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参数资料
| 型号： | AD7778ASZ |
| 厂商： | Analog Devices Inc |
| 文件页数： | 12/12页 |
| 文件大小： | 0K |
| 描述： | IC ADC 10BIT 8CHAN HS 44-MQFP |
| 标准包装： | 1 |
| 位数： | 10 |
| 采样率（每秒）： | 378k |
| 数据接口： | 并联 |
| 转换器数目： | 1 |
| 电压电源： | 单电源 |
| 工作温度： | -40°C ~ 85°C |
| 安装类型： | 表面贴装 |
| 封装/外壳： | 44-QFP |
| 供应商设备封装： | 44-MQFP（10x10） |
| 包装： | 托盘 |
| 输入数目和类型： | 8 个单端，单极 |
AD7776/AD7777/AD7778
–9–
REV. A
Table I. AD7776/AD7777/AD7778 Truth Table for Microprocessor Interfacing
CS
RD
WR
DB0–DB9
Function/Comments
1X
*
X
*
High Z
Data Port High Impedance
01
j
CR Data
Load control register (CR) data to control register and start a conversion.
0
k
1
ADC Data
ADC data placed on data bus. Depending upon location CR6 of the control register, one or two
Read instructions are required.
If CR6 is low, i.e., single-channel conversion selected, a read instruction returns the contents of
ADCREG1. Succeeding read instructions continue to return the contents of ADCREG1.
If CR6 is high, i.e., simultaneous sampling (double conversion) selected, the first read instruction
returns the contents of ADCREG1 while the second read instruction returns the contents of
ADCREG2. A third read instruction returns ADCREG1 again, the fourth ADCREG2, etc.
*X = Don’t Care
DESIGN INFORMATION
Layout Hints
Ensure that the layout for the printed circuit board has the digi-
tal and analog grounds separated as much as possible. Take care
not to run any digital track alongside an analog signal track.
Guard (screen) the analog input(s) with RTN.
Establish a single-point analog ground separate from the logic
system ground and as close as possible to the AD7776/AD7777/
AD7778. Both the RTN and AGND pins on the AD7776/
AD7777/AD7778 and all other signal grounds should be con-
nected to this single point analog ground. In turn, this star
ground should be connected to the digital ground at one point
only—preferably at the low impedance power supply itself.
Low impedance analog and digital power supply common returns
are important for correct operation of the devices, so make the
foil width for these tracks as wide as possible.
To ensure a low impedance +5 V power supply at the actual VCC
pin, it is necessary to use bypass capacitors from the pin itself to
DGND. A 4.7
F tantalum capacitor in parallel with a 0.1 F
ceramic capacitor is sufficient.
ADC Corruption
Executing a read instruction to the AD7776/AD7777/AD7778
while a conversion is in progress immediately halts the conversion
and returns invalid data over the data bus. The
BUSY/ INT
output pin should be monitored closely and all read instructions
to the AD7776/AD7777/AD7778 prevented while this output
shows that a conversion is in progress.
Executing a write instruction to the AD7776/AD7777/AD7778
while a conversion is in progress immediately halts the conversion,
while the falling edge of
WR driving the BUSY/INT output high.
The analog input(s) is sampled as normal, and a new conversion
sequence (dependent upon CR6) is started.
ADC Conversion Time
Although each conversion takes only 14 CLKIN cycles, it can take
between 4.5 and 5.5 CLKIN cycles to acquire the analog input(s)
after the
WR input goes high and before any conversions start.
TERMINOLOGY
Relative Accuracy
For the AD7776/AD7777/AD7778, relative accuracy or endpoint
nonlinearity is the maximum deviation, in LSBs, of the ADC’s
actual code transition points from a straight line drawn between
the endpoints of the ADC transfer function.
Differential Nonlinearity
Differential nonlinearity is the difference between the measured
change and the ideal 1 LSB change between any two adjacent
codes. A specified maximum differential nonlinearity of
±1 LSB
ensures no missed codes.
Bias Offset Error
For an ideal 10-bit ADC, the output code for an input voltage
equal to VBIAS should be midscale. The bias offset error is the
difference between the actual midpoint voltage for midscale code
and VBIAS, expressed in LSBs.
Bias Offset Error Match
This is a measure of how closely the bias offset errors of all
channels track each other. The bias offset error match of any
channel must be no further away than 10 LSBs from the bias
offset error of any other channel, regardless of whether the
channels are independently sampled or simultaneously sampled.
Plus and Minus Full-Scale Error
The input channels of the ADC can be considered to have
bipolar (positive and negative) input ranges, but are referred to
VBIAS (or REFIN) instead of AGND. Positive full-scale error for
the ADC is the difference between the actual input voltage
required to produce the plus full-scale code transition and the ideal
input voltage (VBIAS + VSWING –1.5 LSB), expressed in LSBs.
Minus full-scale error is similarly specified for the minus full-scale
code transition, relative to the ideal input voltage for this
transition (VBIAS – VSWING + 0.5 LSB). Note that the full-scale
errors for the ADC input channels are measured after their
respective bias offset errors have been adjusted out.
Plus and Minus Full-Scale Error Match
This is a measure of how closely the full-scale errors of all
channels track each other. The full-scale error match of any channel
must be no further away than 10 LSBs from the respective
full-scale error of any other channel, regardless of whether the
channels are independently sampled or simultaneously sampled.
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