LT6235IGN datasheet(17/24 Pages) LINER

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623345f

LT6233/LT6233-10/

LT6234/LT6235

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 LT6233/LT6234/

LT6235 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

current 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 through the protec-

tion diodes should be limited to

±40mA. This implies 25Ω

of protection resistance is necessary per volt of overdrive

beyond

±0.7V. These input diodes are rugged enough to

APPLICATIO S I FOR ATIO

Amplifier Characteristics

Figure 1 is a simplified schematic of the LT6233/LT6234/

LT6235, which has a pair of low noise input transistors Q1

and Q2. A simple current mirror Q3/Q4 converts the

differential signal to a single-ended output, and these

transistors are degenerated to reduce their contribution to

the overall noise.

Capacitor C1 reduces the unity cross frequency and

improves the frequency stability without degrading the

gain bandwidth of the amplifier. Capacitor CM sets the

overall amplifier gain bandwidth. The differential drive

generator supplies current to transistors Q5 and Q6 that

swing the output from rail-to-rail.

handle transient currents due to amplifier slew rate over-

drive and clipping without protection resistors.

The photo of Figure 2 shows the output response to an

input overdrive with the amplifier connected as a voltage

follower. With the input signal low, current source I1

saturates and the differential drive generator drives Q6

into saturation so the output voltage swings all the way to

V–. The input can swing positive until transistor Q2 satu-

rates into current mirror Q3/Q4. When saturation occurs,

the output tries to phase invert, but diode D2 conducts

current from the signal source to the output through the

feedback connection. The output is clamped a diode drop

below the input. In this photo, the input signal generator

is limiting at about 20mA.

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

can invert with very heavy overdrive. To avoid this inver-

sion, limit the input overdrive to 0.5V beyond the power

supply rails.

ESD

The LT6233/LT6234/LT6235 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 LT6233/LT6234/LT6235 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.

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

500

µs/DIV

623345 F02

–2.5V

2.5V

Figure 1. Simplified Schematic

ENABLE

DESD6

DESD5

–V

+VIN

–VIN

623345 F01

BIAS

DIFFERENTIAL

DRIVE GENERATOR

VOUT

–V

DESD3

–V

DESD4

DESD1

–V

DESD2

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