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AD7329 Datasheet(PDF) 31 Page - Analog Devices

Part No. AD7329
Description  1 MSPS, 8-Channel, Software-Selectable, True Bipolar Input, 12-Bit Plus Sign ADC
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Maker  AD [Analog Devices]
Homepage  http://www.analog.com
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AD7329 Datasheet(HTML) 31 Page - Analog Devices

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AD7329
Rev. 0 | Page 31 of 40
REFERENCE
The AD7329 can operate with either the internal 2.5 V on-chip
reference or an externally applied reference. The internal
reference is selected by setting the Ref bit in the control register
to 1. On power-up, the Ref bit is 0, which selects the external
reference for the AD7329 conversion. Suitable reference sources
for the AD7329 include AD780, AD1582, ADR431, REF193,
and ADR391.
The internal reference circuitry consists of a 2.5 V band gap
reference and a reference buffer. When operating the AD7329
in internal reference mode, the 2.5 V internal reference is
available at the REFIN/REFOUT pin, which should be decoupled
to AGND using a 680 nF capacitor. It is recommended that the
internal reference be buffered before applying it elsewhere in
the system. The internal reference is capable of sourcing up to
90 μA.
On power-up, if the internal reference operation is required for
the ADC conversion, a write to the control register is necessary
to set the Ref bit to 1. During the control register write, the
conversion result from the first initial conversion is invalid. The
reference buffer requires 500 μs to power up and charge the
680 nF decoupling capacitor during the power-up time.
The AD7329 is specified for a 2.5 V to 3 V reference range.
When a 3 V reference is selected, the ranges are ±12 V, ±6 V,
±3 V, and 0 V to +12 V. For these ranges, the VDD and VSS supply
must be equal to or greater than the maximum analog input
range selected.
VDRIVE
The AD7329 has a VDRIVE feature to control the voltage at which
the serial interface operates. VDRIVE allows the ADC to easily
interface to both 3 V and 5 V processors. For example, if the
AD7329 is operated with a VCC of 5 V, the VDRIVE pin can be
powered from a 3 V supply. This allows the AD7329 to accept
large bipolar input signals with low voltage digital processing.
TEMPERATURE INDICATOR
The AD7329 has an on-chip temperature indicator. The
temperature indicator can be used to provide local temperature
measurements on the AD7329. To access the temperature
indicator, the ADC should be configured in pseudo differential
mode, Mode 1 = Mode 0 = 1, which sets Channel Bits ADD2,
ADD1, and ADD0 to 1. VIN7 must be tied to AGND or to a
small dc voltage within the specified pseudo input range for the
selected analog input range. When a conversion is initiated in
this configuration, the output code represents the temperature
(see Figure 50). When using the temperature indicator on the
AD7329, the part should be operated at low throughput rates, such
as approximately 30 kSPS for the ±2.5 V range. The throughput
rate is reduced for the temperature indicator mode because the
AD7329 requires more acquisition time for this mode.
5450
5050
–40
80
TEMPERATURE (°C)
VCC = VDRIVE = 5V
VDD = 12V, VSS = –12V
±2.5V RANGE
INTERNAL REFERENCE
30kSPS
5400
5350
5300
5250
5200
5150
5100
–20
0
204060
Figure 50. Temperature vs. ADC Output Code for ±2.5 V Range
4420
4340
–40
100
TEMPERATURE (°C)
4410
4400
4390
4380
4370
4360
4350
–20
0
20406080
±10V RANGE, INT REF
VCC =VDRIVE =5V
VDD/VSS = ±12V
50kSPS
Figure 51. Temperature vs. ADC Output Code for ±10 V Range


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