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參數(shù)資料
| 型號： | EVAL-AD7944EBZ |
| 廠商： | Analog Devices Inc |
| 文件頁數(shù)： | 8/28頁 |
| 文件大?。?/td> | 0K |
| 描述： | BOARD EVAL FOR AD7944 |
| 標(biāo)準(zhǔn)包裝： | 1 |
| 系列： | PulSAR® |
| ADC 的數(shù)量： | 1 |
| 位數(shù)： | 14 |
| 采樣率（每秒）： | 2.5M |
| 數(shù)據(jù)接口： | SPI?、QSPI?、MICROWIRE? 和 DSP |
| 輸入范圍： | 0 ~ 5 V |
| 工作溫度： | -40°C ~ 85°C |
| 已用 IC / 零件： | AD7944 |
| 已供物品： | 板 |
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AD7944
Rev. A | Page 16 of 28
VOLTAGE REFERENCE INPUT
The AD7944 allows the choice of a very low temperature drift
internal voltage reference, an external reference, or an external
buffered reference.
The internal reference of the AD7944 provides excellent
performance and can be used in almost all applications.
Internal Reference, REF = 4.096 V (PDREF Low)
To use the internal reference, the PDREF input must be low.
This enables the on-chip band gap reference and buffer, result-
ing in a 4.096 V reference on the REF pin (1.2 V on REFIN).
The internal reference is temperature compensated to
4.096 V ± 15 mV. The reference is trimmed to provide
a typical drift of 10 ppm/°C.
The output resistance of REFIN is 6 k when the internal
reference is enabled. It is necessary to decouple this pin with a
ceramic capacitor of at least 100 nF. The output resistance of
REFIN and the decoupling capacitor form an RC filter, which
helps to reduce noise.
Because the output impedance of REFIN is typically 6 k,
relative humidity (among other industrial contaminants) can
directly affect the drift characteristics of the reference. A guard
ring is typically used to reduce the effects of drift under such
circumstances. However, the fine pitch of the AD7944 makes
this difficult to implement. One solution, in these industrial
and other types of applications, is to use a conformal coating,
such as Dow Corning 1-2577 or HumiSeal 1B73.
External 1.2 V Reference and Internal Buffer (PDREF High)
To use an external reference along with the internal buffer, PDREF
must be high. This powers down the internal reference and allows
the 1.2 V reference to be applied to REFIN, producing 4.096 V
(typically) on the REF pin.
External Reference (PDREF High, REFIN Low)
To apply an external reference voltage directly to the REF pin, tie
PDREF high and tie REFIN low. BVDD should also be driven to
the same potential as REF. For example, if REF = 2.5 V, BVDD
should be tied to 2.5 V.
The advantages of directly using an external voltage reference
are as follows:
SNR and dynamic range improvement (about 1.7 dB)
resulting from the use of a larger reference voltage (5 V)
instead of a typical 4.096 V reference when the internal
reference is used. This is calculated by
=
0
.
5
096
.
4
log
20
SNR
Power savings when the internal reference is powered
down (PDREF high).
Reference Decoupling
The AD7944 voltage reference input, REF, has a dynamic input
impedance that requires careful decoupling between the REF
and REFGND pins. The Layout section describes how this can
be done.
When using an external reference, a very low impedance source
(for example, a reference buffer using the AD8031 or the AD8605)
and a 10 F (X5R, 0805 size) ceramic chip capacitor are appro-
priate for optimum performance.
If an unbuffered reference voltage is used, the decoupling value
depends on the reference used. For example, a 22 F (X5R,
1206 size) ceramic chip capacitor is appropriate for optimum
performance using a low temperature drift ADR43x reference.
If desired, a reference decoupling capacitor with a value as small
as 2.2 F can be used with minimal impact on performance,
especially DNL.
In any case, there is no need for an additional, lower value
ceramic decoupling capacitor (for example, 100 nF) between
the REF and REFGND pins.
POWER SUPPLY
The AD7944 has four power supply pins: an analog supply
(AVDD), a buffer supply (BVDD), a digital supply (DVDD), and
a digital input/output interface supply (VIO). VIO allows a direct
interface to any logic from 1.8 V to 2.7 V. To reduce the number
of supplies needed, the VIO, DVDD, and AVDD pins can be
tied together. The power supplies do not need to be started in a
particular sequence. In addition, the AD7944 is very insensitive
to power supply variations over a wide frequency range.
In normal mode, the AD7944 powers down automatically at
the end of each conversion phase and, therefore, the power
scales linearly with the sampling rate. This makes the part ideal
for low sampling rates (even of a few SPS) and battery-powered
applications.
10
1
0.1
0.01
0.1
1
O
P
E
RAT
ING
CURRE
NT
(
mA)
SAMPLING RATE (MSPS)
IBVDD
IAVDD
IDVDD
IVIO
IVREF
04658-
121
Figure 25. Operating Current vs. Sampling Rate in Normal Mode
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