LT1204CSW datasheet(10/20 Pages) LINER

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LT1204

APPLICATI

I FOR ATIO

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.

Capacitive Loads

The LT1204 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. Alterna-

tively, 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 ampli-

fier bandwidth is only reduced when the capacitive load

is present. The disadvantage is that the gain is a function

of load resistance.

Slew Rate

The slew rate of the current feedback amplifier on the

LT1204 is not independent of the amplifier gain the way

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

input and the output stage have slew rate limitations. In

high gain settings the signal amplitude between the nega-

tive input and any driven positive input is small and the

overall slew rate is that of the output stage. For gains less

than 10, the overall slew rate is limited by the input stage.

The input slew rate of the LT1204 is approximately 135V/

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 10 with

a 1k feedback resistor and

±15 supplies, the output slew

rate is typically 1000V/

µs. Larger feedback resistors will

reduce the slew rate as will lower supply voltages, similar

to the way the bandwidth is reduced.

The graph, Maximum Undistorted Output vs Frequency,

relates the slew rate limitations to sinusoidal inputs for

various gain configurations.

Large-Signal Transient Response

1204 AI02

1204 AI03

VS = ±15V

AV = 2

RF = 1k

RG = 1k

RL = 400Ω

VS = ±15V

AV = 10

RF = 910Ω

RG = 100Ω

RL = 400Ω

Switching Characteristics and Pin 8

Switching between channels is a “make-before-break”

condition where both inputs are on momentarily. The

buffers isolate the inputs when the “make-before-break”

switching occurs. The input with the largest positive

voltage determines the output level. If both inputs are

equal, there is only a 40mV error at the input of the CFA

during the transition. The reference adjust (Pin 8) allows

the user to trade off positive input voltage range for

switching time. For example, on

±15V supplies, setting

the voltage on Pin 8 to – 6.8V reduces the switching

transient to a 50ns duration, and reduces the positive input

range from 6V to 2.35V. The negative input range remains

unchanged at – 6V. When switching video “in picture,” this

short transient is imperceptible even on high quality

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