Analog Devices AN560 Application Notes

AN-560

FREQUENCY – Hz

0.01

0.0001
10 20k

THD + N – %

0.001

0.1

1

100 1k 10k

VS > 3V TO 5V

VS = 1.8V

a

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

Nominal End-of-Voltage

Battery Voltage (V) Discharge (V)

Lead-Acid 2 1.8
Lithium-Ion 2.6–3.6 1.7–2.4
NiMH 1.2 1
NiCd 1.2 1
Carbon-Zinc 1.5 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 V
the THD + N is about 0.001% and 0.03% for V
noninverting configuration with a gain of 1. However,
with an inverting configuration, the THD + N measures

0.001% for all specified supply voltage ranges.

> 3 V,

S

=1.8 V in a

S

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 V
V

= 1.8 V p-p.

IN

VS = 60.9V
VIN = 1.8 V p-p

V

IN

V

OUT

TIME – 200ms/Div

= 1.8 V and

S

Figure 1. Rail-to-Rail Input Output

Figure 2. THD + N vs. Frequency Graph

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.

REV. 0

AN-560

1.8V TO 5V

10kV

0.022mF

1MV

1mF1MV

AD8517

100V

1mF

V

REF

0.9V TO 2.5V

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.

R3

100kV

V

CC

AD8517

V

OUT

2.2kV

ELECTRET

MIC

V

CC

R1

C1

R2

0.1mF

10kV

V

REF

Figure 4. A Microphone Preamplifier

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 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 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.

P1

Tx GAIN

TO

TELEPHONE

LINE

Z

O

600V

MIDCOM
671-8005

1:1

ADJUST

R3

360V

6.2V

6.2V

R9
10kV

2

3

10kV

AD8517

R10

10kV

A3

AD8527

T1

R11
10kV

R12

10kV

2kV

1

1/2

AD8517

R5

7

1/2

1

1/2

9.09kV

A1

R6

10kV

A2

R2

6

5

R13

10kV

C1

R1

0.1mF

10kV

2
3

+1.8V DC

10mF

Rx GAIN
ADJUST

R14

14.3kV
2kV

6

7

A4

5

1/2

AD8527

TRANSMIT
TxA

R7
10kV

R8
10kV

P2

RECEIVE
RxA

C2

0.1mF

Figure 5. Single-Supply Direct Access Arrangement for
Modems

E3760–3–1/00 (rev. 0)

–2–

PRINTED IN U.S.A.

REV. 0