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參數(shù)資料
| 型號： | ADS808 |
| 英文描述： | 12-Bit, 70MHz Sampling ANALOG-TO-DIGITAL CONVERTER |
| 中文描述： | 12位，70MHz的采樣模擬數(shù)字轉(zhuǎn)換器 |
| 文件頁數(shù)： | 9/19頁 |
| 文件大?。?/td> | 364K |
| 代理商： | ADS808 |
ADS808
SBAS179C
9
www.ti.com
By choosing between the three different signal input ranges,
trade-offs can be made between noise and distortion perfor-
mance. In order to maximize the SNR, which is important for
time-domain applications, the 2Vp-p range may be se-
lected. This range may also be used with low-level (
–
6dBFS
to
–
40dBFS) to high-frequency inputs (multi-tone). The
1.5Vp-p range may be considered for achieving a combina-
tion of both low noise and distortion performance. Here the
SNR number is typically 3dB down compared to the 2Vp-p
range, while an improvement in the distortion performance
of the driver amplifier may be realized due to the reduced
output power level required. The third option, 1Vp-p FSR,
may be considered mainly for applications requiring DC-
coupling and/or single-supply operation of the driver and the
converter.
Input Biasing (V
CM
)
The ADS808 operates from a single +5V supply, and re-
quires each of the analog inputs to be externally biased to a
common-mode voltage of typically +2.5V. This allows a
symmetrical signal swing while maintaining sufficient head-
room to either supply rail. Communication systems are usu-
ally AC-coupled in-between signal processing stages, mak-
ing it convenient to set individual common-mode voltages
and allow optimizing the DC operating point for each stage.
Other applications (e.g., imaging) process only unipolar or
DC-restored signals. In this case, the common-mode voltage
may be shifted such that the full-input range of the converter
is utilized.
It should be noted that the CM pin is internally buffered.
However, it is recommended to keep the loading of this pin
to a minimum to avoid an increase in the converter
’
s
nonlinearity. Additionally, the DC voltage at the CM pin is not
exactly +2.5V, but is subject to the tolerance of the top and
bottom references, as well as the resistor ladder.
Input Impedance
The input of the ADS808 is of a capacitive nature and the
driving source needs to provide the slew current to charge or
discharge the input sampling capacitor while the track-and-
hold amplifier is in track mode (see Figure 1). This effectively
results in a dynamic input impedance that is a function of the
sampling frequency. Figure 2 depicts the differential input
impedance of the ADS808 as a function of the input fre-
quency.
For applications that use op amps to drive the ADC, it is
recommended to add a series resistor between the amplifier
output and the converter inputs. This will isolate the converter
’
s
capacitive input from the driving source and avoid gain
peaking, or instability. Furthermore, it will create a 1st-order,
low-pass filter in conjunction with the specified input capaci-
tance of the ADS808. Its cutoff frequency can be adjusted
even further by adding an external shunt capacitor from each
signal input to ground. However, the optimum values of this
RC network depend on a variety of factors, including the
ADS808
’
s sampling rate, the selected op amp, the interface
configuration, and the particular application (time domain
versus frequency domain). Generally, increasing the size of
the series resistor and/or capacitor will improve the signal-to-
noise ratio, however, depending on the signal source, large
resistor values may reduce the harmonic distortion perfor-
mance. In any case, the use of the RC network is optional but
optimizing the values to adapt to the specific application is
encouraged.
INPUT DRIVER CONFIGURATIONS
The following section provides some principal circuit sugges-
tions on how to interface the analog input signal to the
ADS808. A first example of a typical analog interface circuit
is shown in Figure 3. Here it is assumed that the input signal
is already available in differential form (e.g., coming from a
preceding mixer stage). The differential driver performs an
impedance transformation as well as amplifying the signal to
match the selected full-scale input range of the ADS808 (for
example, 2Vp-p). The common-mode voltage (V
CM
) for the
converter input is established by connecting the inputs to the
midpoints of the resistor divider. The input signal is AC-
coupled through capacitors C
IN
to the inputs of the converter
that are set to a V
CM
of approximately +2.5V
DC
.
