AD8515

Rev. D | Page 14 of 16

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 35 shows the schematic for such a circuit.

The two 1 MΩ resistors generate the reference voltages while

drawing only 0.9 μA 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.

AD8515

C3

1µF

C2

0.022µF

C1

1µF

R3

10kΩ

R2

1MΩ

R4

100Ω

1.8V TO 5V

0.9V TO 2.5V

R1

1MΩ

3

4

1

U1

V+

V–

+

–

Figure 35. Micropower Voltage Reference Generator

A 100 kHz SINGLE-SUPPLY SECOND-ORDER

BAND-PASS FILTER

The circuit in Figure 36 is commonly used in portable applica-

tions 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 have a loop gain at 100 kHz 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 37 shows. When the op amp bandwidth is close to the

center frequency of the filter, the amplifier internal phase shift

causes excess phase shift at 100 kHz, altering the filter response.

In fact, if the chosen op amp has a bandwidth close to 100 kHz,

the phase shift of the op amps causes 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 μF capacitor.

C1

R1

f

L

×

×

π

=

2

1

C1

R1

f

H

×

×

π

=

2

1

R2

R1

H

+

= 1

0

V

5

V

8

.

1

−

=

VCC

where:

is the low −3 dB frequency.

is the high −3 dB frequency.

is the midfrequency gain.

400mV

V11

VCC

VCC

VOUT

0

0

R6

1MΩ

R8

1MΩ

C3

1µF

C1

2nF

R1

5kΩ

R2

20kΩ

R5

2kΩ

AD8515

3

4

1

U9

V+

V–

+

–

C6

10pF

Figure 36. Second-Order Band-Pass Filter

2

0

1k

10k

100M

FREQUENCY (Hz)

100k

1M

10M

1.5

1

0.5

Figure 37. Frequency Response of the Band-Pass Filter