AD7656-1 Analog Devices, AD7656-1 Datasheet - Page 21

• Manufacturer Part Number: AD7656-1
• Description: 250 kSPS, 6-Channel, Simultaneous Sampling, Bipolar 16-Bit ADC
• Manufacturer: Analog Devices

Specifications of AD7656-1

• Resolution (bits): 16bit
• # Chan: 6
• Sample Rate: 250kSPS
• Interface: Par,Ser,SPI
• Analog Input Type: SE-Bip
• Ain Range: 10V p-p,20 V p-p,Bip 10V,Bip 5.0V
• Adc Architecture: SAR
• Pkg Type: QFP

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.
The LSB size is dependent on the analog input range selected
(see Table 9).
INTERNAL/EXTERNAL REFERENCE
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.
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,
set the H /S SEL pin to 0 and the REF
internal reference in software mode, set H /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
Parameter
LSB Size
FS Range
Figure 26. AD7656-1/AD7657-1/AD7658-1 Transfer Characteristic
000 ... 001
000 ... 000
100 ... 010
100 ... 001
100 ... 000
011 ... 111
011 ... 110
111 ... 111
–FSR/2 + 1/2LSB
±10 V
0.305 mV
20 V/65,536
AGND – 1LSB
Input Range for AD7656-1
ANALOG INPUT
EN/ DIS
+FSR/2 – 3/2LSB
pin to 1. To enable the
±5 V
0.152 mV
10 V/65,536
Rev. C | Page 21 of 32
±10 V
1.22 mV
20 V/16,384
Input Range for AD7657-1
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/REFBUF
the internal reference and its buffers are disabled, apply an
external buffered reference to the REFCAPx pins.
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 AV
supplies used for the AD7656-1/AD7657-1/AD7658-1 conversion
process; therefore, they should be well decoupled. The AV
which is applied to eight AV
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.
If the same supply is being used for the AV
place a ferrite or small RC filter between the supply pins.
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.
CC
±5 V
0.610 mV
10 V/16,384
supply pins on each part. The AV
REFCAPB
REFOUT
AD7656-1/AD7657-1/AD7658-1
REFIN/
Figure 27. Reference Circuit
REF
CC
±10 V
4.88 mV
20 V/4096
pins can be decoupled using just one
Input Range for AD7658-1
BUF
BUF
BUF
REFCAPA
REFCAPC
CC
and DV
SAR
SAR
SAR
SAR
SAR
SAR
CC
EN /DIS
supplies are the
±5 V
2.44 mV
10 V/4096
CC
pin high. If
supplies,
CC
supply