LT1633CS datasheet(13/16 Pages) LINER

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LT1632/LT1633

APPLICATIONS INFORMATION

They flow in the opposite direction when the NPN input

stage is active. The offset voltage error due to input bias

currents can be minimized by equalizing the noninverting

and inverting input source impedance.

Output

The outputs of the LT1632/LT1633 can deliver large load

currents; the short-circuit current limit is 70mA. Take care

to keep the junction temperature of the IC below the

absolute maximum rating of 150

°C (refer to the Power

Dissipation section). The output of these amplifiers have

reverse-biased diodes to each supply. If the output is

forced beyond either supply, unlimited current will flow

through these diodes. If the current is transient and limited

to several hundred mA, no damage to the part will occur.

Overdrive Protection

To prevent the output from reversing polarity when the

input voltage exceeds the power supplies, two pairs of

crossing diodes D1 to D4 are employed. When the input

voltage exceeds either power supply by approximately

700mV, D1/D2 or D3/D4 will turn on, forcing the output to

the proper polarity. For this phase reversal protection to

work properly, the input current must be limited to less

than 5mA. If the amplifier is to be severely overdriven, an

external resistor should be used to limit the overdrive

current.

The LT1632/LT1633’s input stages are also protected

against large differential input voltages by a pair of back-

to-back diodes D5/D8. When a differential voltage of

more than 1.4V is applied to the inputs, these diodes will

turn on, preventing the emitter-base breakdown of the

input transistors. The current in D5/D8 should be limited

to less than 10mA. Internal 225

Ω resistorsR6andR7will

limit the input current for differential input signals of 4.5V

or less. For larger input levels, a resistor in series with

either or both inputs should be used to limit the current.

Worst-case differential input voltage usually occurs when

the output is shorted to ground. In addition, the amplifier

is protected against ESD strikes up to 3kV on all pins.

Capacitive Load

The LT1632/LT1633 are wideband amplifiers that can

drive capacitive loads up to 200pF on

±15V supplies in a

unity-gain configuration. On a 3V supply, the capacitive

load should be kept to less than 100pF. When there is a

need to drive larger capacitive loads, a resistor of 20

Ω to

Ω should be connected between the output and the

capacitive load. The feedback should still be taken from the

output so that the resistor isolates the capacitive load to

ensure stability.

Feedback Components

The low input bias currents of the LT1632/LT1633 make it

possible to use the high value feedback resistors to set the

gain. However, care must be taken to ensure that the pole

formed by the feedback resistors and the total capacitance

at the inverting input does not degrade stability. For

instance, the LT1632/LT1633 in a noninverting gain of 2,

set with two 20k resistors, will probably oscillate with

10pF total input capacitance (5pF input capacitance and

5pF board capacitance). The amplifier has a 6MHz cross-

ing frequency and a 55

° phase margin at 6dB of gain. The

feedback resistors and the total input capacitance form a

pole at 1.6MHz that induces a phase shift of 75

° at 5MHz!

The solution is simple: either lower the value of the

resistors or add a feedback capacitor of 10pF or more.

TYPICAL APPLICATIONS

Single Supply, 40dB Gain, 550kHz Instrumentation

Amplifier

An instrumentation amplifier with a rail-to-rail output

swing, operating from a 3V supply can be constructed with

the LT1632 as shown in the first page of this data sheet.

The amplifier has a nominal gain of 100, which can be

adjusted with resistor R5. The DC output level is set by the

difference of the two inputs multiplied by the gain of 100.

The voltage gain and the DC output level can be

expressed as follows:

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