AD7730LBRU Analog Devices Inc, AD7730LBRU Datasheet - Page 25

• Manufacturer Part Number: AD7730LBRU
• Description: IC ADC TRANSDUCER BRIDGE 24TSSOP
• Manufacturer: Analog Devices Inc

Specifications of AD7730LBRU

• Rohs Status: RoHS non-compliant
• Number Of Bits: 24
• Sampling Rate (per Second): 600
• Data Interface: DSP, Serial, SPI™
• Number Of Converters: 1
• Power Dissipation (max): 125mW
• Voltage Supply Source: Analog and Digital
• Operating Temperature: -40°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 24-TSSOP (0.173", 4.40mm Width)
• For Use With: EVAL-AD7730LEBZ - BOARD EVALUATION FOR AD7730EVAL-AD7730EBZ - BOARD EVAL FOR AD7730

Burnout Currents
The AD7730 contains two 100 nA constant current generators,
one source current from AV
from AIN(–) to AGND. The currents are switched to the se-
lected analog input pair. Both currents are either on or off,
depending on the BO bit of the Mode Register. These currents
can be used in checking that a transducer is still operational
before attempting to take measurements on that channel. If the
currents are turned on, allowed flow in the transducer, a mea-
surement of the input voltage on the analog input taken and the
voltage measured is full scale, it indicates that the transducer
has gone open-circuit. If the voltage measured is 0 V, it indicates
that the transducer has gone short circuit. For normal operation,
these burnout currents are turned off by writing a 0 to the BO
bit. The current sources work over the normal absolute input
voltage range specifications.
REFERENCE INPUT
The AD7730’s reference inputs, REF IN(+) and REF IN(–),
provide a differential reference input capability. The common-
mode range for these differential inputs is from AGND to
AV
REF IN(–)), for specified operation is +2.5 V with the HIREF
bit at 0 V and +5 V with the HIREF bit at 1. The part is also
functional with V
results in a halving of all input ranges. The resolution in nV will
be unaltered but will appear halved in terms of counts.
Both reference inputs provide a high impedance, dynamic load.
The typical average dc input leakage current over temperature
is 8.5 A with HIREF = 1 and V
HIREF = 0 and V
each reference input is dynamic, external resistance/capacitance
combinations on these inputs may result in gain errors on the
part.
The AD7730 can be operated in either ac or dc mode. If the
bridge excitation is fixed dc, the AD7730 should be operated in
dc mode. If the analog input and the reference inputs are externally
chopped before being applied to the part the AD7730 should be
operated in ac mode and not dc mode. In ac mode, it is assumed
that both the analog inputs and reference inputs are chopped
and as a result change phase every alternate chopping cycle. If
the chopping is synchronized by the AD7730 (using the ACX
signals to control the chopping) the part then takes into account
the reversal of the analog input and reference input signals.
The output noise performance outlined in Tables I through IV
is for an analog input of 0 V and is unaffected by noise on 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 AD7730. If the reference noise in the
bandwidth of interest is excessive, it will degrade the performance
of the AD7730. In applications where the excitation voltage for
the bridge transducer on the analog input also drives the refer-
ence voltage for the part, the effect of the noise in the excita-
tion voltage will be removed as the application is ratiometric.
Figure 7 shows how the reference voltage can be connected in a
ratiometric fashion in a dc-excited bridge application. In this
case, the excitation voltage for the AD7730 and the transducer
is a dc voltage. The HIREF bit of the Mode Register should be
set to 1. Figure 8 meanwhile shows how the reference can be
connected in a ratiometric fashion in an ac-excited bridge
REV. A
DD
. The nominal reference voltage, V
REF
REF
of +2.5 V with the HIREF bit at 1. This
= +2.5 V. Because the input impedance of
DD
to AIN(+) and one sink current
REF
= +5 V, and 2.5 A with
REF
(REF IN(+)—
–25–
application. In this case, both the reference voltage for the part
and the excitation voltage for the transducer are chopped. Once
again, the HIREF bit should be set to 1.
If the AD7730 is not used in a ratiometric application, a low
noise reference should be used. Recommended 2.5 V reference
voltage sources for the AD7730 include the AD780, REF43
and REF192. If any of these references are used as the reference
source for the AD7730, the HIREF bit should be set to 0. It is
generally recommended to decouple the output of these references
to further reduce the noise level.
Reference Detect
The AD7730 includes on-chip circuitry to detect if the part
has a valid reference for conversions or calibrations. If the volt-
age between the REF IN(+) and REF IN(–) pins goes below
0.3 V or either the REF IN(+) or REF IN(–) inputs is open
circuit, the AD7730 detects that it no longer has a valid reference.
In this case, the NO REF bit of the Status Register is set to a 1.
If the AD7730 is performing normal conversions and the NO
REF bit becomes active, the part places all ones in the Data
Register. Therefore, it is not necessary to continuously monitor
the status of the NO REF bit when performing conversions. It is
only necessary to verify its status if the conversion result read
from the Data Register is all 1s.
VOLTAGE = +5V
VOLTAGE = +5V
EXCITATION
EXCITATION
Figure 7. Ratiometric Generation of Reference in DC-
Excited Bridge Application
Figure 8. Ratiometric Generation of Reference in AC-
Excited Bridge Application
OUT+
IN–
IN+
OUT+
OUT–
IN–
IN+
OUT–
REF IN(+)
REF IN(–)
AD7730/AD7730L
AIN1(+)
AIN1(–)
AGND
AV
REF IN(+)
REF IN(–)
AIN1(+)
AIN1(–)
DD
AD7730
ACX
ACX
AGND
DV
AV
DD
DD
EXCITATION
AD7730
CLOCK
DGND
DV
AC
DD
DGND