Data Sheet
AD7656-1/AD7657-1/AD7658-1
The AD7656-1/AD7657-1/AD7658-1 each contain three on-
chip reference buffers as shown in Figure 27. Each of the three
ADC pairs has an associated reference buffer. These reference
buffers require external decoupling capacitors, using 1 µF
capacitors, on the REFCAPA, REFCAPB, and REFCAPC pins.
The internal reference buffers can be disabled in software mode
by writing to Bit DB8 in the internal control register. If a serial
interface is selected, the internal reference buffers can be disabled
in hardware mode by setting the DB14/REFBUFEN/DIS pin high. If
the internal reference and its buffers are disabled, apply an
external buffered reference to the REFCAPx pins.
ADC TRANSFER FUNCTION
The output coding of the AD7656-1/AD7657-1/AD7658-1 is
twos complement. The designed code transitions occur midway
between successive integer LSB values, that is, 1⁄2 LSB, 3⁄2 LSB.
The LSB size is FSR/65,536 for the AD7656-1, FSR/16,384 for
the AD7657-1, and FSR/4096 for the AD7658-1. The ideal
transfer characteristic is shown in Figure 26.
011 ... 111
011 ... 110
000 ... 001
000 ... 000
111 ... 111
REFCAPA
SAR
SAR
BUF
BUF
BUF
REFIN/
100 ... 010
100 ... 001
100 ... 000
AGND – 1LSB
REFOUT
REF
SAR
SAR
–FSR/2 + 1/2LSB
+FSR/2 – 3/2LSB
ANALOG INPUT
REFCAPB
Figure 26. AD7656-1/AD7657-1/AD7658-1 Transfer Characteristic
SAR
SAR
The LSB size is dependent on the analog input range selected
(see Table 9).
REFCAPC
INTERNAL/EXTERNAL REFERENCE
Figure 27. Reference Circuit
The REFIN/REFOUT pin allows access to the 2.5 V reference of
the AD7656-1/AD7657-1/AD7658-1, or it allows an external
reference to be connected to provide the reference source for
conversions.
TYPICAL CONNECTION DIAGRAM
Figure 28 shows the typical connection diagram for the AD7656-1/
AD7657-1/AD7658-1, illustrating the reduction in the number
and value of decoupling capacitors that are required. There are
eight AVCC supply pins on each part. The AVCC supplies are the
supplies used for the AD7656-1/AD7657-1/AD7658-1 conversion
process; therefore, they should be well decoupled. The AVCC supply
which is applied to eight AVCC pins can be decoupled using just one
1 µF capacitor. The AD7656-1/AD7657-1/AD7658-1 can operate
with the internal reference or an externally applied reference. In
this configuration, the parts are configured to operate with the
external reference. The REFIN/REFOUT pin is decoupled with
a 1 µF capacitor. The three internal reference buffers are enabled.
Each of the REFCAPx pins is decoupled with a 1 µF capacitor.
The AD7656-1/AD7657-1/AD7658-1 can each accommodate a
2.5 V external reference. When applying an external reference via
the REFIN/REFOUT pin, the internal reference must be disabled
and the reference buffers must be enabled. Alternatively, an external
refernce can be applied via the REFCAPx pins, in which case the
internal reference should be disabled and it is recommended
to disable the reference buffers to save power and minimize
crosstalk. After a reset, the AD7656-1/AD7657-1/AD7658-1
default to operating in external reference mode with the internal
reference disabled and the reference buffers enabled.
The internal reference can be enabled in either hardware or
software mode. To enable the internal reference in hardware mode,
If the same supply is being used for the AVCC and DVCC supplies,
place a ferrite or small RC filter between the supply pins.
H
set the /S SEL pin to 0 and the REFEN/DIS pin to 1. To enable the
AGND pins are connected to the AGND plane of the system.
The DGND pins are connected to the digital ground plane in
the system. Connect the AGND and DGND planes together at
one place in the system. This connection should be as close as
possible to the AD7656-1/AD7657-1/AD7658-1 in the system.
H
internal reference in software mode, set /S SEL to 1 and write to
the control register to set DB9 of the register to 1. For the internal
reference mode, decouple the REFIN/REFOUT pin using a 1 µF
capacitor.
Table 9. LSB Size for Each Analog Input Range
Input Range for AD7656-1
Input Range for AD7657-1
Input Range for AD7658-1
Parameter
LSB Size
FS Range
10 V
5 V
10 V
5 V
10 V
5 V
0.305 mV
20 V/65,536
0.152 mV
10 V/65,536
1.22 mV
20 V/16,384
0.610 mV
10 V/16,384
4.88 mV
20 V/4096
2.44 mV
10 V/4096
Rev. D | Page 21 of 32
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