AD8515 Analog Devices, AD8515 Datasheet - Page 14

• Manufacturer Part Number: AD8515
• Description: 1.8 V Low Power CMOS Rail-to-Rail Input/Output Operational Amplifier
• Manufacturer: Analog Devices

Specifications of AD8515

• -3db Bandwidth: 5MHz
• Slew Rate: 2.7V/µs
• Vos: 1mV
• Ib: 2pA
• # Opamps Per Pkg: 1
• Input Noise (nv/rthz): 22nV/rtHz
• Vcc-vee: 1.8V to 6V
• Isy Per Amplifier: 500µA
• Packages: SC70,SOT

AD8515
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.
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.
1µF
C3
Figure 35. Micropower Voltage Reference Generator
R2
1MΩ
R1
1MΩ
3
4
+
–
1.8V TO 5V
V+
V–
U1
AD8515
0.022µF
10kΩ
1
R3
C2
100Ω
R4
C1
1µF
0.9V TO 2.5V
Rev. D | Page 14 of 16
where:
f
f
H
1µF
L
H
C3
0
is the low −3 dB frequency.
is the high −3 dB frequency.
is the midfrequency gain.
VCC
H
f
f
L
H
0
1.5
0.5
=
1MΩ
1MΩ
=
= 1
2
1
0
1k
Figure 37. Frequency Response of the Band-Pass Filter
R6
R8
2
0
=
2
VCC
π
π
+
1
×
8 .
×
R2
R1
400mV
Figure 36. Second-Order Band-Pass Filter
R1
R1
1
V
1
−
×
10k
×
C1
5
C1
V
V11
2nF
C1
FREQUENCY (Hz)
100k
2kΩ
5kΩ
R5
R1
1M
3
4
+
–
VCC
V+
V–
20kΩ
10pF
R2
C6
10M
U9
0
AD8515
1
100M
VOUT