9

LT1207

S

APPLICATI

I FOR ATIO

FREQUENCY (MHz)

1

–8

–4

0

4

8

10

100

LT1207 • F01

–6

–2

2

6

10

12

COMPENSATION

NO COMPENSATION

COMPENSATION

Figure 1.

0pF to 1000pF. The graph of Maximum Capacitive Load vs

Feedback Resistor can be used to select the appropriate

value of the feedback resistor. The values shown are for

0.5dB and 5dB peaking at a gain of 2 with no resistive load.

This is a worst-case condition, as the amplifier is more stable

at higher gains and with some resistive load in parallel with

the capacitance. Also shown is the –3dB bandwidth with the

suggested feedback resistor vs the load capacitance.

Although the optional compensation works well with

capacitive loads, it simply reduces the bandwidth when it

is connected with resistive loads. For instance, with a 30

Ω

load, the bandwidth drops from 55MHz to 35MHz when

the compensation is connected. Hence, the compensation

was made optional. To disconnect the optional compensa-

tion, leave the COMP pin open.

Shutdown/Current Set

If the shutdown feature is not used, the Shutdown pins

Each amplifier has a separate Shutdown pin which can be

used to either turn off the amplifier, which reduces the

amplifier supply current to less than 200

µA, or to control

the supply current in normal operation.

The supply current in each amplifier is controlled by the

current flowing out of the Shutdown pin. When the Shut-

down pin is open or driven to the positive supply, the

amplifier is shut down. In the shutdown mode, the output

looks like a 40pF capacitor and the supply current is

typically 100

µA. Each Shutdown pin is referenced to the

positive supply through an internal bias circuit (see the

Simplified Schematic). An easy way to force shutdown is

to use open drain (collector) logic. The circuit shown in

Figure 2 uses a 74C904 buffer to interface between 5V

logic and the LT1207. The switching time between the

active and shutdown states is less than 1

µs. A 24k pull-up

resistor speeds up the turn-off time and insures that the

amplifier is completely turned off. Because the pin is

referenced to the positive supply, the logic used should

have a breakdown voltage of greater than the positive

supply voltage. No other circuitry is necessary as the

internal circuit limits the Shutdown pin current to about

500

µA. Figure 3 shows the resulting waveforms.

Figure 2. Shutdown Interface

–

+

1/2 LT1207

SHDN

15V

–15V

5V

24k

ENABLE

LT1207 • F02

15V

74C906

For applications where the full bandwidth of the amplifier

is not required, the quiescent current may be reduced by

connecting a resistor from the Shutdown pin to ground.

Figure 3. Shutdown Operation

LT1207 • F3