LT6222IGN datasheet(15/20 Pages) LINER

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LT6220/LT6221/LT6222

sn622012 622012fs

severely overdriven, an external resistor should be used to

limit the overdriven current.

The LT6220/LT6221/LT6222’s input stages are also pro-

tected against a large differential input voltage of 1.4V or

higher by a pair of back-to-back diodes, D5/D8, to prevent

the emitter-base breakdown of the input transistors. The

current in these diodes should be limited to less than

10mA when they are active. The worse-case differential

input voltage usually occurs when the input is driven while

the output is shorted to ground in a unity-gain configura-

tion. In addition, the amplifier is protected against ESD

strikes up to 3kV on all pins by a pair of protection diodes

on each pin that are connected to the power supplies as

shown in Figure 1.

Capacitive Load

The LT6220/LT6221/LT6222 are optimized for high band-

width, low power and precision applications. They can

drive a capacitive load up to 100pF in a unity-gain configu-

ration and more for higher gain. When driving a larger

APPLICATIO S I FOR ATIO

capacitive load, a resistor of 10Ω to 50Ω should be

connected between the output and the capacitive load to

avoid ringing or oscillation. The feedback should still be

taken from the output so that the resistor will isolate the

capacitive load to ensure stability. Graphs on capacitive

loads show the transient response of the amplifier when

driving capacitive load with specified series resistors.

Feedback Components

When feedback resistors are used to set up gain, 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 LT6220/

LT6221/LT6222, set up with a noninverting gain of 2, two

5k resistors and a capacitance of 5pF (part plus PC board),

will probably oscillate. The pole is formed at 12.7MHz that

will reduce phase margin by 52 degrees when the cross-

over frequency of the amplifier is around 10MHz. A capaci-

tor of 10pF or higher connecting across the feedback

resistor will eliminate any ringing or oscillation.

TYPICAL APPLICATIO S

Stepped-Gain Photodiode Amplifier

The circuit of Figure 2 is a stepped gain transimpedance

photodiode amplifier. At low signal levels, the circuit has

a high 100kΩ gain, but at high signal levels the circuit

automatically and smoothly changes to a low 3.2kΩ gain.

The benefit of a stepped gain approach is that it maximizes

dynamic range, which is very useful on limited supplies.

Put another way, in order to get 100kΩ sensitivity and still

handle a 1mA signal level without resorting to gain reduc-

tion, the circuit would need a 100V negative voltage

supply.

The operation of the circuit is quite simple. At low photo-

diode currents (below 10µA) the output and inverting

input of the op amp will be no more than 1V below ground.

The LT1634 in parallel with R3 and Q2 keep a constant

current though Q2 of about 20µA. R4 maintains quiescent

current through the LT1634 and pulls Q2’s emitter above

ground, so Q1 is reverse biased and no current flows

through R2. So for small signals, the only feedback path

is R1 (and C1) and the circuit is a simple transimpedance

amplifier with 100kΩ gain.

Figure 2. Stepped-Gain Photodiode Amplifier

–

IPD

PHOTODIODE

~4pF

VS+

VS–

LT6220

100k

3.24k

33k

10k

1pF

30pF

PHILIPS

BCV62

LT1634-1.25

VOUT

VS = ±1.5V TO ±5V

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