LT1739CFE datasheet(13/20 Pages) LINER

13 / 20 page

LT1739

1739fas, sn1739

In differential driver applications, as shown on the first

page of this data sheet, it is recommended that the gain

setting resistor be comprised of two equal value resistors

connected to a good AC ground at high frequencies. This

ensures that the feedback factor of each amplifier remains

less than 0.1 at any frequency. The midpoint of the

resistors can be directly connected to ground, with the

resulting DC gain to the VOS of the amplifiers, or just

bypassed to ground with a 1000pF or larger capacitor.

Line Driving Back-Termination

The standard method of cable or line back-termination is

shown in Figure 13. The cable/line is terminated in its

characteristic impedance (50

Ω, 75Ω, 100Ω, 135Ω, etc.).

A back-termination resistor also equal to the chararacteristic

impedance should be used for maximum pulse fidelity of

outgoing signals, and to terminate the line for incoming

signals in a full-duplex application. There are three main

drawbacks to this approach. First, the power dissipated in

the load and back-termination resistors is equal so half of

the power delivered by the amplifier is wasted in the

termination resistor. Second, the signal is halved so the

gain of the amplifer must be doubled to have the same

overall gain to the load. The increase in gain increases

noise and decreases bandwidth (which can also increase

distortion). Third, the output swing of the amplifier is

doubled which can limit the power it can deliver to the load

for a given power supply voltage.

An alternate method of back-termination is shown in

Figure 14. Positive feedback increases the effective back-

termination resistance so RBT can be reduced by a factor

of n. To analyze this circuit, first ground the input. As RBT =

RL/n, and assuming RP2>>RL we require that:

∆VA = ∆VO (1 – 1/n) to increase the effective value of

RBT by n.

∆VP = ∆VO (1 – 1/n)/(1 + RF/RG)

∆VO = ∆VP (1 + RP2/RP1)

APPLICATIO S I FOR ATIO

1739 F13

RBT

CABLE OR LINE WITH

CHARACTERISTIC IMPEDANCE RL

(1 + RF/RG)

RBT = RL

Figure 13. Standard Cable/Line Back Termination

1739 F14

RBT

RP2

RP1

1 +

= 1 –

–

FOR RBT =

()

1 +

()

RP1

RP1 + RP2

RP1

RP2 + RP1

RP2/(RP2 + RP1)

()

1 + 1/n

Figure 14. Back Termination Using Postive Feedback

Eliminating

∆VP, we get the following:

(1 + RP2/RP1) = (1 + RF/RG)/(1 – 1/n)

For example, reducing RBT by a factor of n = 4, and with an

amplifer gain of (1 + RF/RG) = 10 requires that RP2/RP1

= 12.3.

Note that the overall gain is increased:

nR R

()

() +

()

[]−+

()

[]

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