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參數(shù)資料
| 型號： | ADS7842E |
| 英文描述： | 12-Bit, 4-Channel Parallel Output Sampling ANALOG-TO-DIGITAL CONVERTER |
| 中文描述： | 12位，4通道并行輸出采樣模擬到數(shù)字轉換器 |
| 文件頁數(shù)： | 13/13頁 |
| 文件大小： | 139K |
| 代理商： | ADS7842E |
13
ADS7842
REFERENCE INPUT
The external reference sets the analog input range. The
ADS7842 will operate with a reference in the range of 100mV
to +V
CC
.
There are several critical items concerning the reference input
and its wide voltage range. As the reference voltage is re-
duced, the analog voltage weight of each digital output code
is also reduced. This is often referred to as the LSB (least
significant bit) size and is equal to the reference voltage
divided by 4096. Any offset or gain error inherent in the A/D
converter will appear to increase, in terms of LSB size, as the
reference voltage is reduced. For example, if the offset of a
given converter is 2 LSBs with a 2.5V reference, then it will
typically be 10 LSBs with a 0.5V reference. In each case, the
actual offset of the device is the same, 1.22mV.
Likewise, the noise or uncertainty of the digitized output will
increase with lower LSB size. With a reference voltage of
100mV, the LSB size is 24
μ
V. This level is below the
internal noise of the device. As a result, the digital output
code will not be stable and vary around a mean value by a
number of LSBs. The distribution of output codes will be
gaussian and the noise can be reduced by simply averaging
consecutive conversion results or applying a digital filter.
With a lower reference voltage, care should be taken to
provide a clean layout including adequate bypassing, a clean
(low noise, low ripple) power supply, a low-noise reference,
and a low-noise input signal. Because the LSB size is lower,
the converter will also be more sensitive to nearby digital
signals and electromagnetic interference.
The voltage into the V
REF
input is not buffered and directly
drives the capacitor digital-to-analog converter (CDAC)
portion of the ADS7842. Typically, the input current is
13
μ
A with a 2.5V reference. This value will vary by
microamps depending on the result of the conversion. The
reference current diminishes directly with both conversion
rate and reference voltage. As the current from the reference
is drawn on each bit decision, clocking the converter more
quickly during a given conversion period will not reduce
overall current drain from the reference.
Data Format
The ADS7842 output data is in Straight Offset Binary
format as shown in Table IV. This figure shows the ideal
output code for the given input voltage and does not include
the effects of offset, gain, or noise.
LAYOUT
For optimum performance, care should be taken with the
physical layout of the ADS7842 circuitry. This is particu-
larly true if the reference voltage is low and/or the conver-
sion rate is high.
The basic SAR architecture is sensitive to glitches or sudden
changes on the power supply, reference, ground connec-
tions, and digital inputs that occur just prior to latching the
output of the analog comparator. Thus, during any single
conversion for an n-bit SAR converter, there are n “win-
dows” in which large external transient voltages can easily
affect the conversion result. Such glitches might originate
from switching power supplies, nearby digital logic, and
high power devices. The degree of error in the digital output
depends on the reference voltage, layout, and the exact
timing of the external event. The error can change if the
external event changes in time with respect to the DCLK
input.
With this in mind, power to the ADS7842 should be clean
and well bypassed. A 0.1
μ
F ceramic bypass capacitor should
be placed as close to the device as possible. In addition, a
1
μ
F to 10
μ
F capacitor and a 5
or 10
series resistor may
be used to lowpass filter a noisy supply.
The reference should be similarly bypassed with a 0.1
μ
F
capacitor. Again, a series resistor and large capacitor can be
used to lowpass filter the reference voltage. If the reference
voltage originates from an op amp, make sure that it can
drive the bypass capacitor without oscillation (the series
resistor can help in this case). The ADS7842 draws very
little current from the reference on average, but it does place
larger demands on the reference circuitry over short periods
of time (on each rising edge of CLK during a conversion).
The ADS7842 architecture offers no inherent rejection of
noise or voltage variation in regards to the reference input.
This is of particular concern when the reference input is tied
to the power supply. Any noise and ripple from the supply
will appear directly in the digital results. While high fre-
quency noise can be filtered out as discussed in the previous
paragraph, voltage variation due to line frequency (50Hz or
60Hz) can be difficult to remove.
The GND pin should be connected to a clean ground point.
In many cases, this will be the “analog” ground. Avoid
connections which are too near the grounding point of a
microcontroller or digital signal processor. If needed, run a
ground trace directly from the converter to the power supply
entry point. The ideal layout will include an analog ground
plane dedicated to the converter and associated analog
circuitry.
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相關代理商/技術參數(shù) |
參數(shù)描述 |
|---|---|
| ADS7842E/1K | 功能描述:模數(shù)轉換器 - ADC 12-Bit 4-Ch Parallel Output Sampling RoHS:否 制造商:Texas Instruments 通道數(shù)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 |
| ADS7842E/1KG4 | 功能描述:模數(shù)轉換器 - ADC 12-Bit 4-Ch Parallel Output Sampling RoHS:否 制造商:Texas Instruments 通道數(shù)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 |
| ADS7842EB | 功能描述:模數(shù)轉換器 - ADC 12-Bit 4-Ch Parallel Output Sampling RoHS:否 制造商:Texas Instruments 通道數(shù)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 |
| ADS7842EB | 制造商:Texas Instruments 功能描述:IC 12BIT ADC 200KHZ SSOP28 制造商:Texas Instruments 功能描述:IC, 12BIT ADC, 200KHZ, SSOP28 制造商:Texas Instruments 功能描述:IC, 12BIT ADC, 200KHZ, SSOP28; Resolution (Bits):12bit; Sampling Rate:200kSPS; Supply Voltage Type:Single; Supply Voltage Min:2.7V; Supply Voltage Max:5.25V; Supply Current:550A; Digital IC Case Style:SSOP; No. of Pins:28; Input ;RoHS Compliant: Yes |
| ADS7842EB | 制造商:Texas Instruments 功能描述:IC's |
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