Analog Devices AN594-a Application Notes

AN-594

APPLICATION NOTE

One Technology Way • P.O. Box 9106 • Norwood, MA 02062-9106 • Tel: [phone] • Fax: [phone] • 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.

REV. A

Figure 1. Simplifi ed Analog Input

AN-594

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.

–2–

REV. A