AD7657 Analog Devices, AD7657 Datasheet - Page 21

• Manufacturer Part Number: AD7657
• Description: 250 kSPS, 6-Channel, Simultaneous Sampling, Bipolar, 14-Bit A/D Converter
• Manufacturer: Analog Devices

Specifications of AD7657

• Resolution (bits): 14bit
• # Chan: 6
• Sample Rate: 250kSPS
• Interface: Par,Ser,SPI
• Analog Input Type: SE-Bip
• Ain Range: Bip (Vref) x 2,Bip (Vref) x 4,Bip 10V,Bip 5.0V
• Adc Architecture: SAR
• Pkg Type: QFP

ADC TRANSFER FUNCTION
The output coding of the AD7656/AD7657/AD7658 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/65536 for the AD7656, FSR/16384 for the
AD7657, and FSR/4096 for the AD7658. The ideal transfer
characteristic is shown in Figure 25.
The LSB size is dependent on the analog input range selected
(see Table 9).
REFERENCE SECTION
The RFIN/REFOUT pin either allows access to the AD7656/
AD7657/AD7658’s 2.5 V reference or it allows an external
reference to be connected, providing the reference source for
each part’s conversions. The AD7656/AD7657/AD7658 can
accommodate a 2.5 V to 3 V external reference range. When
using an external reference, the internal reference needs to be
disabled. After a reset, the AD7656/AD7657/AD7658 default to
operating in external reference mode with the internal reference
buffers enabled. The internal reference can be enabled in either
hardware or software mode. To enable the internal reference in
hardware mode, the H /S SEL pin = 0 and the REF
enable the internal reference in software mode, H /S SEL = 1 and
a write to the control register is necessary to make DB9 of the
register = 1. For the internal reference mode, the REFIN/REFOUT
pin should be decoupled using 10 μF and 100 nF capacitors.
The AD7656/AD7657/AD7658 contain three on-chip reference
buffers. Each of the three ADC pairs has an associated reference
buffer. These reference buffers require external decoupling
capacitors on REFCAPA, REFCAPB, and REFCAPC pins,
and 10 μF and 100 nF decoupling capacitors should be placed
on these REFCAP pins. The internal reference buffers can be
disabled in software mode by writing to Bit DB8 in the internal
control register. If operating the devices in serial mode,
Table 9. LSB Size for Each Analog Input Range
Range
Input Range
LSB Size
FS Range
Figure 25. AD7656/AD7657/AD7658 Transfer Characteristic
–FSR/2 + 1/2LSB
000...001
000...000
100...010
100...001
100...000
011...110
011...111
111...111
±10 V
0.305 mV
20 V/65536
AGND – 1LSB
ANALOG INPUT
AD7656
+FSR/2 – 3/2LSB
±5 V
0.152 mV
10 V/65536
EN/DIS
pin = 1. To
Rev. C | Page 21 of 32
±10 V
1.22 mV
20 V/16384
the internal reference buffers can be disabled in hardware
mode by setting the DB14/REFBUF
internal reference and its buffers are disabled, an external
buffered reference should be applied to the REFCAP pins.
TYPICAL CONNECTION DIAGRAM
Figure 26 shows the typical connection diagram for the
AD7656/AD7657/AD7658. There are eight AV
on the parts. The AV
AD7656/AD7657/AD7658 conversion process; therefore, it
should be well decoupled. Each AV
individually decoupled with a 10 μF tantalum capacitor and a
100 nF ceramic capacitor. The AD7656/AD7657/AD7658 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 10 μF and 100 nF capacitor pair. The three
internal reference buffers are enabled. Each of the REFCAP
pins are decoupled with the 10 μF and 100 nF capacitor pair.
Six of the AV
ADC cores on the AD7656/AD7657/AD7658 and, as a result,
are used for the conversion process. Each analog input pin is
surrounded by an AV
AV
vidual ADC cores. For example, Pin 33 is V1, Pin 34 is the AV
supply for ADC Core 1, and Pin 32 is the AGND for ADC Core
1. An alternative reduced decoupling solution is to group these
six AV
and Pin 41, and Pin 46 and Pin 47.
For the AD7656, a 100 μF decoupling capacitor can be placed
on each of the pin pairs. All of the other supply and reference
pins should be decoupled with a 10 μF decoupling capacitor.
When the AD7657 is configured in this reduced decoupling
configuration, each of the three AV
decoupled with a 33 μF capacitor. When the AD7658 is con-
figured in this same configuration, each of the three AV
pin pairs should be decoupled with a 22 μF capacitor.
If the same supply is being used for the AV
supply, a ferrite or small RC filter should be placed between the
supply pins.
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. The AGND and DGND planes should be connected
together at one place in the system. This connection should be
made as close as possible to the AD7656/AD7657/AD7658 in
the system.
CC
AD7657
and AGND pins are the supply and ground for the indi-
CC
±5 V
0.610 mV
10 V/16384
supply pins into three pairs, Pin 34 and Pin 35, Pin 40
CC
supply pins are used as the supply to the six
CC
CC
supply is the supply that is used for the
supply pin and an AGND pin. These
AD7656/AD7657/AD7658
±10 V
4.88 mV
20 V/4096
CC
CC
EN/DIS
supply pin should be
pin pairs should be
pin high. If the
AD7658
CC
supply and DV
CC
±5 V
2.44mV
10 V/4096
supply pins
CC
CC
CC