a
AN-560
APPLICATION NOTE
One Technology Way • P.O. Box 9106 • Norwood, MA 02062-9106 • 781/329-4700 • World Wide Web Site: http://www.analog.com
Low Voltage Amplifier
By Olivier Betancourt
BATTERY VOLTAGE DISCHARGE
The AD8517 operates at supply voltages as low as 1.8 V.
This amplifier is ideal for battery-powered applications
since it can operate at the end of discharge voltage of
most popular batteries. Table I lists the Nominal and
End of Discharge Voltages of several typical batteries.
Table I. Typical Battery Life Voltage Range
Battery
Nominal
Voltage (V)
End-of-Voltage
Discharge (V)
Lead-Acid
Lithium-Ion
NiMH
NiCd
Carbon-Zinc
2
2.6–3.6
1.2
1.2
1.5
1.8
1.7–2.4
1
1
1.1
The rail-to-rail feature of the AD8517 can be observed
over all voltage supply ranges for which the part is being
specified, which is from 1.8 V to 5 V.
TOTAL HARMONIC DISTORTION + NOISE
The AD85x7 family offers a low total harmonic distortion, which makes this amplifier ideal for audio applications. Figure 2 shows a graph of THD + N; for a VS > 3 V,
the THD + N is about 0.001% and 0.03% for VS =1.8 V in a
noninverting configuration with a gain of 1. However,
with an inverting configuration, the THD + N measures
0.001% for all specified supply voltage ranges.
1
0.1
0.01
VS > 3V TO 5V
0.001
0.0001
10
100
1k
FREQUENCY – Hz
10k
20k
Figure 2. THD + N vs. Frequency Graph
VS = 60.9V
VIN = 1.8 V p-p
VIN
VOUT
TIME – 200ms/Div
Figure 1. Rail-to-Rail Input Output
REV. 0
VS = 1.8V
THD + N – %
RAIL-TO-RAIL INPUT AND OUTPUT
The AD8517 features an extraordinary rail-to-rail input
and output with supply voltages as low as 1.8 V. With
the amplifier’s supply range set to 1.8 V, the commonmode voltage can be set to 1.8 V p-p, allowing the output to swing to both rails without clipping. Figure 1
shows a scope picture of both input and output taken at
unity gain, with a frequency of 22 kHz, at VS = 1.8 V and
VIN = 1.8 V p-p.
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 3
shows the schematic for such a circuit. The two 1 MΩ
resistors generate the reference voltages while drawing
only 900 nA 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.
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 AD8517’s 5-lead SOT-23
package or the AD8527’s 8-lead MSOP, and 8-lead SOIC
footprint, and this circuit offers a compact solution.
1.8V TO 5V
10kV
0.022mF
1MV
AD8517
100V
VREF
0.9V TO 2.5V
1mF
1MV
1mF
Figure 3. A Micropower Reference Voltage Generator
MICROPHONE PREAMPLIFIER
The AD8517 is ideal to use as a microphone preamplifier. Figure 4 shows this implementation. The gain of the
amplifier is set as R3/R2. R1 is used to bias an electret
microphone and C1 block dc voltage from the amplifier.
C1
0.1mF
R2
10kV
6.2V
R5
R6
10kV 10kV
T1
MIDCOM
671-8005
7
R9
10kV
VREF
R11
10kV
R12
10kV
A2
R7
10kV
5
1/2
AD8517
A3
1
1/2
AD8527
R8
10kV
10mF
R10
10kV
2
3
DIRECT ACCESS ARRANGEMENT FOR TELEPHONE LINE
INTERFACE
Figure 5 illustrates a 1.8 V transmit/receive telephone
line interface for 600 Ω transmission systems. It allows
full duplex transmission of signals on a transformercoupled 600 Ω line in a differential manner. Amplifier A1
provides gain that can be adjusted to meet the modem
+1.8V DC
6
VOUT
Figure 4. A Microphone Preamplifier
TRANSMIT
TxA
3
1/2
AD8517
VCC
AD8517
ELECTRET
MIC
A1
C1
0.1mF
R1
10kV
2
1
6.2V
ZO
600V
R2
9.09kV
2kV
R3
360V
TO
TELEPHONE
LINE 1:1
R3
100kV
VCC
R1
2.2kV
P1
Tx GAIN
ADJUST
E3760–3–1/00 (rev. 0)
AN-560
R13
10kV
R14
14.3kV
2kV
6
5
P2
Rx GAIN
ADJUST
A4
7
C2
0.1mF
RECEIVE
RxA
1/2
AD8527
PRINTED IN U.S.A.
Figure 5. Single-Supply Direct Access Arrangement for
Modems
–2–
REV. 0