REV. B

AD8515

–12–

Full Power Bandwidth

The slew rate of an amplifier determines the maximum frequency

at which it can respond to a large input signal. This frequency

(known as full power bandwidth, FPBW) can be calculated

from the equation

FPBW

SR

V

PEAK

=

¥

2

p

for a given distortion. The FPBW of AD8515 is shown in Figure 3

to be close to 200 kHz.

TIME (2 s/DIV)

0

0

0

00

0

0000

0

000

0

0

0

0

0

0

Figure 3. Full Power Bandwidth

A MICROPOWER REFERENCE VOLTAGE GENERATOR

Many single-supply circuits are configured with the circuit biased

to one-half of the supply voltage. In these cases, a false ground

reference can be created by using a voltage divider buffered by

an amplifier. Figure 4 shows the schematic for such a circuit. The

two 1 M

W resistors generate the reference voltages while drawing

only 0.9

mA of current from a 1.8 V supply. A capacitor connected

from the inverting terminal to the output of the op amp provides

compensation to allow for a bypass capacitor to be connected at

the reference output. This bypass capacitor helps establish an ac

ground for the reference output.

0.9V TO 2.5V

AD8515

1

3

2

C2

0.022 F

R4

100

C1

1 F

V–

V+

1.8V TO 5V

U1

R2

1M

C3

1 F

R1

1M

R3

10k

Figure 4. Micropower Voltage Reference Generator

A 100 kHz Single-Supply Second Order Band-Pass Filter

The circuit in Figure 5 is commonly used in portable applications

where low power consumption and wide bandwidth are required.

This figure shows a circuit for a single-supply band-pass filter

with a center frequency of 100 kHz. It is essential that the op

amp has a loop gain at 100 kHz in order to maintain an accurate

center frequency. This loop gain requirement necessitates the

choice of an op amp with a high unity gain crossover frequency,

such as the AD8515. The 4.5 MHz bandwidth of the AD8515

is sufficient to accurately produce the 100 kHz center frequency,

as the response in Figure 6 shows. When the op amp’s bandwidth

is close to the filter’s center frequency, the amplifier’s internal

phase shift causes excess phase shift at 100 kHz, which alters

the filter’s response. In fact, if the chosen op amp has a bandwidth

close to 100 kHz, the phase shift of the op amps will cause the

loop to oscillate.

A common-mode bias level is easily created by connecting the

noninverting input to a resistor divider consisting of two resistors

connected between VCC and ground. This bias point is also

decoupled to ground with a 1

mF capacitor.

f

RC

f

RC

H

R

R

VCC

V

V

L

H

=

¥¥

=

¥¥

=+

=-

1

21

1

1

21

1

1

2

1

18

5

0

p

p

.

where:

VOUT

AD8515

1

3

4

C6

10pF

V–

V+

VCC

U9

R6

1M

R8

1M

R2

20k

R5

2k

R1

5k

C1

2nF

V11

400mV

VCC

C3

1 F

0

0

Figure 5. Second Order Band-Pass Filter

FREQUENCY (Hz)

2

0

1k

100M

10k

100k

1M

10M

1

Figure 6. Frequency Response of the Band-Pass Filter