8

LT1227

S

APPLICATI

I FOR ATIO

and inverting input bias current will change. The offset

voltage changes about 500

µV per volt of supply mis-

match. The inverting bias current can change as much as

5.0

µA per volt of supply mismatch, though typically the

change is less than 0.5

µA per volt.

Slew Rate

The slew rate of a current feedback amplifier is not

independent of the amplifier gain configuration the way

slew rate is in a traditional op amp. This is because both the

input stage and the output stage have slew rate limitations.

In the inverting mode, and for higher gains in the

noninverting mode, the signal amplitude between the

input pins is small and the overall slew rate is that of the

output stage. For gains less than ten in the noninverting

mode, the overall slew rate is limited by the input stage.

The input stage slew rate of the LT1227 is approximately

125V/

µs and is set by internal currents and capacitances.

The output slew rate is set by the value of the feedback

resistors and the internal capacitances. At a gain of ten

with a 1k feedback resistor and

±15V supplies, the output

slew rate is typically 1100V/

µs. Larger feedback resistors

will reduce the slew rate as will lower supply voltages,

similar to the way the bandwidth is reduced.

The graph of Maximum Undistorted Output vs Frequency

relates the slew rate limitations to sinusoidal inputs for

various gain configurations.

AI01

AI02

Capacitance on the Inverting Input

Current feedback amplifiers require resistive feedback

from the output to the inverting input for stable operation.

Take care to minimize the stray capacitance between the

output and the inverting input. Capacitance on the invert-

ing input to ground will cause peaking in the frequency

response (and overshoot in the transient response), but it

does not degrade the stability of the amplifier.

Capacitive Loads

The LT1227 can drive capacitive loads directly when the

proper value of feedback resistor is used. The graph of

Maximum Capacitive Load vs Feedback Resistor should

be used to select the appropriate value. The value shown

is for 5dB peaking when driving a 1k load at a gain of 2. This

is a worst case condition, the amplifier is more stable at

higher gains and driving heavier loads. Alternatively, a

small resistor (10

Ω to 20Ω) can be put in series with the

output to isolate the capacitive load from the amplifier

output. This has the advantage that the amplifier band-

width is only reduced when the capacitive load is present

and the disadvantage that the gain is a function of the load

resistance.

Power Supplies

The LT1227 will operate from single or split supplies from

±2V (4V total) to ±15V (30V total). It is not necessary to

use equal value split supplies, however the offset voltage