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| 型号： | AD7658BSTZ |
| 厂商： | Analog Devices Inc |
| 文件页数： | 14/32页 |
| 文件大小： | 0K |
| 描述： | IC ADC 12BIT 6CH 250KSPS 64LQFP |
| 标准包装： | 1 |
| 位数： | 12 |
| 采样率（每秒）： | 250k |
| 数据接口： | 串行，并联 |
| 转换器数目： | 6 |
| 功率耗散（最大）： | 143mW |
| 电压电源： | 模拟和数字，双 ± |
| 工作温度： | -40°C ~ 85°C |
| 安装类型： | 表面贴装 |
| 封装/外壳： | 64-LQFP |
| 供应商设备封装： | 64-LQFP（10x10） |
| 包装： | 托盘 |
| 输入数目和类型： | 6 个单端，双极 |
| 产品目录页面： | 778 (CN2011-ZH PDF) |
| 配用： | EVAL-AD7658CBZ-ND - BOARD EVAL FOR AD7658 |
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Data Sheet
AD7656/AD7657/AD7658
Rev. D | Page 21 of 32
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.
011...111
011...110
000...001
000...000
111...111
–FSR/2 + 1/2LSB
+FSR/2 – 3/2LSB
AGND – 1LSB
ANALOG INPUT
ADC
CO
DE
100...010
100...001
100...000
05020-
005
Figure 25. AD7656/AD7657/AD7658 Transfer Characteristic
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 REFEN/DIS pin = 1. To
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 F 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 F 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,
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, 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 AVCC supply pins
on the parts. The AVCC supply is the supply that is used for the
AD7656/AD7657/AD7658 conversion process; therefore, it
should be well decoupled. Each AVCC supply pin should be
individually decoupled with a 10 F 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 F and 100 nF capacitor pair. The three
internal reference buffers are enabled. Each of the REFCAP
pins are decoupled with the 10 F and 100 nF capacitor pair.
Six of the AVCC supply pins are used as the supply to the six
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 AVCC supply pin and an AGND pin. These
AVCC and AGND pins are the supply and ground for the indi-
vidual ADC cores. For example, Pin 33 is V1, Pin 34 is the AVCC
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 AVCC supply pins into three pairs, Pin 34 and Pin 35, Pin 40
and Pin 41, and Pin 46 and Pin 47.
For the AD7656, a 100 F 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 F decoupling capacitor.
When the AD7657 is configured in this reduced decoupling
configuration, each of the three AVCC pin pairs should be
decoupled with a 33 F capacitor. When the AD7658 is con-
figured in this same configuration, each of the three AVCC
pin pairs should be decoupled with a 22 F capacitor.
If the same supply is being used for the AVCC supply and DVCC
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.
Table 9. LSB Size for Each Analog Input Range
Range
AD7656
AD7657
AD7658
Input Range
±10 V
±5 V
±10 V
±5 V
±10 V
±5 V
LSB Size
0.305 mV
0.152 mV
1.22 mV
0.610 mV
4.88 mV
2.44mV
FS Range
20 V/65536
10 V/65536
20 V/16384
10 V/16384
20 V/4096
10 V/4096
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