AD7730LBRU Analog Devices Inc, AD7730LBRU Datasheet - Page 32

• 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

AD7730/AD7730L
The range of input span in both the unipolar and bipolar modes
has a minimum value of 0.8 FS and a maximum value of
2.1 FS. However, the span (which is the difference between
the bottom of the AD7730’s input range and the top of its input
range) has to take into account the limitation on the positive
full-scale voltage. The amount of offset which can be accommo-
dated depends on whether the unipolar or bipolar mode is being
used. Once again, the offset has to take into account the limita-
tion on the positive full-scale voltage. In unipolar mode, there is
considerable flexibility in handling negative (with respect to
AIN(–)) offsets. In both unipolar and bipolar modes, the range
of positive offsets that can be handled by the part depends on
the selected span. Therefore, in determining the limits for sys-
tem zero-scale and full-scale calibrations, the user has to ensure
that the offset range plus the span range does exceed 1.05 FS.
This is best illustrated by looking at a few examples.
If the part is used in unipolar mode with a required span of
0.8 FS, the offset range the system calibration can handle is
from –1.05 FS to +0.25 FS. If the part is used in unipolar
mode with a required span of FS, the offset range the system cali-
bration can handle is from –1.05 FS to +0.05 FS. Similarly, if
the part is used in unipolar mode and required to remove an
offset of 0.2 FS, the span range the system calibration can
handle is 0.85 FS.
If the part is used in bipolar mode with a required span of
from –0.65 FS to +0.65 FS. If the part is used in bipolar
mode with a required span of FS, the offset range the system
calibration can handle is from –0.05 FS to +0.05
larly, if the part is used in bipolar mode and required to remove
an offset of 0.2
handle is 0.85
ranges.
Power-Up and Calibration
On power-up, the AD7730 performs an internal reset which sets
the contents of the internal registers to a known state. There are
default values loaded to all registers after a power-on or reset.
The default values contain nominal calibration coefficients for
the calibration registers. To ensure correct calibration for the
device, a calibration routine should be performed after power-up.
INPUT RANGE
0.4 FS, the offset range the system calibration can handle is
(0.8
2.1
AD7730
FS TO
FS)
Figure 16. Span and Offset Limits
0V DIFFERENTIAL
–1.05
1.05
FS. Figure 16 summarizes the span and offset
FS, the span range the system calibration can
FS.
FS.
UPPER LIMIT. AD7730’s INPUT
VOLTAGE CANNOT EXCEED THIS
GAIN CALIBRATIONS EXPAND OR
CONTRACT THE AD7730’s INPUT
RANGE
NOMINAL ZERO-SCALE POINT
ZERO-SCALE CALIBRATIONS
MOVE INPUT RANGE UP OR DOWN
LOWER LIMIT. AD7730’s INPUT
VOLTAGE CANNOT EXCEED THIS
FS. Simi-
–32–
The power dissipation and temperature drift of the AD7730 are
low and no warm-up time is required before the initial calibra-
tion is performed. If, however, an external reference is being
used, this reference must have stabilized before calibration is
initiated. Similarly, if the clock source for the part is generated
from a crystal or resonator across the MCLK pins, the start-up
time for the oscillator circuit should elapse before a calibration
is initiated on the part (see below).
Drift Considerations
The AD7730 uses chopper stabilization techniques to minimize
input offset drift. Charge injection in the analog multiplexer and
dc leakage currents at the analog input are the primary sources
of offset voltage drift in the part. The dc input leakage current is
essentially independent of the selected gain. Gain drift within
the converter depends primarily upon the temperature tracking
of the internal capacitors. It is not affected by leakage currents.
When operating the part in CHOP mode (CHP = 1), the signal
chain including the first-stage filter is chopped. This chopping
reduces the overall offset drift to 5 nV/ C. Integral and differen-
tial linearity errors are not significantly affected by temperature
changes.
Care must also be taken with external drift effects in order to
achieve optimum drift performance. The user has to be espe-
cially careful to avoid, as much as possible, thermocouple effects
from junctions of different materials. Devices should not be
placed in sockets when evaluating temperature drift, there
should be no links in series with the analog inputs and care
must be taken as to how the input voltage is applied to the input
pins. The true offset drift of the AD7730 itself can be evaluated
by performing temperature drift testing of the part with the
AIN(–)/AIN(–) input channel arrangement (i.e., internal shorted
input, test mode).
USING THE AD7730
Clocking and Oscillator Circuit
The AD7730 requires a master clock input, which may be an
external CMOS compatible clock signal applied to the MCLK IN
pin with the MCLK OUT pin left unconnected. Alternatively, a
crystal or ceramic resonator of the correct frequency can be
connected between MCLK IN and MCLK OUT in which case
the clock circuit will function as an oscillator, providing the
clock source for the part. The input sampling frequency, the
modulator sampling frequency, the –3 dB frequency, output
update rate and calibration time are all directly related to the
master clock frequency, f
frequency by a factor of two will halve the above frequencies and
update rate and double the calibration time.
The crystal or ceramic resonator is connected across the MCLK
IN and MCLK OUT pins, as per Figure 17. Capacitors C1 and
C2 may or may not be required and may vary in value depend-
ing on the crystal/resonator manufacturer's recommendations.
The AD7730 has a capacitance of 5 pF on MCLK IN and 13 pF
on MCLK OUT so, in most cases, capacitors C1 and C2 will
not be required to get the crystal/resonator operating at its cor-
rect frequency.
CLK IN
. Reducing the master clock
REV. A