10

LT1886

APPLICATIO S I FOR ATIO

The dissipation for the amplifiers is:

The total package power dissipation is 0.6W. When a 2500

sq. mm PC board with 2oz copper on top and bottom is

used, the thermal resistance is 80

°C/W. The junction

The maximum junction temperature for the LT1886 is

150

°C so the heat sinking capability of the board is

adequate for the application.

If the copper area on the PC board is reduced to 180 sq.

mm the thermal resistance increases to 122

°C/W and the

junction temperature becomes:

which is above the maximum junction temperature indi-

cating that the heat sinking capability of the board is

inadequate and should be increased.

Capacitive Loading

The LT1886 is stable with a 1000pF capacitive load. The

photo of the small-signal response with 1000pF load in a

gain of 10 shows 50% overshoot. The photo of the large-

signal response with a 1000pF load shows that the output

slew rate is not limited by the short-circuit current. The

Typical Performance Curve of Frequency Response vs

Capacitive Load shows the peaking for various capacitive

loads.

This stability is useful in the case of directly driving a

coaxial cable or twisted pair that is inadvertently

unterminated. For best pulse fidelity, however, a termina-

tion resistor of value equal to the characteristic impedance

of the cable or twisted pair (i.e., 50

Ω/75Ω/100Ω/135Ω)

should be placed in series with the output. The other end

of the cable or twisted pair should be terminated with the

same value resistor to ground.

Compensation

The LT1886 is stable in a gain 10 or higher for any supply

and resistive load. It is easily compensated for lower gains

with a single resistor or a resistor plus a capacitor.

Figure 2 shows that for inverting gains, a resistor from the

inverting node to AC ground guarantees stability if the

should be less than 15MHz to minimize peaking. The

Typical Curve of Frequency Response vs Supply Voltage,

frequency of 12.8MHz.

–

+

6V

–6V

909

Ω

100

Ω

100

Ω

1886 F01

1K

50

Ω

–4V

4V

f = 1MHz

Figure 1. Thermal Calculation Example

(OPTIONAL)

1886 F02

=

< 15MHz

1

2

Figure 2. Compensation for Inverting Gains

Figure 3 shows compensation in the noninverting configu-

case. The input impedance is not reduced because the

network is bootstrapped. This network can also be placed

between the inverting input and an AC ground.

Another compensation scheme for noninverting circuits is

shown in Figure 4. The circuit is unity gain at low frequency

offset is reduced by a factor of ten. The techniques of