參數資料
| 型號： | AD9748ACPZRL7 |
| 廠商： | Analog Devices Inc |
| 文件頁數： | 4/24頁 |
| 文件大?。?/td> | 0K |
| 描述： | IC DAC 8BIT 210MSPS 32LFCSP |
| 標準包裝： | 1,500 |
| 系列： | TxDAC® |
| 設置時間： | 11ns |
| 位數： | 8 |
| 數據接口： | 并聯 |
| 轉換器數目： | 1 |
| 電壓電源： | 模擬和數字 |
| 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 |
| 供應商設備封裝： | 32-LFCSP-VQ（5x5） |
| 包裝： | 帶卷 (TR) |
| 輸出數目和類型： | 2 電流，單極；2 電流，雙極 |
| 采樣率（每秒）： | 210M |
AD9748
Data Sheet
Rev. B | Page 12 of 24
The control amplifier allows a wide (10:1) adjustment span of
IOUTFS over a 2 mA to 20 mA range by setting IREF between 62.5
A and 625 A. The wide adjustment span of IOUTFS provides
several benefits. The first relates directly to the power
the Power Dissipation section). The second relates to a 20 dB
adjustment, which is useful for system gain control purposes.
The small signal bandwidth of the reference control amplifier is
approximately 500 kHz and can be used for low frequency small
signal multiplying applications.
DAC TRANSFER FUNCTION
The AD9748 provides complementary current outputs, IOUTA
and IOUTB. IOUTA provides a near full-scale current output,
IOUTFS, when all bits are high (that is, DAC CODE = 255), while
IOUTB, the complementary output, provides no current. The
current output appearing at IOUTA and IOUTB is a function of
both the input code and IOUTFS and can be expressed as:
IOUTA = (DAC CODE/256) × IOUTFS
(1)
IOUTB = (255 DAC CODE)/256 × IOUTFS
(2)
where DAC CODE = 0 to 255 (that is, decimal representation).
As mentioned previously, IOUTFS is a function of the reference
current IREF, which is nominally set by a reference voltage,
VREFIO, and external resistor, RSET. It can be expressed as:
IOUTFS = 32 × IREF
(3)
where
IREF = VREFIO/RSET
(4)
The two current outputs typically drive a resistive load directly
or via a transformer. If dc coupling is required, then IOUTA
and IOUTB should be directly connected to matching resistive
loads, RLOAD, that are tied to analog common, ACOM. Note
that RLOAD can represent the equivalent load resistance seen by
IOUTA or IOUTB as would be the case in a doubly terminated
50
or 75
-ended voltage output appearing
at the IOUTA and IOUTB nodes is simply
VOUTA = IOUTA × RLOAD
(5)
VOUTB = IOUTB × RLOAD
(6)
Note that the full-scale value of VOUTA and VOUTB should not
exceed the specified output compliance range to maintain
specified distortion and linearity performance.
VDIFF = (IOUTA IOUTB) × RLOAD
(7)
Substituting the values of IOUTA, IOUTB, IREF, and VDIFF can be
expressed as:
VDIFF = {(2 × DAC CODE 255)/256}
(32 × RLOAD/RSET) × VREFIO
(8)
Equation 7 and Equation 8 highlight some of the advantages of
operating the AD9748 differentially. First, the differential
operation helps cancel common-mode error sources associated
with IOUTA and IOUTB, such as noise, distortion, and dc
offsets. Second, the differential code dependent current and
subsequent voltage, VDIFF, is twice the value of the single-ended
voltage output (that is, VOUTA or VOUTB), thus providing twice the
signal power to the load.
Note that the gain drift temperature performance for a single-
ended (VOUTA and VOUTB) or differential output (VDIFF) of the
AD9748 can be enhanced by selecting temperature tracking
resistors for RLOAD and RSET due to their ratiometric relationship,
as shown in Equation 8.
ANALOG OUTPUTS
The complementary current outputs in each DAC, IOUTA,
and IOUTB can be configured for single-ended or differential
operation. IOUTA and IOUTB can be converted into
complementary single-ended voltage outputs, VOUTA and VOUTB,
Function section by Equation 5 through Equation 8. The
differential voltage, VDIFF, existing between VOUTA and VOUTB, can
also be converted to a single-ended voltage via a transformer or
differential amplifier configuration. The ac performance
of the AD9748 is optimum and specified using a differential
transformer-coupled output in which the voltage swing at
IOUTA and IOUTB is limited to ±0.5 V.
The distortion and noise performance of the AD9748 can be
enhanced when it is configured for differential operation. The
common-mode error sources of both IOUTA and IOUTB can
be significantly reduced by the common-mode rejection of a
transformer or differential amplifier. These common-mode
error sources include even-order distortion products and noise.
The enhancement in distortion performance becomes more
significant as the frequency content of the reconstructed
waveform increases and/or its amplitude decreases. This is due
to the first-order cancellation of various dynamic common-
mode distortion mechanisms, digital feedthrough, and noise.
Performing a differential-to-single-ended conversion via a
transformer also provides the ability to deliver twice the
reconstructed signal power to the load (assuming no source
termination). Because the output currents of IOUTA and
IOUTB are complementary, they become additive when
processed differentially. A properly selected transformer allows
the AD9748 to provide the required power and voltage levels to
different loads.
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