VIN

–VS

+VS

VOFFSET

AD8129/

AD8130

Figure 13. Adding an attenuator at the offset input causes

it to appear at the output with unity gain.

Resistorless Gain-of-Two

The voltage applied to the REF input (Pin 4) can also be a high

bandwidth signal. If a unity-gain AD8130 has both +IN and

REF driven with the same signal, there will be unity gain from

VIN and unity gain from VREF. Thus, the circuit will have a gain

of two, and requires no resistors. (See Figure 14.)

VOUT

0.1 F10 F

–V

0.1 F

10 F

VIN

–VS

+VS

AD8130

Figure 14. Gain-of-Two Connections with No Resistors

Summer

A general summing circuit can be made by the above technique.

A unity-gain configured AD8130 has one signal applied to +IN,

while the other signal is applied to REF. The output will be the

sum of the two input signals. (See Figure 15.)

VOUT = V1 + V2

0.1 F10 F

–V

0.1 F

10 F

–VS

+VS

AD8130

Figure 15. A Summing Circuit that is Noninverting with

High Input Impedance

This circuit offers several advantages over a conventional op

amp inverting summing circuit. First, the inputs are both high-

impedance and the circuit is noninverting. It would require

significant additional circuitry to make an op amp summing

circuit that has high input impedance and is noninverting.

Another advantage is that the AD8130 circuit still preserves the

full bandwidth of the part. In a conventional summing circuit,

the noise gain is increased for every additional input, so the

bandwidth response decreases accordingly. By this technique,

four signals can be summed by applying them to two AD8130s,

and then summing the two outputs by a third AD8130.

Cable-Tap Amplifier

It is often desirable to have a video signal drive several different

pieces of equipment. However, the cable should only be termi-

nated once at its end point, so it is not appropriate to have a

termination at each device. A “loop-through” connection allows

a device to tap the video signal while not disturbing it by any

excessive loading.

Such a connection, also referred to as a cable-tap amplifier, can

be simply made with an AD8130. (See Figure 16.) The circuit is

configured with unity gain, and if no output offset is desired,

the REF pin is grounded. The negative differential input is

connected directly to the shield of the cable (or an associated

connector) at the point at which it wants to be “tapped.”

VIDEO

VOUT

0.1 F10 F

–V

0.1 F

10 F

–VS

+VS

AD8130

Figure 16. The AD8130 can tap the video signal at any

point along the cable without loading the signal.

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