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參數(shù)資料
| 型號： | AD9748ACPZRL7 |
| 廠商： | Analog Devices Inc |
| 文件頁數(shù)： | 3/24頁 |
| 文件大?。?/td> | 0K |
| 描述： | IC DAC 8BIT 210MSPS 32LFCSP |
| 標準包裝： | 1,500 |
| 系列： | TxDAC® |
| 設(shè)置時間： | 11ns |
| 位數(shù)： | 8 |
| 數(shù)據(jù)接口： | 并聯(lián) |
| 轉(zhuǎn)換器數(shù)目： | 1 |
| 電壓電源： | 模擬和數(shù)字 |
| 功率耗散（最大）： | 145mW |
| 工作溫度： | -40°C ~ 85°C |
| 安裝類型： | 表面貼裝 |
| 封裝/外殼： | 32-VFQFN 裸露焊盤，CSP |
| 供應(yīng)商設(shè)備封裝： | 32-LFCSP-VQ（5x5） |
| 包裝： | 帶卷 (TR) |
| 輸出數(shù)目和類型： | 2 電流，單極；2 電流，雙極 |
| 采樣率（每秒）： | 210M |
Data Sheet
AD9748
Rev. B | Page 11 of 24
FUNCTIONAL DESCRIPTION
Figure 15 shows a simplified block diagram of the AD9748. The
AD9748 consists of a DAC, digital control logic, and full-scale
output current control. The DAC contains a PMOS current
source array capable of providing up to 20 mA of full-scale
current (IOUTFS). The array is divided into 31 equal currents that
make up the five most significant bits (MSBs). The next three
bits consist of seven equal current sources whose value is of
an MSB current source. Implementing the lower bits with
current sources, instead of an R-2R ladder, enhances its
dynamic performance for multitone or low amplitude signals
and helps maintain the DAC’s high output impedance (that is,
>100 k).
All of these current sources are switched to one or the other of
the two output nodes (that is, IOUTA or IOUTB) via PMOS
differential current switches. The switches are based on the
architecture that was pioneered in the AD9764 family, with
further refinements to reduce distortion contributed by the
switching transient. This switch architecture also reduces
various timing errors and provides matching complementary
drive signals to the inputs of the differential current switches.
The analog and digital sections of the AD9748 have separate
power supply inputs (that is, AVDD and DVDD) that can
operate independently over a 2.7 V to 3.6 V range. The digital
section, which is capable of operating at a rate of up to 210 MSPS,
consists of edge-triggered latches and segment decoding logic
circuitry. The analog section includes the PMOS current sources,
the associated differential switches, a 1.2 V band gap voltage
reference, and a reference control amplifier.
The DAC full-scale output current is regulated by the reference
control amplifier and can be set from 2 mA to 20 mA via an
external resistor, RSET, connected to the full-scale adjust (FS ADJ)
pin. The external resistor, in combination with both the reference
control amplifier and voltage reference, VREFIO, sets the reference
current, IREF, which is replicated to the segmented current
sources with the proper scaling factor. The full-scale current,
IOUTFS, is 32 times IREF.
REFERENCE OPERATION
The AD9748 contains an internal 1.2 V band gap reference. The
internal reference cannot be disabled but can be easily overridden
by an external reference with no effect on performance. Figure 16
shows an equivalent circuit of the band gap reference. REFIO
serves as either an output or an input depending on whether the
internal or an external reference is used. To use the internal
reference, simply decouple the REFIO pin to ACOM with a
0.1 F capacitor and connect REFLO to ACOM via a resistance
less than 5 . The internal reference voltage are present at
REFIO. If the voltage at REFIO is to be used anywhere else in
the circuit, than an external buffer amplifier with an input bias
current of less than 100 nA should be used. An example of the
use of the internal reference is shown in Figure 17.
AVDD
7k
84A
REFLO
REFIO
0321
1-044
Figure 16. Equivalent Circuit of Internal Reference
150pF
1.2V REF
AVDD
REFLO
CURRENT
SOURCE
ARRAY
3.3V
REFIO
FS ADJ
AD9748
ADDITIONAL
LOAD
OPTIONAL
EXTERNAL
REF BUFFER
032
1
1-045
0.1
F
2k
Figure 17. Internal Reference Configuration
An external reference can be applied to REFIO, as shown in
Figure 18. The external reference can provide either a fixed
reference voltage to enhance accuracy and drift performance or
a varying reference voltage for gain control. Note that the 0.1 F
compensation capacitor is not required because the internal
reference is overridden, and the relatively high input impedance of
REFIO minimizes any loading of the external reference.
150pF
1.2V REF
AVDD
REFLO
CURRENT
SOURCE
ARRAY
REFIO
FS ADJ
AD9748
REFERENCE
CONTROL
AMPLIFIER
3.3V
032
1
1-
046
Figure 18. External Reference Configuration
REFERENCE CONTROL AMPLIFIER
The AD9748 contains a control amplifier that is used to regulate
the full-scale output current, IOUTFS. The control amplifier is
configured as a V-I converter, as shown in Figure 17, so that its
current output, IREF, is determined by the ratio of the VREFIO and
an external resistor, RSET, as stated in Equation 4. IREF is copied
to the segmented current sources with the proper scale factor to
set IOUTFS, as stated in Equation 3.
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