LF400AMH datasheet(9/12 Pages) NSC

9 / 12 page

Typical Performance Characteristics (Continued)

CommonMode Voltage Transfer Characteristic

TLH9414 – 19

TLH9414 – 20

Application Hints

The LF400 is a high-speed low input bias current Bi-FET

operational amplifier capable of settling to 001% of a 10V

output swing in less than 400 ns The rugged JFET inputs

allow differential input voltages as high as 32V without a

large increase in input current However the inputs should

never be driven to voltages lower than the negative supply

as this can result in input currents large enough to damage

the device To prevent this from occurring when power is

first applied always turn the positive and negative power

supplies on simultaneously or turn the negative supply on

first

Exceeding the common-mode input range will not damage

the device as long as the Absolute Maximum Ratings are

not violated but it will result in a high output voltage Latch-

ing will not occur however and when the offending signal is

removed the LF400 will recover quickly

The nominal power supply voltage is g15V but the LF400

will operate satisfactorily from g10V to g16V The LF400 is

functional down to g5V but performance will be degraded

(See Typical Performance curves)

Settling Time Considerations

The settling performance of any high-speed operational am-

plifier is highly dependent on the external components and

circuit board layout Capacitance between the amplifier

summing junction and ground affects the closed-loop trans-

fer function and should be minimized The compensation

capacitor CC between the output and the inverting input

should be carefully chosen to counteract the effect of the

input capacitance Since input capacitance is made up of

several stray capacitances that are difficult to predict the

compensation capacitor will generally have to be deter-

mined empirically for best settling time A good starting

point is around 10 pF for AV eb1

Settling time may be verified using a circuit similar to the

one in

Figure 1 The LF400 is connected for inverting opera-

tion and the output voltage is summed with the input volt-

age step When the LF400’s output voltage is equal to the

input voltage the voltage on the gate of Q1 will be zero Any

voltage appearing at this point will represent an error The

FET source follower output is observed on an oscilloscope

and the settling time is equal to the time required for the

error signal displayed on the oscilloscope to decay to less

than one-half the necessary accuracy (see oscilloscope

photos of ‘‘Settling TimePositive Output Swing’’ and ‘‘Set-

tling TimeNegative Output Swing’’) For a 10V input sig-

nal settling time to 001% (1 mV) will occur when the dis-

played error is less than

mV Since settling time is strong-

ly dependent on slew rate settling will be faster for smaller

signal swings The LF400’s inverting slew rate is faster than

its non-inverting slew rate so settling will be faster for in-

verting applications as well

It is important to note that the oscilloscope input amplifier

will be overdriven during a settling time measurement so

the oscilloscope must be capable of recovering from over-

drive very quickly Very few oscilloscopes are suitable for

this sort of measurement The signal generator used for set-

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