LT6204IGN datasheet(16/24 Pages) LINER

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LT6202/LT6203/LT6204

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Input Protection

There are back-to-back diodes, D1 and D2, across the

+ and – inputs of these amplifiers to limit the differential

input voltage to

±0.7V. The inputs of the LT6202/LT6203/

LT6304 do not have internal resistors in series with the

input transistors. This technique is often used to protect

the input devices from over voltage that causes excessive

currents to flow. The addition of these resistors would

significantly degrade the low noise voltage of these ampli-

fiers. For instance, a 100

Ω resistor in series with each

input would generate 1.8nV/

√Hz of noise, and the total

amplifier noise voltage would rise from 1.9nV/

√Hz to

2.6nV/

√Hz. Once the input differential voltage exceeds

±0.7V, steady state current conducted though the protec-

tion diodes should be limited to

±40mA. This implies 25Ω

of protection resistance per volt of continuous overdrive

beyond

±0.7V. The input diodes are rugged enough to

handle transient currents due to amplifier slew rate over-

drive or momentary clipping without these resistors.

Figure 2 shows the input and output waveforms of the

amplifier driven into clipping while connected in a gain of

AV = 1. When the input signal goes sufficiently beyond the

power supply rails, the input transistors will saturate.

When saturation occurs, the amplifier loses a stage of

phase inversion and the output tries to change states.

Diodes D1 and D2 forward bias and hold the output within

a diode drop of the input signal. In this photo, the input

signal generator is clipping at

±35mA, and the output

transistors supply this generator current through the

protection diodes.

With the amplifier connected in a gain of AV ≥ 2, the output

can invert with very heavy input overdrive. To avoid this

inversion, limit the input overdrive to 0.5V beyond the

power supply rails.

ESD

The LT6202/LT6203/LT6204 have reverse-biased ESD

protection diodes on all inputs and outputs as shown in

Figure 1. If these pins are forced beyond either supply,

unlimited current will flow through these diodes. If the

current is transient and limited to one hundred milliamps

or less, no damage to the device will occur.

Noise

The noise voltage of the LT6202/LT6203/LT6204 is equiva-

lent to that of a 225

Ω resistor, and for the lowest possible

noise it is desirable to keep the source and feedback

resistance at or below this value, i.e. RS + RG||RFB ≤ 225Ω.

With RS + RG||RFB = 225Ω the total noise of the amplifier

is: en = √(1.9nV)2 + (1.9nV)2 = 2.7nV. Below this resistance

value, the amplifier dominates the noise, but in the resis-

tance region between 225

Ω and approximately 10kΩ, the

noise is dominated by the resistor thermal noise. As the

total resistance is further increased, beyond 10k, the noise

current multiplied by the total resistance eventually domi-

nates the noise.

The product of en • √ISUPPLY is an interesting way to gauge

low noise amplifiers. Many low noise amplifiers with low

en have high ISUPPLY current. In applications that require

low noise with the lowest possible supply current, this

product can prove to be enlightening. The LT6202/LT6203/

LT6204 have an en, √ISUPPLY product of 3.2 per amplifier,

yet it is common to see amplifiers with similar noise

specifications have an en • √ISUPPLY product of 4.7 to 13.5.

For a complete discussion of amplifier noise, see the

LT1028 data sheet.

Figure 2. VS = ±2.5V, AV = 1 with Large Overdrive

LT6202/03/04 F02

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