參數(shù)資料
型號(hào)： AD7641BCPZ
廠(chǎng)商： Analog Devices Inc
文件頁(yè)數(shù)： 11/28頁(yè)
文件大?。?/td> 0K
描述： IC ADC 18BIT 2MSPS SAR 48-LFCSP
產(chǎn)品培訓(xùn)模塊： ADC Applications
ADC Architectures
ADC DC/AC Performance
標(biāo)準(zhǔn)包裝： 1
位數(shù)： 18
采樣率（每秒）： 2M
數(shù)據(jù)接口： 串行，并聯(lián)
轉(zhuǎn)換器數(shù)目： 1
功率耗散（最大）： 92mW
電壓電源： 模擬和數(shù)字
工作溫度： -40°C ~ 85°C
安裝類(lèi)型： 表面貼裝
封裝/外殼： 48-VFQFN 裸露焊盤(pán)，CSP
供應(yīng)商設(shè)備封裝： 48-LFCSP-VQ（7x7）
包裝： 托盤(pán)
輸入數(shù)目和類(lèi)型： 1 個(gè)差分，雙極
配用： EVAL-AD7641CBZ-ND - BOARD EVALUATION FOR AD7641
AD7641
Rev. 0 | Page 19 of 28
However, because the AD7641 has a fine lead pitch, guarding
this node is not practical. Therefore, in these industrial and
other types of applications, it is recommended to use a conformal
coating, such as Dow Corning 1-2577 or HumiSeal 1B73.
For applications that use multiple AD7641 devices, it is more
effective to use an external reference with the internal reference
buffer to buffer the reference voltage. However, because the
reference buffers are not unity gain, ratiometric, simultaneously
sampled designs should use an external reference and external
buffer, such as the
External 1.2 V Reference and Internal Buffer (PDBUF =
Low, PDREF = High)
AD8031/AD8032; therefore, preserving the
same reference level for all converters.
To use an external reference along with the internal buffer,
PDREF should be high and PDBUF should be low. This powers
down the internal reference and allows the 1.2 V reference to
be applied to REFBUFIN, producing 2.048 V (typically) on
the REF pin.
The voltage reference temperature coefficient (TC) directly
impacts full scale; therefore, in applications where full-scale
accuracy matters, care must be taken with the TC. For instance,
a ±4 ppm/°C TC of the reference changes full scale by ±1 LSB/°C.
External 2.5 V Reference (PDBUF = High, PDREF = High)
Note that VREF can be increased to AVDD + 0.1 V. Because the
input range is defined in terms of VREF, this would essentially
increase the range to 0 V to 2.8 V with an AVDD = 2.7 V.
To use an external 2.5 V reference directly on the REF pin,
PDREF and PDBUF should both be high.
For improved drift performance, an external reference, such as
the
Temperature Sensor
AD780 or ADR431, can be used. The advantages of directly
using the external voltage reference are:
The TEMP pin measures the temperature of the AD7641. To
improve the calibration accuracy over the temperature range,
the output of the TEMP pin is applied to one of the inputs of
the analog switch (such as,
The SNR and dynamic range improvement (about 1.7 dB)
resulting from the use of a reference voltage very close to
the supply (2.5 V) instead of a typical 2.048 V reference
when the internal reference is used. This is calculated by
ADG779), and the ADC itself is
used to measure its own temperature. This configuration is
shown in Figure 27.
=
50
.
2
048
.
2
log
20
SNR
04
76
1-
02
8
ADG779
AD8021
CC
ANALOG INPUT
(UNIPOLAR)
AD7641
IN+
TEMPERATURE
SENSOR
TEMP
The power savings when the internal reference is powered
down (PDREF high).
PDREF and PDBUF power down the internal reference and
the internal reference buffer, respectively. The input current
of PDREF and PDBUF should never exceed 20 mA. This can
occur when the driving voltage is above AVDD (for instance, at
power-up). In this case, a 125 Ω series resistor is recommended.
Figure 27. Use of the Temperature Sensor
Reference Decoupling
Whether using an internal or external reference, the AD7641
voltage reference input (REF) has a dynamic input impedance;
therefore, it should be driven by a low impedance source with
efficient decoupling between the REF and REFGND inputs.
This decoupling depends on the choice of the voltage reference
but usually consists of a low ESR capacitor connected to REF
and REFGND with minimum parasitic inductance. A 10 μF
(X5R, 1206 size) ceramic chip capacitor (or 47 μF tantalum
capacitor) is appropriate when using either the internal
reference or one of the recommended reference voltages.
The placement of the reference decoupling is also important to
the performance of the AD7641. The decoupling capacitor
should be mounted on the same side as the ADC right at the
REF pin with a thick PCB trace. The REFGND should also connect
to the reference decoupling capacitor with the shortest distance.
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