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AD7714ARS-5 参数 Datasheet PDF下载

AD7714ARS-5图片预览
型号: AD7714ARS-5
PDF下载: 下载PDF文件 查看货源
内容描述: 3 V / 5 V , CMOS , 500微安信号调理ADC [3 V/5 V, CMOS, 500 uA Signal Conditioning ADC]
分类和应用: 转换器光电二极管
文件页数/大小: 40 页 / 308 K
品牌: ADI [ ADI ]
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AD7714  
ANALOG FILTERING  
value which, when normalized, is subtracted from all conversion  
results. The full-scale calibration register contains a value  
which, when normalized, is multiplied by all conversion results.  
The offset calibration coefficient is subtracted from the result  
prior to the multiplication by the full-scale coefficient. This  
means that the full-scale coefficient is effectively a span or gain  
coefficient.  
The digital filter does not provide any rejection at integer mul-  
tiples of the input sampling frequency, as outlined earlier. How-  
ever, due to the AD7714’s high oversampling ratio, these bands  
occupy only a small fraction of the spectrum and most broad-  
band noise is filtered. This means that the analog filtering re-  
quirements in front of the AD7714 are considerably reduced  
versus a conventional converter with no on-chip filtering. In  
addition, because the part’s common-mode rejection perfor-  
mance of 100 dB extends out to several kHz, common-mode  
noise in this frequency range will be substantially reduced.  
2
The AD7714 offers self-calibration, system calibration and  
background calibration facilities. For full calibration to occur  
on the selected channel, the on-chip microcontroller must record  
the modulator output for two different input conditions. These  
are “zero-scale” and “full-scale” points. These points are de-  
rived by performing a conversion on the different input voltages  
provided to the input of the modulator during calibration. As a  
result, the accuracy of the calibration can only be as good as the  
noise level which the part provides in normal mode. The result  
of the “zero-scale” calibration conversion is stored in the Zero  
Scale Calibration Register for the appropriate channel. The  
result of the “full-scale” calibration conversion is stored in the  
Full-Scale Calibration Register for the appropriate channel. With  
these readings, the microcontroller can calculate the offset and  
the gain slope for the input to output transfer function of the  
converter. Internally, the part works with 33 bits of resolution  
to determine its conversion result of either 16 bits or 24 bits.  
Depending on the application, however, it may be necessary to  
provide attenuation prior to the AD7714 in order to eliminate  
unwanted frequencies from these bands which the digital filter  
will pass. It may also be necessary in some applications to pro-  
vide analog filtering in front of the AD7714 to ensure that dif-  
ferential noise signals outside the band of interest do not  
saturate the analog modulator.  
If passive components are placed in front of the AD7714, in  
unbuffered mode, care must be taken to ensure that the source  
impedance is low enough so as not to introduce gain errors in  
the system. This significantly limits the amount of passive anti-  
aliasing filtering which can be provided in front of the AD7714  
when it is used in unbuffered mode. However, when the part is  
used in buffered mode, large source impedances will simply  
result in a small dc offset error (a 10 ksource resistance will  
cause an offset error of less than 10 µV). Therefore, if the sys-  
tem requires any significant source impedances to provide pas-  
sive analog filtering in front of the AD7714, it is recommended  
that the part be operated in buffered mode.  
Self-Calibration  
A self-calibration is initiated on the AD7714 by writing the  
appropriate values (0, 0, 1) to the MD2, MD1 and MD0 bits of  
the Mode Register. In the self-calibration mode with a unipolar  
input range, the zero-scale point used in determining the cali-  
bration coefficients is with the inputs of the differential pair  
internally shorted on the part (i.e., AIN(+) = AIN(–) = Internal  
Bias Voltage). The PGA is set for the selected gain (as per G2,  
G1, G0 bits in the Mode Register) for this zero-scale calibration  
conversion. The full-scale calibration conversion is performed at  
CALIBRATION  
The AD7714 provides a number of calibration options which  
can be programmed via the MD2, MD1 and MD0 bits of the  
Mode Register. The different calibration options are outlined  
in the Mode Register and Calibration Sequences sections. A  
calibration cycle may be initiated at any time by writing to these  
bits of the Mode Register. Calibration on the AD7714 removes  
offset and gain errors from the device. A calibration routine  
should be initiated on the device whenever there is a change in  
the ambient operating temperature or supply voltage. It should  
also be initiated if there is a change in the selected gain, filter  
notch or bipolar/unipolar input range.  
the selected gain on an internally-generated voltage of VREF  
/
Selected Gain.  
The duration time of the calibration is 6 × 1/Output Rate. This  
is made up of 3 × 1/Output Rate for the zero-scale calibration  
and 3 × 1/Output Rate for the full-scale calibration. At this time  
the MD2, MD1 and MD0 bits in the Mode Register return to  
0, 0, 0. This gives the earliest indication that the calibration  
sequence is complete. The DRDY line goes high when calibra-  
tion is initiated and does not return low until there is a valid  
new word in the data register. The duration time from the cali-  
bration command being issued to DRDY going low is 9 × 1/  
Output Rate. This is made up of 3 × 1/Output Rate for the zero-  
scale calibration, 3 × 1/Output Rate for the full-scale calibration  
and 3 × 1/Output Rate for a conversion on the analog input. If  
DRDY is low before (or goes low during) the calibration com-  
mand write to the Mode Register, it may take up to one modu-  
lator cycle (MCLK IN/128) before DRDY goes high to indicate  
that calibration is in progress. Therefore, DRDY should be  
ignored for up to one modulator cycle after the last bit of the  
calibration command is written to the Mode Register.  
The AD7714 gives the user access to the on-chip calibration  
registers allowing the microprocessor to read the device’s cali-  
bration coefficients and also to write its own calibration coeffi-  
cients to the part from prestored values in E2PROM. This gives  
the microprocessor much greater control over the AD7714’s  
calibration procedure. It also means that the user can verify  
that the device has performed its calibration correctly by com-  
paring the coefficients after calibration with prestored values in  
E2PROM. The values in these calibration registers are 24-bit  
wide. In addition, the span and offset for the part can be  
adjusted by the user.  
There is a significant variation in the value of these coefficients  
across the different output update rates, gains and unipolar/  
bipolar operation. Internally in the AD7714, these coefficients  
are normalized before being used to scale the words coming out  
of the digital filter. The offset calibration register contains a  
For bipolar input ranges in the self-calibrating mode, the se-  
quence is very similar to that just outlined. In this case, the two  
points are exactly the same as above but since the part is config-  
ured for bipolar operation, the output code for zero differential  
input is 800000 Hex in 24-bit mode.  
REV. C  
–23–  
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