AD7730BNZ Analog Devices Inc, AD7730BNZ Datasheet - Page 30

• Manufacturer Part Number: AD7730BNZ
• Description: IC ADC TRANSDUCER BRIDGE 24-DIP
• Manufacturer: Analog Devices Inc

Specifications of AD7730BNZ

• Data Interface: DSP, Serial, SPI™
• Number Of Bits: 24
• Sampling Rate (per Second): 1.2k
• Number Of Converters: 1
• Power Dissipation (max): 125mW
• Voltage Supply Source: Analog and Digital
• Operating Temperature: -40°C ~ 85°C
• Mounting Type: Through Hole
• Package / Case: 24-DIP (0.300", 7.62mm)
• Resolution (bits): 24bit
• Sampling Rate: 1.2kSPS
• Input Channel Type: Differential, Single Ended
• Supply Voltage Range - Analog: 4.75V To 5.25V
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• For Use With: EVAL-AD7730LEBZ - BOARD EVALUATION FOR AD7730EVAL-AD7730EBZ - BOARD EVAL FOR AD7730
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant, Lead free / RoHS Compliant

AD7730/AD7730L
Internally in the AD7730, the coefficients are normalized before
being used to scale the words coming out of the digital filter.
The offset calibration register contains a value which, when
normalized, is subtracted from all conversion results. The gain
calibration register contains a value which, when normalized, is
multiplied by all conversion results. The offset calibration coeffi-
cient is subtracted from the result prior to the multiplication by
the gain coefficient.
The AD7730 offers self-calibration or system calibration facili-
ties. For full calibration to occur on the selected channel, the on-
chip microcontroller must record the modulator output for two
different input conditions. These are “zero-scale” and “full-
scale” points. These points are derived by performing a conver-
sion on the different input voltages provided to the input of the
modulator during calibration. The result of the “zero-scale”
calibration conversion is stored in the Offset Calibration Regis-
ter for the appropriate channel. The result of the “full-scale”
calibration conversion is stored in the Gain Calibration Register
for the appropriate channel. With these readings, the microcon-
troller can calculate the offset and the gain slope for the input to
output transfer function of the converter. Internally, the part
works with 33 bits of resolution to determine its conversion
result of either 16 bits or 24 bits.
The sequence in which the zero-scale and full-scale calibration
occurs depends upon the type of full-scale calibration being
performed. The internal full-scale calibration is a two-step cali-
bration that alters the value of the Offset Calibration Register.
Thus, the user must perform a zero-scale calibration (either
internal or system) after an internal full-scale calibration to correct
the Offset Calibration Register contents. When using system
full-scale calibration, it is recommended that the zero-scale
calibration (either internal or system) is performed first.
Since the calibration coefficients are derived by performing a
conversion on the input voltage provided, the accuracy of the
calibration can only be as good as the noise level the part pro-
vides in normal mode. To optimize the calibration accuracy, it
is recommended to calibrate the part at its lowest output rate
where the noise level is lowest. The coefficients generated at any
output update rate will be valid for all selected output update
rates. This scheme of calibrating at the lowest output update
rate does mean that the duration of calibration is longer.
Internal Zero-Scale Calibration
An internal zero-scale calibration is initiated on the AD7730 by
writing the appropriate values (1, 0, 0) to the MD2, MD1 and
MD0 bits of the Mode Register. In this calibration mode with a
unipolar input range, the zero-scale point used in determining
the calibration coefficients is with the inputs of the differential
pair internally shorted on the part (i.e., AIN(+) = AIN(–) =
Externally-Applied AIN(–) voltage). The PGA is set for the
selected gain (as per the RN1, RN0 bits in the Mode Register)
for this internal zero-scale calibration conversion.
The calibration is performed with dc excitation regardless of the
status of the ac bit. The duration time of the calibration de-
pends upon the CHP bit of the Filter Register. With CHP = 1,
the duration is 22
is 24
bits in the Mode Register return to 0, 0, 0 (Sync or Idle Mode
for the AD7730). The RDY line goes high when calibration is
initiated and returns low when calibration is complete. Note
that the part has not performed a conversion at this time; it has
1/Output Rate. At this time the MD2, MD1 and MD0
1/Output Rate; with CHP = 0, the duration
–30–
simply performed a zero-scale calibration and updated the Off-
set Calibration Register for the selected channel. The user must
write either 0, 0, 1 or 0, 1, 0 to the MD2, MD1, MD0 bits of the
Mode Register to initiate a conversion. If RDY is low before (or
goes low during) the calibration command write to the Mode
Register, it may take up to one modulator cycle (MCLK IN/32)
before RDY goes high to indicate that calibration is in progress.
Therefore, RDY should be ignored for up to one modulator
cycle after the last bit of the calibration command is written to
the Mode Register.
For bipolar input ranges in the internal zero-scale calibrating
mode, the sequence is very similar to that just outlined. In this
case, the zero-scale point is exactly the same as above but since
the part is configured for bipolar operation, the output code for
zero differential input is 800000 Hex in 24-bit mode.
The internal zero-scale calibration needs to be performed as
one part of a two part full calibration. However, once a full
calibration has been performed, additional internal zero-scale
calibrations can be performed by themselves to adjust the
part’s zero-scale point only. When performing a two step full
calibration care should be taken as to the sequence in which the
two steps are performed. If the internal zero-scale calibration is
one part of a full self-calibration, then it should take place after
an internal full-scale calibration. If it takes place in association
with a system full-scale calibration, then this internal zero-scale
calibration should be performed first.
Internal Full-Scale Calibration
An internal full-scale calibration is initiated on the AD7730 by
writing the appropriate values (1, 0, 1) to the MD2, MD1 and
MD0 bits of the Mode Register. In this calibration mode, the
full-scale point used in determining the calibration coefficients is
with an internally-generated full-scale voltage. This full-scale
voltage is derived from the reference voltage for the AD7730
and the PGA is set for the selected gain (as per the RN1, RN0
bits in the Mode Register) for this internal full-scale calibration
conversion.
In order to meet the post-calibration numbers quoted in the
specifications, it is recommended that internal full-scale calibra-
tions be performed on the 80 mV range. This applies even if the
subsequent operating mode is on the 10 mV, 20 mV or 40 mV
input ranges.
The internal full-scale calibration is a two-step sequence that
runs when an internal full-scale calibration command is written
to the AD7730. One part of the calibration is a zero-scale cali-
bration and as a result, the contents of the Offset Calibration
Register are altered during this Internal Full-Scale Calibration.
The user must therefore perform a zero-scale calibration (either
internal or system) AFTER the internal full-scale calibration.
This zero-scale calibration should be performed at the operating input
range.
performed in isolation.
The calibration is performed with dc excitation regardless of the
status of the ac bit. The duration time of the calibration de-
pends upon the CHP bit of the Filter Register. With CHP = 1,
the duration is 44
is 48
bits in the Mode Register return to 0, 0, 0 (Sync or Idle Mode
for the AD7730). The RDY line goes high when calibration is
initiated and returns low when calibration is complete. Note
that the part has not performed a conversion at this time. The
This means that internal full-scale calibrations cannot be
1/Output Rate. At this time the MD2, MD1 and MD0
1/Output Rate; with CHP = 0, the duration
REV. A