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AD7707 Datasheet(PDF) 28 Page - Analog Devices

Part No. AD7707
Description  3-Channel 16-Bit, Sigma-Delta ADC
Download  53 Pages
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Maker  AD [Analog Devices]
Homepage  http://www.analog.com
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AD7707 Datasheet(HTML) 28 Page - Analog Devices

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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.


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