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APPLICATION NOTE
One Technology Way • P.O. Box 9106 • Norwood, MA 02062-9106 • Tel: 781/329-4700 • Fax: 781/326-8703 • www.analog.com
Use (Almost) Any Input Voltage Range with an AD7671, AD7665, or AD7663
PulSAR 16-Bit ADC
By Alain Guery
SUMMARY
The AD7671, AD7665, and AD7663 are specifi ed for six
analog input voltage ranges: ±10 V, ±5 V, ±2.5 V, 0 V to
10 V, 0 V to 5 V, and 0 V to 2.5 V. These ADCs, however,
can handle other input voltage ranges not described
in the data sheet. For instance, 0 V to 4 V, 0 V to 3.3 V,
±3.3 V, or 0 V to 24 V ranges are available; and these
ranges, as well others, may be more suitable for some
specifi c applications.
This application note assists the user in determining the
confi guration of the AD7671, AD7665, or AD7663 to use
the analog input range of preference.
Also, a software tool called “PulSAR input range” helps
the user optimize the input voltage range choice and
provides all the necessary information to use it. This
software can be downloaded from the application note
link from the AD7671, AD7665, and AD7663 product web
pages.
DISCUSSION
As described in Table I of the data sheet, the different
input voltage ranges are selected by connecting the four
separate input pins called IND, INC, INB, and INA to three
different signals comprising the analog signal itself (V
),
IN
the signal ground (GND), and the reference voltage (REF).
Obviously, these four inputs offer more combinations
than the six input ranges mentioned in the data sheet.
These four inputs connect to an internal resistive divider,
as shown in Figure 1. This resistive divider scales down
and shifts the external signals. The output of this resistive
divider is tied through the switch to the internal ADC
which always has a range from 0 V to REF (Cs).
AVDD
0V TO REF
RANGE HERE
IND
INC
INB
INA
AGND
4R
4R
2R
R
R = 1.28k⍀ FOR AD7665
or AD7663
R = 355.5⍀ FOR AD7671
R
1
C
s
Table I gives the range the user can obtain as a function
of the connection of the four inputs. Negative full-scale
and positive full-scale columns are the ratio between the
full-scale ends and the reference voltage. For instance,
by connecting the V
signal to both INA and IND, and
IN
grounding INB and INC, the input voltage range is 0 V
to 4 V with a 2.5 V reference. This table also provides the
input impedance of the ADC.
The input voltage range of the AD7671, AD7665, and
AD7663 can also be scaled by changing the reference
voltage itself. These ADCs work well with a reference
voltage as low as 2 V and up to AVDD – 1.85 V. Different
considerations can drive the reference value choice. For a
given input voltage range, the LSB value can be lowered,
which provides an improved analog resolution. Also, an
increase in the reference voltage reduces the weight of
the internal ADC noise sources.
Although reference voltage and pin confi guration allow
many possible input voltage ranges, some are limited
because of the absolute ratings of the ADC. Also, a high
reference voltage such as 3 V reduces the operating AVDD
power supply range to maintain a headroom of 1.85 V.
SOFTWARE
The best tool to calculate the optimum input voltage
range is the PulSAR input range software provided with
this application note. This software calculates the list of
input ranges that match the user criteria. The user criteria
are the desired reference voltage and the voltage range.
The results are sorted in the order of recommendation,
as shown in Figure 2.
This software runs under Windows
®
9x, Windows NT®,
and Windows 2000. To install it, run setup.exe and follow
the instructions.
Figure 2 gives a quick view of how to operate this
software.
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Figure 1. Simplifi ed Analog Input
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Ta ble I. Analog Voltage Input
Negative Full Scale Positive Full Scale IND INC INB INA AD7663/AD7665 Z
IN
AD7671 Z
IN
0.000 1.000 V
IN
0.000 1.143 GND V
–0.143 1.000 REF V
0.000 1.333 GND GND V
–0.167 1.167 GND REF V
–0.333 1.000 REF REF V
0.000 1.600 V
IN
–0.200 1.400 REF V
–0.400 1.200 GND V
–0.600 1.000 V
0.000 2.000 V
IN
IN
–0.250 1.750 REF GND GND V
–0.500 1.500 REF REF GND V
–0.750 1.250 GND REF REF V
–1.000 1.000 V
IN
0.000 2.667 GND V
–0.333 2.333 REF V
–1.333 1.333 GND V
–1.667 1.000 REF V
0.000 4.000 V
IN
–0.500 3.500 REF GND V
–1.000 3.000 V
–2.000 2.000 V
IN
IN
–2.500 1.500 REF GND V
–3.000 1.000 V
0.000 8.000 V
–1.000 7.000 V
–2.000 6.000 V
–3.000 5.000 V
–4.000 4.000 V
–5.000 3.000 V
–6.000 2.000 V
–7.000 1.000 V
IN
IN
IN
IN
IN
IN
IN
IN
IN
V
IN
IN
IN
GND GND V
IN
IN
REF REF V
V
IN
V
IN
IN
IN
IN
IN
V
IN
V
IN
V
IN
V
IN
V
IN
V
IN
V
IN
IN
IN
IN
GND V
REF V
V
IN
V
IN
V
IN
V
IN
V
IN
V
IN
V
IN
V
IN
V
IN
V
IN
V
IN
V
IN
IN
IN
IN
IN
00
5851 1625
5851 1625
3413 948
3413 948
3413 948
2731 758
2731 758
2731 758
2731 758
GND 2560 711
IN
IN
IN
2560 711
2560 711
2560 711
REF 2560 711
GND 2731 758
GND 2731 758
REF 2731 758
REF 2731 758
GND GND 3413 948
IN
GND 3413 948
REF GND 3413 948
GND REF 3413 948
IN
REF 3413 948
REF REF 3413 948
GND GND GND 5851 1625
REF GND GND 5851 1625
GND REF GND 5851 1625
REF REF GND 5851 1625
GND GND REF 5851 1625
REF GND REF 5851 1625
GND REF REF 5851 1625
REF REF REF 5851 1625
Multiply the values in the negative and positive full-scale column by the reference voltage to determine the actual analog voltage range.
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Step 1. The Run button starts the software. This button is
pressed fi rst.
Step 2. Enter the reference voltage and the full scale
range that you want to use.
Step 3. This table displays all the possible input ranges
compatible with the necessary criteria. The results are
sorted in the order of recommendation.
Figure 2. PulSAR Input Range Software Front Panel
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Step 4. Different commands may be chosen from
this menu.
Save or F5 allows the user to save the table of results in
an Excel
spreadsheet.
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E02982–0–1/03(A)PRINTED IN U.S.A.
© 2003 Analog Devices, Inc. All rights reserved. Trademarks and registered trademarks are the property of their respective companies.
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