AD8067 datasheet(19/24 Pages) AD

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Data Sheet

AD8067

Rev. B | Page 19 of 24

Test data for the preamp is shown in Figure 52 and Figure 53.

FREQUENCY – MHz

100

0.01

0.1

100

Figure 52. Photodiode Preamplifier Frequency Response

M 50ns CH1

830mV

CH1 500mV

C1 RISE

31.2ns

C1 FALL

31.6ns

Figure 53. Photodiode Preamplifier Pulse Response

USING THE AD8067 AT GAINS OF LESS THAN 8

A common technique used to stabilize de-compensated

amplifiers is to increase the noise gain, independent of the

signal gain. The AD8067 can be used in applications where the

signal gain is less than 8, if proper care is taken to ensure that

the noise gain of the amplifier is set to at least the recommended

minimum signal gain of 8 (see Figure 54).

The signal and noise gain equations for a noninverting

amplifier are:

Gain

Signal

= 1

Gain

Noise

The addition of resistor R2 modifies the noise gain equation.

Note the signal gain equation has not changed.

Gain

Noise

1 +

AD8067

0.1

µF

+5V

Ω

µF

0.1

µF

–5V

VOUT

Ω

600

Ω

VIN

301

Ω

Figure 54. Gain = 3 Schematics

This technique allows the designer to use the AD8067 in gain

configurations of less than 8. The drawback to this type of

compensation is that the input noise and offset voltages are

also amplified by the value of the noise gain. In addition, the

distortion performance is degraded. To avoid excessive

overshoot and ringing when driving a capacitive load, the

AD8067 should be buffered by a small series resistor; in this

case, a 51 Ω resistor was used.

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