Datasheet

Data Sheet AD7328
Rev. C | Page 27 of 36
REFERENCE
The AD7328 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 AD7328 conversion. Suitable reference sources for the AD7328
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 AD7328
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 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 con-
version 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 AD7328 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, a n d
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.
V
DRIVE
The AD7328 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
AD7328 is operated with a V
CC
o f 5 V, t h e V
DRIVE
pin can be
powered from a 3 V supply. This allows the AD7328 to accept
large bipolar input signals with low voltage digital processing.
TEMPERATURE INDICATOR
The AD7328 has an on-chip temperature indicator. The tem-
perature indicator can be used to provide local temperature
measurements on the AD7328. 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. V
IN
7 must be tied to AGND or to a small dc
voltage within the specified pseudo differential 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 AD7328, 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 AD7328
requires more acquisition time for this mode.
442
0
4340
–40
100
TEMPERATURE (°C)
ADC OUTPUT CODE
441
0
4400
4390
4380
4370
4360
435
0
–20 0 2
0
40 60
80
±10V RANGE, INT REF
V
C
C
= V
DR
IV
E
=
5
V
V
DD
/V
SS
= ±12V
50k
SPS
04852-033
Figure 46. ADC Output Code vs. Temperature 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
04852-034
Figure 47. ADC Output Code vs. Temperature for ±2.5 V Range