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參數資料
| 型號： | SPT7935SCT |
| 廠商： | FAIRCHILD SEMICONDUCTOR CORP |
| 元件分類： | ADC |
| 英文描述： | 1-CH 12-BIT PROPRIETARY METHOD ADC, SERIAL ACCESS, PQFP44 |
| 封裝： | TQFP-44 |
| 文件頁數： | 6/8頁 |
| 文件大?。?/td> | 77K |
| 代理商： | SPT7935SCT |
SPT
6
6/7/99
SPT7935
COMMON MODE VOLTAGE REFERENCE
CIRCUIT
The SPT7935 has an on-board common mode voltage refer-
ence circuit (VCM). It is typically one-half of the supply voltage
and can drive loads of up to 20
A. This circuit is commonly
used to drive the center tap of the RF transformer in fully
differential applications. For single-ended applications, this
output can be used to provide the level shifting required for
the single-to-differential converter conversion circuit.
BIAS CURRENT CIRCUITS
The bias currents suggested (Bias 1 and Bias 2 in figure 2)
optimize device performance for the stated sample rate of
20 MSPS. To achieve the best dynamic performance when
operating the device at sample rates other than 20 MSPS, the
bias current levels should be adjusted. Table I shows the
settings for Bias 1 and Bias 2 for selected sample rates.
Please refer to the application note for more information.
Table I - Sample Rate Settings
Sample Rate (MHz)
Bias 1 (
A)
Bias 2 (
A)
1
20
3.5
5
50
6.5
10
80
8.0
20
90
9.5
CLOCK
The SPT7935 accepts a low voltage CMOS logic level at
the CLK input. The duty cycle of the clock should be kept as
close to 50% as possible. Because consecutive stages in the
ADC are clocked in opposite phase to each other, a non-50%
duty cycle reduces the settling time available for every other
stage and thus potentially causing a degradation of dynamic
performance.
The AC coupled input is most conveniently implemented
using a transformer with a center tapped secondary winding.
The center tap is connected to the VCM pin as shown in
figure 2. To obtain low distortion, it is important that the
selected transformer does not exhibit core saturation at the
full-scale voltage. Proper termination of the input is important
for input signal purity. A small capacitor across the inputs
attenuates kickback noise from the internal sample and hold.
Figure 3 illustrates a solution (based on operational amplifiers) that
can be used if a a DC coupled single-ended input is desired.
The selection criteria of the buffer op-amps is as follows:
- Open loop gain >75 dB
- Gain bandwidth product >50 MHz
- Total harmonic distortion
≤-75 dB
- Signal to Noise ratio >75 dB
POWER SUPPLIES AND GROUNDING
The SPT7935 is operated from a single power supply in the
range of 2.8 to 3.6 volts. Nominal operation is suggested to
be 3.3 volts. All power supply pins should be bypassed as
close to the package as possible. The analog and digital
grounds should be connected together with a ferrite bead as
shown in the typical interface circuit and as close to the ADC
as possible.
REFERENCES
The SPT7935 has a differential analog input. The voltages
applied to the VREF+ and VREF- pins determine the input
voltage range and are equal to
±(VREF+ - VREF-). This
voltage range will be symmetric about the common mode
voltage. Externally generated reference voltages must be
connected to these pins. (See figure 2, Typical Interface
Circuit.) For best performance, these voltages should be
symmetrical about the midpoint of the supply voltage.
VCM
R3
(R3)/2
+
–
Input
Voltage
(
±0.5 V)
R3
R
R2
+
–
+
R
RR
51
51
R
ADC
VIN+
VIN–
15 pF
51
Figure 3 - DC-Coupled Single Ended to Differential Conversion (Power Supplies and Bypassing are Not Shown)
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相關代理商/技術參數 |
參數描述 |
|---|---|
| SPT7935SCT_D | 功能描述:模數轉換器 - ADC RoHS:否 制造商:Texas Instruments 通道數量:2 結構:Sigma-Delta 轉換速率: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 安裝風格:SMD/SMT 封裝 / 箱體:VQFN-32 |
| SPT7935SCTX | 功能描述:模數轉換器 - ADC RoHS:否 制造商:Texas Instruments 通道數量:2 結構:Sigma-Delta 轉換速率: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 安裝風格:SMD/SMT 封裝 / 箱體:VQFN-32 |
| SPT7935SIT | 功能描述:模數轉換器 - ADC RoHS:否 制造商:Texas Instruments 通道數量:2 結構:Sigma-Delta 轉換速率: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 安裝風格:SMD/SMT 封裝 / 箱體:VQFN-32 |
| SPT7935SITX | 功能描述:模數轉換器 - ADC RoHS:否 制造商:Texas Instruments 通道數量:2 結構:Sigma-Delta 轉換速率: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 安裝風格:SMD/SMT 封裝 / 箱體:VQFN-32 |
| SPT7936 | 制造商:FAIRCHILD 制造商全稱:Fairchild Semiconductor 功能描述:12-BIT, 28 MSPS SAMPLING A/D CONVERTER |
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