LT1886CS8 datasheet(11/16 Pages) LINER

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LT1886

APPLICATIO S I FOR ATIO

Figures 3 and 4 can be combined as shown in Figure 5. The

gain is unity at low frequencies, 1 + RF/RG at mid-band and

for stability, a gain of 10 or greater at high frequencies.

termination resistor is used, a capacitor to ground at the

load can eliminate ringing.

Line Driving Back-Termination

The standard method of cable or line back-termination is

shown in Figure 6. The cable/line is terminated in its

characteristic impedance (50

Ω, 75Ω, 100Ω, 135Ω, etc.).

A back-termination resistor also equal to 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 ampli-

fier is doubled which can limit the power it can deliver to

the load for a given power supply voltage.

(OPTIONAL)

1886 F03

= 1 +

< 15MHz

πRCCC

(RC || RG) ≤ RF/9

Output Loading

The LT1886 output stage is very wide bandwidth and able

to source and sink large currents. Reactive loading, even

isolated with a back-termination resistor, can cause ring-

ing at frequencies of hundreds of MHz. For this reason, any

design should be evaluated over a wide range of output

conditions. To reduce the effects of reactive loading, an

optional snubber network consisting of a series RC across

the load can provide a resistive load at high frequency.

Another option is to filter the drive to the load. If a back-

Figure 3. Compensation for Noninverting Gains

1886 F04

< 15MHz

πRGCC

RG ≤ RF/9

= 1 (LOW FREQUENCIES)

(HIGH FREQUENCIES)

= 1 +

Figure 4. Alternate Noninverting Compensation

1886 F05

CBIG

= 1 AT LOW FREQUENCIES

= 1 +

AT MEDIUM FREQUENCIES

(RC || RG)

= 1 +

AT HIGH FREQUENCIES

Figure 5. Combination Compensation

1886 F06

RBT

CABLE OR LINE WITH

CHARACTERISTIC IMPEDANCE RL

(1 + RF/RG)

RBT = RL

An alternate method of back-termination is shown in

Figure 7. 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)

Eliminating Vp, we get the following:

(1 + RP2/RP1) = (1 + RF/RG)/(1 – 1/n)

Figure 6. Standard Cable/Line Back-Termination

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