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AD7715 Datasheet(PDF) 22 Page - Analog Devices |
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AD7715 Datasheet(HTML) 22 Page - Analog Devices |
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22 / 41 page  Data Sheet AD7715 Rev. E | Page 21 of 40 REFERENCE INPUT The reference inputs of the AD7715, 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 AD7715-5 and 1.25 V for the AD7715-3. 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 AD7715. Both reference inputs provide a high impedance, dynamic load similar to the analog inputs in unbuffered mode. The maximum 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 kΩ typical and the reference capacitor (CREF) varies with gain. The sample rate on the refer- ence inputs is fCLKIN/64 and does not vary with gain. For gains of 1 and 2, CREF is 8 pF; for a gain of 32, it is 4.25 pF, and for a gain of 128, it is 3.3125 pF. The output noise performance outlined in Table 15 through Table 22 is for an analog input of 0 V which effectively removes the effect of 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 AD7715. If the reference noise in the bandwidth of interest is excessive, it will degrade the performance of the AD7715. In applications where the excitation voltage for the bridge transducer on the analog input also derives 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 AD7715-5 include the AD780, REF43 and REF192, while the recommended reference sources for the AD7715-3 include the AD589 and AD1580. It is generally recommended to decouple the output of these references to further reduce the noise level. DIGITAL FILTERING The AD7715 contains an on-chip low-pass digital filter that processes the output of the part’s Σ-Δ modulator. Therefore, the part not only provides the analog-to-digital conversion function but it also provides a level of filtering. Users should be aware that there are a number of system differences when the filtering function is provided in the digital domain rather than the analog domain. 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 super- imposed 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 AD7715 has overrange headroom built into the Σ-Δ 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 provides an overrange capability greater than 100% at the expense of reducing the dynamic range by 1 bit (50%). In addition, the digital filter does not provide any rejection at integer multiples of the digital filter’s sample frequency. However, the input sampling on the part provides attenuation at multiples of the digital filter’s sampling frequency so that the unattenuated bands actually occur around multiples of the sampling frequency fS (as defined in Table 25). Thus the unattenuated bands occur at n × fS (where n = 1, 2, 3 … ). At these frequencies, there are frequency bands, ±f3dB wide (f3dB is the cutoff frequency of the digital filter) at either side where noise passes unattenuated to the output. |
Similar Part No. - AD7715_17 |
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Similar Description - AD7715_17 |
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