FIGURE 2. Differential Input Impedance versus Input
Frequency.
Differential
Driver
ADS808
0.1
μ
F
0.1
μ
F
REFT
REFB
IN
IN
1k
1k
1k
C
IN
C
IN
1k
V
CM
= +2.5V
V
IN
V
IN
NOTE: Reference bypassing omitted for clarity.
FIGURE 3. AC Coupling Allows for Easy DC Biasing of the
ADS808 Inputs While the Input Signal is Applied
by the Differential Input Driver.
1000
100
10
1
0.1
0.01
0.1
1
10
100
1000
ADS808 INPUT IMPEDANCE vs INPUT FREQUENCY
f
IN
(MHz)
Z
I
)
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相關(guān)代理商/技術(shù)參數(shù) |
參數(shù)描述 |
|---|---|
| ADS808Y/250 | 功能描述:模數(shù)轉(zhuǎn)換器 - ADC 12-Bit 70MHz Sampling ADC RoHS:否 制造商:Texas Instruments 通道數(shù)量:2 結(jié)構(gòu):Sigma-Delta 轉(zhuǎn)換速率:125 SPs to 8 KSPs 分辨率:24 bit 輸入類型:Differential 信噪比:107 dB 接口類型:SPI 工作電源電壓:1.7 V to 3.6 V, 2.7 V to 5.25 V 最大工作溫度:+ 85 C 安裝風(fēng)格:SMD/SMT 封裝 / 箱體:VQFN-32 |
| ADS808Y/250G4 | 功能描述:模數(shù)轉(zhuǎn)換器 - ADC 12-Bit 70MHz Sampling ADC RoHS:否 制造商:Texas Instruments 通道數(shù)量:2 結(jié)構(gòu):Sigma-Delta 轉(zhuǎn)換速率:125 SPs to 8 KSPs 分辨率:24 bit 輸入類型:Differential 信噪比:107 dB 接口類型:SPI 工作電源電壓:1.7 V to 3.6 V, 2.7 V to 5.25 V 最大工作溫度:+ 85 C 安裝風(fēng)格:SMD/SMT 封裝 / 箱體:VQFN-32 |
| ADS808Y/2K | 功能描述:模數(shù)轉(zhuǎn)換器 - ADC 12-Bit 70MHz Sampling ADC RoHS:否 制造商:Texas Instruments 通道數(shù)量:2 結(jié)構(gòu):Sigma-Delta 轉(zhuǎn)換速率:125 SPs to 8 KSPs 分辨率:24 bit 輸入類型:Differential 信噪比:107 dB 接口類型:SPI 工作電源電壓:1.7 V to 3.6 V, 2.7 V to 5.25 V 最大工作溫度:+ 85 C 安裝風(fēng)格:SMD/SMT 封裝 / 箱體:VQFN-32 |
| ADS808Y/2KG4 | 功能描述:模數(shù)轉(zhuǎn)換器 - ADC 12-Bit 70MHz Sampling ADC RoHS:否 制造商:Texas Instruments 通道數(shù)量:2 結(jié)構(gòu):Sigma-Delta 轉(zhuǎn)換速率:125 SPs to 8 KSPs 分辨率:24 bit 輸入類型:Differential 信噪比:107 dB 接口類型:SPI 工作電源電壓:1.7 V to 3.6 V, 2.7 V to 5.25 V 最大工作溫度:+ 85 C 安裝風(fēng)格:SMD/SMT 封裝 / 箱體:VQFN-32 |
| ADS809 | 制造商:BB 制造商全稱:BB 功能描述:12-Bit, 80MHz Sampling ANALOG-TO-DIGITAL CONVERTER |
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