Datasheet

Data Sheet AD7327
Rev. B | Page 27 of 36
REFERENCE
The AD7327 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 AD7327 conversion. Suitable reference sources
for the AD7327 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 AD7327
in internal reference mode, the 2.5 V internal reference is available
at the REFIN/OUT 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 A.
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 AD7327 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 V
DD
and V
SS
supply
must be equal to or greater than the maximum analog input
range selected (see Table 6).
V
DRIVE
The AD7327 has a V
DRIVE
feature to control the voltage at which
the serial interface operates. V
DRIVE
allows the ADC to easily
interface to both 3 V and 5 V processors. For example, if the
AD7327 is operated with a V
CC
of 5 V, the V
DRIVE
pin can be
powered from a 3 V supply. This allows the AD7327 to accept
large bipolar input signals with low voltage digital processing.
TEMPERATURE INDICATOR
The AD7327 has an on-chip temperature indicator. The
temperature indicator can be used to give local temperature
measurements on the AD7327. To access the temperature
indicator, the ADC should be configured in pseudo differential
mode, Mode 1 = Mode 0 = 1, and channel Bit ADD2, Bit ADD1,
and Bit ADD0 should be set to 1. V
IN
7 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 46 and Figure 47). When using the temperature
indicator on the AD7327, the part should be operated at low
throughput rates, such as approximately 50 kSPS for the ±10 V
range and 30 kSPS for the ±2.5 V range. The throughput rate is
reduced for the temperature indicator mode because the AD7327
requires more acquisition time for this mode.
4420
4340
–40 100
TEMPERATURE (°C)
ADC OUTPUT CODE
4410
4400
4390
4380
4370
4360
4350
–20 0 20 40 60 80
±10V RANGE, INT REF
V
CC
=V
DRIVE
=5V
V
DD
/V
SS
= ±12V
50kSPS
05401-033
Figure 46. Temperature vs. ADC Output Code for ±10 V Range
5450
5100
–40 80
TEMPERATURE (°C)
ADC OUTPUT CODE
5400
5350
5300
5250
5200
5150
–20 0 20 40 60
V
CC
=V
DRIVE
=5V
V
DD
/V
SS
=±12V
±2.5V RANGE
INT REFERENCE
30kSPS
05401-034
Figure 47. Temperature vs. ADC Output Code for ±2.5 V Range