Data Sheet
AD7328
Rev. C | Page 27 of 36
REFERENCE
Th
e 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 internal reference circuitry consists of a 2.5 V band gap
reference and a reference buffer. When operating th
e AD7328in 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 s to power up and charge the
680 nF decoupling capacitor during the power-up time.
T
he 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, 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 AD7328 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
AD7328 is operated with a VCC of 5 V, the VDRIVE 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
Th
e AD7328 has an on-chip temperature indicator. The tem-
perature indicator can be used to provide local temperature
measurements on th
e 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. VIN7 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
indicator on th
e 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 th
e AD7328requires more acquisition time for this mode.
4420
4340
–40
100
TEMPERATURE (°C)
ADC
O
UT
P
UT
CO
DE
4410
4400
4390
4380
4370
4360
4350
–20
0
20
40
60
80
±10V RANGE, INT REF
VCC = VDRIVE = 5V
VDD/VSS = ±12V
50kSPS
04852-
033
Figure 46. ADC Output Code vs. Temperature for ±10 V Range
5450
5100
–40
80
TEMPERATURE (°C)
ADC
O
UT
P
UT
CO
DE
5400
5350
5300
5250
5200
5150
–20
0
20
40
60
VCC = VDRIVE = 5V
VDD/VSS = ±12V
±2.5V RANGE
INT REFERENCE
30kSPS
04852-
034
Figure 47. ADC Output Code vs. Temperature for ±2.5 V Range