7

LT1217

S

APPLICATI

I FOR ATIO

Offset Adjust

Output offset voltage is equal to the input offset voltage

times the gain plus the inverting input bias current times

the feedback resistor. The LT1217 output offset voltage

can be nulled by pulling approximately 30

µA from pin 1 or

5. The easy way to do this is to use a 100k

Ω pot between

pin 1 and 5 with a 430k

Ω resistor from the wiper to ground

for 15V supply applications. Use a 110k resistor when

operating on a 5V supply.

Shutdown

Pin 8 activates a shutdown control function. Pulling more

than 50

µA from pin 8 drops the supply current to less than

350

µA, and puts the output into a high impedance state.

The easy way to force shutdown is to ground pin 8, using

an open collector (drain) logic stage. An internal resistor

limits current, allowing direct interfacing with no addi-

tional parts. When pin 8 is open, the LT1217 operates

normally.

Slew Rate

The slew rate of a current feedback amplifier is not

independent of the amplifier gain configuration the way it

is in a traditional op amp. This is because the input stage

and the output stage both have slew rate limitations.

Inverting amplifiers do not slew the input and are therefore

limited only by the output stage. High gain, non-inverting

amplifiers are similar. The input stage slew rate of the

LT1217 is about 50V/

µs before it becomes non-linear and

is enhanced by the normally reverse biased emitters on the

input transistors. The output slew rate depends on the size

of the feedback resistors. The output slew rate is about

850V/

µs with a 3k feedback resistor and drops propor-

tionally for larger values. The photos show the LT1217

with a 20V peak-to-peak output swing for three different

gain configurations.

Settling Time

The characteristic curves show that the LT1217 settles to

within 10mV of final value in less than 300ns for any output

step up to 10V. Settling to 1mV of final value takes less

than 500ns.

Information furnished by Linear Technology Corporation is believed to be accurate and reliable.

However, no responsibility is assumed for its use. Linear Technology Corporation makes no represen-

tation that the interconnection of its circuits as described herein will not infringe on existing patent rights.

Slew Rate

500V/

µs

Slew Rate

850V/

µs

Slew Rate

150V/

µs