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AD7707 Datasheet(PDF) 28 Page - Analog Devices |
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AD7707 Datasheet(HTML) 28 Page - Analog Devices |
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28 / 53 page  AD7707 Rev. B | Page 27 of 52 REFERENCE INPUT The AD7707 reference inputs, REF IN(+) and REF IN(−), provide a differential reference input capability. The common- mode range for these differential inputs is from GND to AVDD. The nominal reference voltage, VREF REF IN(+) − REF IN(−), for specified operation is +2.5 V for the AD7707 operated with an AVDD of 5 V and 1.225 V for the AD7707 operated with an AVDD of 3 V. The part is functional with VREF voltages down to 1 V, but with degraded performance because the LSB size is smaller. REF IN(+) must always be greater than REF IN(−) for correct operation of the AD7707. 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 reference inputs is fCLKIN/64 and does not vary with gain. For gains of 1 and 2, 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. The output noise performance outlined in Table 7 through Table 13 is for an analog input of 0 V, which effectively removes the effect of noise from 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 AD7707. If the reference noise in the bandwidth of interest is excessive, it degrades the performance of the AD7707. In bridge transducer applications where the reference voltage for the ADC is derived from the excitation voltage, the effect of the noise in the excitation voltage is removed because the application is ratiometric. Rec- ommended reference voltage sources for the AD7707 with an AVDD of 5 V include the AD780, REF43, and REF192, and the recommended reference sources for the AD7707 operated with an AVDD of 3 V include the AD589 and AD1580. It is generally recommended to decouple the output of these references to further reduce the noise level. |
Similar Part No. - AD7707_17 |
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Similar Description - AD7707_17 |
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