Datasheet

Data Sheet AD8067
Rev. B | Page 19 of 24
Test data for the preamp is shown in Figure 52 and Figure 53.
FREQUENCY – MHz
TRANSIMPEDANCE GAIN – dB
100
65
70
75
80
85
90
95
60
0.01 0.1 1 10 100
Figure 52. Photodiode Preamplifier Frequency Response
M 50ns CH1 830mVCH1 500mV
C1 RISE
31.2ns
C1 FALL
31.6ns
T
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:
R1
R3
GainSignal +=1
R1
R3
GainNoise +=1
The addition of resistor R2 modifies the noise gain equation.
Note the signal gain equation has not changed.
R2R1
R3
GainNoise
||
1+=
AD8067
C2
0.1µF
C1
10
µF
+5V
R2
50Ω
C3
10µF
C4
0.1µF
–5V
V
OUT
R
L
R4
51Ω
R3
600Ω
V
IN
1
5
4
3
2
R1
301Ω
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.