LT6232 datasheet(18/30 Pages) AD

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LTC6228/LTC6229

Rev. B

For more information www.analog.com

APPLICATIONS INFORMATION

without input bias cancellation. Additionally when input

bias current cancellation is enabled, the current noise

increases. The decision to use input bias cancellation

should be made with the end application’s specifications

and conditions in mind.

If the SHDN pin is left floating, input bias cancellation is

notenabled,whichmaybesuitableformanyapplications.

Output

The LTC6228 family has excellent output drive capability.

The amplifiers can typically deliver more than 90mA of

output current at a total supply of 10V, and can typically

swing to within 320mV of the supply with load currents

as high as 25mA. As the supply voltage to the amplifier

decreases, the output current capability also decreases.

Attention must be paid to keep the junction temperature

oftheICbelow150°C(refertoPowerDissipationsection)

whentheoutputisincontinuousshort-circuit.Theoutput

of the amplifier has reverse-biased diodes connected to

each supply. If the output is forced beyond either supply,

extremely high currents will flow through those diodes,

which may result in damage to the device.

Input Protection

The LTC6228 has a pair of back to back diodes (D5 and

D7) to prevent the emitter base breakdown of the input

transistors and limit the differential input to ±700mV.

Unlike many other high performance amplifiers, the

bases of the input pair transistors Q1 and Q2 are not

connected to the pins through internal resistors to limit

input current, since doing so would cause the noise to

increase. For instance, a 100Ω resistor in series with

each input generates 1.8nV/√Hz of noise, and the total

amplifier noise voltage would rise from 0.88nV/√Hz to

2nV/√Hz. If the input differential voltage exceeds ±0.7V,

current conducted though the protection diodes D5 and

D7 should be limited to under 10mA. This implies 25Ω of

protectionresistanceperquartervolt(250mV)ofoverdrive

beyond ±0.7V. In addition, the input and shutdown pins

have reverse biased diodes connected to the supplies. The

current in these diodes must be limited to under 10mA.

The amplifiers should not be used as comparators or in

other open loop applications.

ESD

The LTC6228 family has reverse biased ESD protection

diodes on all inputs as shown in Figure 1. There is an

additional clamp between the positive and negative sup-

plies that further protects the device during ESD strikes.

Hot plugging of the device into a powered socket should

be avoided since this can trigger the clamp resulting in

larger currents flowing between the supply pins.

Capacitive Loads

Because the LTC6228/LTC6229 is designed for high

bandwidth applications, the output has not been designed

to drive capacitive loads directly. Load capacitance at the

output creates a non-dominant pole in the open loop fre-

quency response, worsening the phase margin. When

driving capacitive loads, a resistor of 10Ω to 100Ω should

be connected between the amplifier output and the capac-

itive load to avoid ringing or oscillation. The feedback

should be taken directly from the amplifier output. Higher

voltage gain configurations tend to have better capaci-

tive drive capability than lower gain configurations due

to lower closed loop bandwidth. The graphs titled Series

Output Resistor vs Capacitive Load demonstrate the tran-

sient response of the amplifier when driving capacitive

loads with various series resistors.

Feedback Components

When feedback resistors are used to set up gain, care

must be taken to ensure that the non-dominant pole

formed by the feedback resistors and the parasitic capaci-

tance at the inverting input does not degrade stability. For

example if the amplifier is set up in a gain of +2 configu-

ration with gain and feedback resistors of 1k, a parasitic

capacitance of 7pF (device + PC board) at the amplifier’s

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