LT1225

6

S

APPLICATI

I FOR ATIO

The LT1225 may be inserted directly into HA2541, HA2544,

AD847, EL2020 and LM6361 applications, provided that

the amplifier configuration is a noise gain of 5 or greater,

and the nulling circuitry is removed. The suggested nulling

circuit for the LT1225 is shown below.

Offset Nulling

The large-signal response in both inverting and noninvert-

ing gain shows symmetrical slewing characteristics. Nor-

mally the noninverting response has a much faster rising

edge than falling edge due to the rapid change in input

common-mode voltage which affects the tail current of the

input differential pair. Slew enhancement circuitry has

been added to the LT1225 so that the noninverting slew

rate response is balanced.

Input Considerations

Resistors in series with the inputs are recommended for

the LT1225 in applications where the differential input

voltage exceeds

±6V continuously or on a transient basis.

An example would be in noninverting configurations with

high input slew rates or when driving heavy capacitive

loads. The use of balanced source resistance at each input

is recommended for applications where DC accuracy must

be maximized.

Capacitive Loading

The LT1225 is stable with all capacitive loads. This is

accomplished by sensing the load induced output pole and

adding compensation at the amplifier gain node. As the

capacitive load increases, both the bandwidth and phase

margin decrease so there will be peaking in the frequency

Layout and Passive Components

As with any high speed operational amplifier, care must be

taken in board layout in order to obtain maximum perfor-

mance. Key layout issues include: use of a ground plane,

minimization of stray capacitance at the input pins, short

lead lengths, RF-quality bypass capacitors located close

to the device (typically 0.01

µF to 0.1µF), and use of low

ESR bypass capacitors for high drive current applications

(typically 1

µF to 10µF tantalum). Sockets should be

avoided when maximum frequency performance is

required, although low profile sockets can provide

reasonable performance up to 50MHz. For more details

see Design Note 50. Feedback resistor values greater than

5k are not recommended because a pole is formed with the

input capacitance which can cause peaking. If feedback

resistors greater than 5k are used, a parallel

capacitor of 5pF to 10pF should be used to cancel the input

pole and optimize dynamic performance.

Transient Response

The LT1225 gain-bandwidth is 150MHz when measured at

1MHz. The actual frequency response in gain of 5 is

considerably higher than 30MHz due to peaking caused by

a second pole beyond the gain of 5 crossover point. This

is reflected in the small-signal transient response. Higher

noise gain configurations exhibit less overshoot as seen in

the inverting gain of 5 response.

–

+

3

2

1

8

5k

0.1

µF

7

6

4

0.1

µF

LT1225

LT1225 AI01

LT1225 AI02

LT1225 AI03