7
LT1217
U
S
A
O
PP
L
IC
AT
I
WU
U
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 30µA from pin 1 or
5. The easy way to do this is to use a 100kΩ pot between
pin 1 and 5 with a 430kΩ 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 50µA from pin 8 drops the supply current to less than
350µA, 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.
Large Signal Response, A
V
= 2, R
F
= R
G
= 3k,
Slew Rate 500V/µs
Large Signal Response, A
V
= –2, R
F
= 3k, R
G
= 1.5k,
Slew Rate 850V/µs
Large Signal Response, A
V
= 10, R
F
= 3k, R
G
= 330Ω,
Slew Rate 150V/µs
