AD8067

Rev. 0 | Page 21 of 24

AD8067

0.1

µF

10

µF

+5V

49.9k

Ω

50

Ω

49.9k

Ω

0.33pF

0.1

µF

10

µF

–5V

EPM 605 LL

–5V

0.33pF

NOTES

Using the AD8067 at Gains of Less Than 8

A common technique used to stabilize decompensated amplifiers is

to increase the noise gain, independent of the signal gain. The

AD8067 can be used for signal gains of less than 8, provided that

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 shown below.

1

3

1

R

R

Gain

Signal

+

=

1

3

1

R

R

Gain

Noise

+

=

Figure 51. Photodiode Preamplifier

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

The addition of resistor R2 modifies the noise gain equation, as

shown below. Note the signal gain equation has not changed.

FREQUENCY – MHz

100

65

70

75

80

85

90

95

60

0.01

0.1

1

10

100

2

1

3

1

R

||

R

R

Gain

Noise

+

=

AD8067

C2

0.1

µF

C1

10

µF

+5V

R2

50

Ω

C4

0.1

µF

C3

10

µF

–5V

R4

51

Ω

R3

600

Ω

1

5

4

3

2

R1

301

Ω

Figure 52. Photodiode Preamplifier Frequency Response

Figure 54. Gain of Less than 2 Schematic

M 50ns CH1 830mV

C1 RISE

31.2ns

C1 FALL

31.6ns

T

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 will be 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.

Figure 53. Photodiode Preamplifier Pulse Response