參數(shù)資料
型號： AD7641BCPZRL
廠商： Analog Devices Inc
文件頁數(shù)： 10/28頁
文件大小： 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： 2,500
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
安裝類型： 表面貼裝
封裝/外殼： 48-VFQFN 裸露焊盤，CSP
供應(yīng)商設(shè)備封裝： 48-LFCSP-VQ（7x7）
包裝： 帶卷 (TR)
輸入數(shù)目和類型： 1 個差分，雙極
配用： EVAL-AD7641CBZ-ND - BOARD EVALUATION FOR AD7641
AD7641
Rev. 0 | Page 18 of 28
Single-to-Differential Driver
The noise generated by the driver amplifier needs to be
kept as low as possible to preserve the SNR and transition
noise performance of the AD7641. The noise coming from
the driver is filtered by the AD7641 analog input circuit
1-pole, low-pass filter made by RIN and CIN or by the
external filter, if one is used. The SNR degradation due
to the amplifier is
For applications using unipolar analog signals, a single-ended-
to-differential driver, as shown in Figure 26, allows for a
differential input into the part. This configuration, when
provided an input signal of 0 to VREF, produces a differential
±VREF with midscale at VREF/2. The 1-pole filter using R = 10 Ω
and C = 1 nF provides a corner frequency of 16 MHz.
()
()
+
=
+
2
π
2
π
900
30
20log
N
3dB
N
3dB
LOSS
Ne
f
Ne
f
SNR
If the application can tolerate more noise, the AD8139
differential driver can be used.
0
47
61
-02
7
AD8021
ANALOG INPUT
(UNIPOLAR 0V TO 2.048V)
AD8021
IN+
IN–
AD7641
REF
10F
15
100nF
2.7nF
U2
U1
10pF
5k
590
where:
f–3dB is the input bandwidth of the AD7641 (50 MHz) or
the cutoff frequency of the input RC filter shown in Figure 23
(3.9 MHz), if one is used.
N is the noise factor of the amplifier (1 in buffer
configuration).
eN+ and eN are the equivalent input voltage noise densities
of the op amps connected to IN+ and IN, in nV/√Hz.
This approximation can be used when the resistances used
around the amplifier are small. If larger resistances are
used, their noise contributions should also be root-sum
squared.
Figure 26. Single-Ended-to-Differential Driver Circuit
(Internal Reference Buffer Used)
VOLTAGE REFERENCE INPUT
The AD7641 allows the choice of either a very low temperature
drift internal voltage reference or an external reference.
For instance, when using op amps with an equivalent input
noise density of 2.1 nV/√Hz, such as the AD8021, with a
noise gain of 1 when configured as a buffer, degrades the
SNR by only 0.25 dB when using the RC filter in
Unlike many ADCs with internal references, the internal
reference of the AD7641 provides excellent performance and
can be used in almost all applications.
and by 2.5 dB without it.
Internal Reference (PDBUF = Low, PDREF = Low)
The driver needs to have a THD performance suitable to
that of the AD7641. Figure 13 gives the THD vs. frequency
that the driver should exceed.
To use the internal reference, the PDREF and PDBUF inputs
must both be low. This produces a 1.2 V band gap output on
REFBUFIN, which is amplified by the internal buffer and
results in a 2.048 V reference on the REF pin.
The AD8021 meets these requirements and is appropriate for
almost all applications. The AD8021 needs a 10 pF external
compensation capacitor that should have good linearity as an
NPO ceramic or mica type. Moreover, the use of a noninverting
1 gain arrangement is recommended and helps to obtain the
best signal-to-noise ratio.
The internal reference is temperature compensated to 2.048 V ±
10 mV. The reference is trimmed to provide a typical drift of
10 ppm/°C. This typical drift characteristic is shown in Figure 7.
The output resistance of REFBUFIN is 6.33 kΩ (minimum)
when the internal reference is enabled. It is necessary to
decouple this with a ceramic capacitor greater than 100 nF.
Therefore, the capacitor provides an RC filter for noise reduction.
The AD8022 can also be used when a dual version is needed
and a gain of 1 is present. The AD829 is an alternative in
applications where high frequency (above 100 kHz) performance is
not required. In applications with a gain of 1, an 82 pF
compensation capacitor is required. The
Because the output impedance of REFBUFIN is typically
6.33 kΩ, relative humidity (among other industrial contaminates)
can directly affect the drift characteristics of the reference.
Typically, a guard ring is used to reduce the effects of drift
under such circumstances.
AD8610 is an option
when low bias current is needed in low frequency applications.
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