AD8631 データシートの表示（PDF） - Analog Devices
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AD8631 Datasheet PDF : 12 Pages
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AD8631/AD8632
Table II. Snubber Network Values for Large Capacitive Loads
CLOAD
600 pF
1 nF
10 nF
Rx
300 Ω
300 Ω
90 Ω
Cx
1 nF
1 nF
8 nF
TOTAL HARMONIC DISTORTION + NOISE
The AD863x family offers a low total harmonic distortion, which
makes this amplifier ideal for audio applications. Figure 27 shows
a graph of THD + N, which is ~0.02% @ 1 kHz, for a 1.8 V supply.
At unity gain in an inverting configuration the value of the Total
Harmonic Distortion + Noise stays consistently low over all volt-
ages supply ranges.
10
INVERTING
AV = 1
1
0.1
VS = 1.8V
0.01
VS = 5V
0.001
10
100
1k
FREQUENCY – Hz
10k 20k
Figure 27. THD + N vs. Frequency Graph
AD8632 Turn-On Time
The low voltage, low power AD8632 features an extraordinary turn
on time. This is about 500 ns for VSY = 5 V, which is impressive
considering the low supply current (300 µA typical per amplifier).
Figure 28 shows a scope picture of the AD8632 with both channels
configured as followers. Channel A has an input signal of 2.5 V and
channel B has the input signal at ground. The top waveform shows
the supply voltage and the bottom waveform reflects the response
of the amplifier at the output of Channel A.
0
VS = 5V
0
AV = 1
VIN = 2.5V STEP
0V0
0
0
0V
0
0
TIME – 200ns/DIV
Figure 28. AD8632 Turn-On Time
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 28 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 a bypass capacitor to be
connected at the reference output. This bypass capacitor helps
establish an ac ground for the reference output.
1.8V TO 5V
10k⍀
0.022F
1M⍀
1M⍀
AD8631
1F
100⍀
1F
VREF
0.9V TO 2.5V
Figure 29. A Micropower Reference Voltage Generator
MICROPHONE PREAMPLIFIER
The AD8631 is ideal to use as a microphone preamplifier.
Figure 30 shows this implementation.
1.8V
R1
2.2k⍀
VIN
C1 R2
0.1F 22k⍀
ELECTRET
MIC
R3
220k⍀
1.8V
AD8631
VREF = 0.9V
AV
= R3
R2
VOUT
Figure 30. A Microphone Preamplifier
R1 is used to bias an electret microphone and C1 blocks dc voltage
from the amplifier. The magnitude of the gain of the amplifier is
approximately R3/R2 when R2 ≥ 10 ϫ R1. VREF should be equal to
1/2 ϫ 1.8 V for maximum voltage swing.
Direct Access Arrangement for Telephone Line Interface
Figure 31 illustrates a 1.8 V transmit/receive telephone line interface
for 600 Ω transmission systems. It allows full duplex transmission of
signals on a transformer-coupled 600 Ω line in a differential manner.
Amplifier A1 provides gain that can be adjusted to meet the modem
output drive requirements. Both A1 and A2 are configured to apply
the largest possible signal on a single supply to the transformer.
Amplifier A3 is configured as a difference amplifier for two reasons:
(1) It prevents the transmit signal from interfering with the receive
signal and (2) it extracts the receive signal from the transmission line
for amplification by A4. A4’s gain can be adjusted in the same
manner as A1’s to meet the modem’s input signal requirements.
Standard resistor values permit the use of SIP (Single In-line
Package) format resistor arrays. Couple this with the AD8631/
REV. 0
–11–
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