Datasheet LT1223 (Analog Devices) - 9

LT1223
U U W U APPLICATIO S I FOR ATIO Shutdown Output Slew Rate of 500V/
µ
s
Pin 8 activates a shutdown control function. Pulling more than 200mA from Pin 8 drops the supply current to less than 3mA, 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 LT1223 operates normally.
Slew Rate
The slew rate of a current feedback amplifier is not inde- pendent of the amplifier gain configuration the way it is in 1223 A01 a traditional op amp. This is because the input stage and
Output Slew Rate of 1000V/
µ
s
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, noninverting amplifi- ers are similar. The input stage slew rate of the LT1223 is about 350V/µs before it becomes nonlinear and is en- hanced by the normally reverse-biased emitters on the input transistors. The output slew rate depends on the size of the feedback resistors. The peak output slew rate is about 2000V/µs with a 1k feedback resistor and drops proportionally for larger values. At an output slew rate of 1000V/µs or more, the transistors in the “mirror circuits” will begin to saturate due to the large feedback currents. This causes the output to have slew induced overshoot and 1223 A02 is somewhat unusual looking; it is in no way harmful or dangerous to the device. The photos show the LT1223 in
Output Slew Rate at 2000V/
µ
s Shows Aberrations (See Text)
a noninverting gain of three (RF = 1k, RG = 500Ω) with a 20V peak-to-peak output slewing at 500V/µs, 1000V/µs and 2000V/µs.
Settling Time
The Inverting Amplifier Settling Time versus Output Step curve shows that the LT1223 will settle to within 1mV of final value in less than 100ns for all output changes of 10V or less. When operated as an inverting amplifier there is less than 500µV of thermal settling in the amplifier. However, when operating the LT1223 as a noninverting amplifier, there is an additional thermal settling compo- 1223 A03 nent that is about 200µV for every volt of input common mode change. So a noninverting gain of one amplifier will 1223fb 9