AD8515AKS Analog Devices, AD8515AKS Datasheet - Page 12

• Manufacturer Part Number: AD8515AKS
• Description: 1.8 V Low Power CMOS Rail-to-rail Input/output Single Operational Amp
• Manufacturer: Analog Devices

AD8515
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
for a given distortion. The FPBW of AD8515 is shown in Figure 3
to be close to 200 kHz.
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 MW 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.
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
1 F
C3
Figure 4. Micropower Voltage Reference Generator
0
0
0
0
0
0
0
0
0
0
R2
1M
R1
1M
Figure 3. Full Power Bandwidth
0
V
V
OUT
IN
0
3
2
FPBW
1.8V TO 5V
0
V+
V–
U1
AD8515
TIME (2 s/DIV)
0
10k
=
0.022 F
R3
1
2p
C2
0
¥
SR
V
PEAK
0
100
R4
0
0
0
0.9V TO 2.5V
C1
1 F
0
–12–
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.
where:
1 F
f
f
H
C3
Figure 6. Frequency Response of the Band-Pass Filter
L
H
0
is the low –3 db frequency.
is the high –3 db frequency.
is the midfrequency gain.
2
1
0
1M
1M
Figure 5. Second Order Band-Pass Filter
1k
R6
R8
0
VCC
400mV
10k
VCC
H
f
f
L
H
V11
0
2nF
C1
=
=
= +
100k
FREQUENCY (Hz)
2
=
1
2
p
5k
p
1 8
2k
R1
R5
¥
.
¥
R
R
R
R
1
V
2
1
1
1
1
¥
-
¥
1M
C
5
C
3
4
1
V
1
VCC
V+
V–
20k
10pF
0
R2
C6
U9
10M
AD8515
1
100M
REV. B
VOUT