AD8067

Data Sheet

Rev. B | Page 20 of 24

M 50ns CH1

288mV

CH1 200mV

CH2 200mV

T

VIN

VOUT

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

Bandwidth

dB

3

–

VINPUT

π

=

2

1

C5

R

Bandwidth

dB

3

–

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:

(

)C2

R3

R2

Bandwidth

dB

3

V 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

0.1

µF

C3

10

µF

+5V

R4

2.7k

Ω

R1

300

Ω

R3

30k

Ω

R2

70k

Ω

RL

1k

Ω

C5

15

µF

VOUT

1

2

3

4

5

VIN

C1

47

µF

C2

6.8

µF

+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.