AD8067 Data Sheet
Rev. B | Page 20 of 24
M 50ns CH1 288mV
CH1 200mV CH2 200mV
T
V
IN
V
OUT
Figure 55. Gain of 3 Pulse Response
SINGLE-SUPPLY OPERATION
The AD8067 is well suited for low voltage single-supply
applications, given its N-channel JFET input stage and rail-to-
rail output stage. It is fully specified for 5 V supplies. Successful
single-supply applications require attention to keep signal
voltages within the input and output headroom limits of the
amplifier. The input stage headroom extends to 1.7 V
(minimum) on a 5 V supply. The center of the input range is
0.85 V. The output saturation limit defines the hard limit of the
output headroom. This limit depends on the amount of current
the amplifier is sourcing or sinking, as shown in Figure 29.
Traditionally, an offset voltage is introduced in the input
network replacing ground as a reference. This allows the output
to swing about a dc reference point, typically midsupply.
Attention to the required headroom of the amplifier is
important, in this case, the required headroom from the
positive supply is 3 V; therefore, 1.5 V was selected as a
reference, which allows for a 100 mV signal at the input. Figure 56
shows the AD8067 configured for 5 V supply operation with a
reference voltage of 1.5 V. Capacitors C1 and C5 ac couple the
signal into and out of the amplifier and partially determine the
bandwidth of the input and output structures.
R1C1
BandwidthdB3–V
INPUT
π
=
2
1
C5R
BandwidthdB3–V
L
OUTPUT
π
=
2
1
Resistors R2 and R3 set a 1.5 V output bias point for the output
signal to swing about. It is critical to have adequate bypassing to
provide a good ac ground for the reference voltage. Generally,
the bandwidth of the reference network (R2, R3, and C2) is
selected to be one tenth that of the input bandwidth. This
ensures that any frequencies below the input bandwidth do not
pass through the reference network into the amplifier.
Reference network:
( )
C2R3R2
BandwidthdB3V
REF
||2
1
π
=−
+
Resistors R4 and R1 set the gain, in this case, an inverting gain
of 10 was selected. In this application, the input and output
bandwidths were set for approximately 10 Hz. The reference
network was set for a tenth of the input and output bandwidth,
at approximately 1 Hz.
AD8067
C4
0.1µF
C3
10µF
+5V
R4
2.7kΩ
R1
300Ω
R3
30kΩ
R2
70kΩ
R
L
1kΩ
C5
15µF
V
OUT
1
2
3
4
5
V
IN
C1
47µF
C2
6.8µF
+5V
Figure 56. Single-Supply Operation Schematic
HIGH GAIN, HIGH BANDWIDTH COMPOSITE
AMPLIFIER
The composite amplifier takes advantage of combining key
parameters that can otherwise be mutually exclusive of a
conventional single amplifier. For example, most precision
amplifiers have good dc characteristics but lack high speed ac
characteristics. Composite amplifiers combine the best of both
amplifiers to achieve superior performance over their single op
amp counterparts. The AD8067 and the AD8009 are well suited
for a composite amplifier circuit, combining dc precision with
high gain and bandwidth. The circuit runs off a ±5 V power
supply at approximately 20 mA of bias current. With a gain of
approximately 40 dB, the composite amplifier offers <1 pA
input current, a gain bandwidth product of 6.1 GHz, and a slew
rate of 630 V/µs.
