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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  
Table XIV. Input Sampling Frequency vs. Gain  
Input Sampling Freq (fS)  
for the AD7714-3. The part is functional with VREF voltages  
down to 1 V but with degraded performance as the output noise  
will, in terms of LSB size, be larger. REF IN(+) must always be  
greater than REF IN(–) for correct operation of the AD7714.  
Gain  
1
fCLK IN/64 (38.4 kHz @ fCLK IN = 2.4576 MHz)  
2
4
8
16  
32  
64  
128  
2 × fCLK IN/64 (76.8 kHz @ fCLK IN = 2.4576 MHz)  
4 × fCLK IN/64 (153.6 kHz @ fCLK IN = 2.4576 MHz)  
8 × fCLK IN/64 (307.2 kHz @ fCLK IN = 2.4576 MHz)  
8 × fCLK IN/64 (307.2 kHz @ fCLK IN = 2.4576 MHz)  
8 × fCLK IN/64 (307.2 kHz @ fCLK IN = 2.4576 MHz)  
8 × fCLK IN/64 (307.2 kHz @ fCLK IN = 2.4576 MHz)  
8 × fCLK IN/64 (307.2 kHz @ fCLK IN = 2.4576 MHz)  
Both reference inputs provide a high impedance, dynamic load  
similar to the analog inputs in unbuffered mode. The maxi-  
mum dc input leakage current is ±1 nA over temperature and  
source resistance may result in gain errors on the part. In this  
case, the sampling switch resistance is 5 ktyp and the refer-  
ence capacitor (CREF) varies with gain. The sample rate on the  
reference inputs is fCLK IN/64 and does not vary with gain. For  
gains of 1 to 8, CREF is 8 pF; for a gain of 16, it is 5.5 pF, for a  
gain of 32, it is 4.25 pF, for a gain of 64, it is 3.625 pF and for a  
gain of 128, it is 3.3125 pF.  
2
Burnout Current  
The AD7714 contains two 1 µA currents, one source current  
from AVDD to AIN(+) and one sink from AIN(–) to AGND. The  
currents are either both on or off depending on the BO bit of the  
Mode Register. These currents can be used in checking that a  
transducer has not burned out nor gone open-circuit before  
attempting to take measurements on that channel. If the cur-  
rents are turned on, allowed flow in the transducer, a measure-  
ment of the input voltage on the analog input taken and the  
voltage measured is full scale, it indicates that the transducer has  
gone open-circuit; if the voltage measured is zero, it indicates  
that the transducer has gone short-circuit. For normal opera-  
tion, these burnout currents are turned off by writing a 0 to the  
BO bit. For the source current to work correctly, the applied  
voltage on AIN(+) should not go within 500 mV of AVDD. For  
the sink current to work correctly, the applied voltage on the  
AIN(–) input should not go within 500 mV of AGND.  
The output noise performance outlined in Tables I through IV  
is for an analog input of 0 V and is unaffected by noise on the  
reference. To obtain the same noise performance as shown in  
the noise tables over the full input range requires a low noise  
reference source for the AD7714. If the reference noise in the  
bandwidth of interest is excessive, it will degrade the perfor-  
mance of the AD7714. In applications where the excitation  
voltage for the bridge transducer on the analog input also de-  
rives the reference voltage for the part, the effect of the noise in  
the excitation voltage will be removed as the application is  
ratiometric. Recommended reference voltage sources for the  
AD7714-5 and AD7714Y grade with AVDD = 5 V include the  
AD780, REF43 and REF192 while the recommended reference  
sources for the AD7714-3 and AD7714Y with AVDD = 3 V  
include the AD589 and AD1580. It is generally recommended  
to decouple the output of these references to further reduce the  
noise level.  
Bipolar/Unipolar Inputs  
The analog inputs on the AD7714 can accept either unipolar or  
bipolar input voltage ranges. Bipolar input ranges do not imply  
that the part can handle negative voltages on its analog inputs,  
since the analog input cannot go more negative than –30 mV to  
ensure correct operation of the part. The input channels are  
either fully differential or pseudo-differential (all other channels  
referenced to AIN6). In either case, the input channels are  
arranged in pairs with an AIN(+) and AIN(–). As a result, the  
voltage to which the unipolar and bipolar signals on the AIN(+)  
input are referenced is the voltage on the respective AIN(–)  
input. For example, if AIN(–) is +2.5 V and the AD7714 is  
configured for unipolar operation with a gain of 2 and a VREF of  
+2.5 V, the input voltage range on the AIN(+) input is +2.5 V to  
+3.75 V. If AIN(–) is +2.5 V and the AD7714 is configured for  
bipolar mode with a gain of 2 and a VREF of +2.5 V, the analog  
input range on the AIN(+) input is +1.25 V to +3.75 V (i.e.,  
2.5 V ± 1.25 V). If AIN(–) is at AGND, the part cannot be con-  
figured for bipolar ranges in excess of ±30 mV.  
DIGITAL FILTERING  
The AD7714 contains an on-chip low-pass digital filter which  
processes the output of the part’s sigma-delta modulator. There-  
fore, the part not only provides the analog-to-digital conversion  
function but it also provides a level of filtering. There are a  
number of system differences when the filtering function is  
provided in the digital domain rather than the analog domain  
and the user should be aware of these.  
First, since digital filtering occurs after the A-to-D conversion  
process, it can remove noise injected during the conversion  
process. Analog filtering cannot do this. Also, the digital filter  
can be made programmable far more readily than an analog  
filter. Depending on the digital filter design, this gives the user  
the capability of programming cutoff frequency and output  
update rate.  
On the other hand, analog filtering can remove noise superim-  
posed on the analog signal before it reaches the ADC. Digital  
filtering cannot do this and noise peaks riding on signals near  
full scale have the potential to saturate the analog modulator  
and digital filter, even though the average value of the signal is  
within limits. To alleviate this problem, the AD7714 has over-  
range headroom built into the sigma-delta modulator and digital  
filter which allows overrange excursions of 5% above the analog  
input range. If noise signals are larger than this, consideration  
should be given to analog input filtering, or to reducing the  
input channel voltage so that its full scale is half that of the  
analog input channel full scale. This will provide an overrange  
capability greater than 100% at the expense of reducing the  
dynamic range by 1 bit (50%).  
Bipolar or unipolar options are chosen by programming the B/U  
bit of the Filter High Register. This programs the selected chan-  
nel for either unipolar or bipolar operation. Programming the  
channel for either unipolar or bipolar operation does not change  
any of the input signal conditioning; it simply changes the data  
output coding and the points on the transfer function where  
calibrations occur.  
REFERENCE INPUT  
The AD7714’s reference inputs, REF IN(+) and REF IN(–),  
provide a differential reference input capability. The common-  
mode range for these differential inputs is from AGND to AVDD  
The nominal reference voltage, VREF (REF IN(+) –REF IN(–)),  
for specified operation is +2.5 V for the AD7714-5 and +1.25 V  
.
REV. C  
–21–  
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