
AD7730/AD7730L
–20–
REV. A
DAC Register (RS2–RS0 = 1, 0, 0); Power On/Reset Status: 20 Hex
T he DAC Register is an 8-bit register from which data can either be read or to which data can be written. T his register provides
the code for the offset-compensation DAC on the part. T able X VI outlines the bit designations for the DAC Register. DR0
through DR7 indicate the bit location, DR denoting the bits are in the DAC Register. DR7 denotes the first bit of the data
stream. T he number in brackets indicates the power-on/reset default status of that bit. Figure 5 shows a flowchart for reading
from the registers on the AD7730 and Figure 6 shows a flowchart for writing to the registers on the part.
T able X VI. DAC Register
DR7
DR6
DR5
DR4
DR3
DR2
DR1
DR0
ZERO (0)
ZERO (0)
DAC5 (1)
DAC4 (0)
DAC3 (0)
DAC2 (0)
DAC1 (0)
DAC0 (0)
Bit
Location
DR7–DR6
DR5–DR0
Bit
Mnemonic
ZERO
DAC5–DAC0
Description
A zero
must
be written to these bits to ensure correct operation of the AD7730.
DAC Selection Bits. T hese bits program the output of the offset DAC. T he DAC is effectively
6 bits with one sign bit (DAC5) and five magnitude bits. With DAC5 at 1, the DAC output
subtracts from the analog input before it is applied to the PGA. With DAC5 at 0, the DAC
output adds to the analog input before it is applied to the PGA. T he DAC output is given by
(V
REF
/62.5)
×
(D/32) = (V
REF
/2000)
×
D where D is the decimal equivalent of bits DAC4 to
DAC0. T hus, for a 5 V reference applied across the REF IN pins, the DAC resolution is 2.5 mV
and offsets in the range –77.5 mV to +77.5 mV can be removed from the analog input signal
before it is applied to the PGA. Note, that the HIREF bit has no effect on the DAC range or
resolution, it controls the ADC range only.
Offset Calibration Register (RS2–RS0 = 1, 0, 1); Power-On/Reset Status: 800000 Hex
T he AD7730 contains three 24-bit Offset Calibration Registers, labelled Offset Calibration Register 0 to Offset Calibration Reg-
ister 2, to which data can be written and from which data can be read. T he three registers are totally independent of each other.
T he Offset Calibration Register is used in conjunction with the associated Gain Calibration Register to form a register pair. T he
calibration register pair used to scale the output is as outlined in T able X III. T he Offset Calibration Register is updated after an
offset calibration routine (1, 0, 0 or 1, 1, 0 loaded to the MD2, MD1, MD0 bits of the Mode Register). During subsequent
conversions, the contents of this register are subtracted from the filter output prior to gain scaling being performed on the word.
Figure 5 shows a flowchart for reading from the registers on the AD7730 and Figure 6 shows a flowchart for writing to the regis-
ters on the part.
Gain Calibration Register (RS2–RS0 = 1, 1, 0); Power-On/Reset Status: 593CE A
T he AD7730 contains three 24-bit Gain Calibration Registers, labelled Gain Calibration Register 0 to Gain Calibration Register
2, to which data can be written and from which data can be read. T he three registers are totally independent of each other. T he
Gain Calibration Register is used in conjunction with the associated Offset Calibration Register to form a register pair. T he
calibration register pair used to scale the output is as outlined in T able X III. T he Gain Calibration Register is updated after a
gain calibration routine (1, 0, 1 or 1, 1, 1 loaded to the MD2, MD1, MD0 bits of the Mode Register). During subsequent con-
versions, the contents of this register are used to scale the number which has already been offset corrected with the Offset Cali-
bration Register contents. Figure 5 shows a flowchart for reading from the registers on the AD7730 and Figure 6 shows a
flowchart for writing to the registers on the part.
T est Register (RS2–RS0 = 1, 1, 1); Power-On/Reset Status: 000000Hex
T he AD7730 contains a 24-bit T est Register to which data can be written and from which data can be read. T he contents of this
T est Register are used in testing the device. T he user is advised not to change the status of any of the bits in this register from the
default (Power-On or RESET ) status of all 0s as the part will be placed in one of its test modes and will not operate correctly. If the
part enters one of its test modes, exercising
RESET
or writing 32 successive 1s to the part will exit the AD7730 from the mode and
return all register contents to their power-on/reset status. Note, if the part is placed in one of its test modes, it may not be possible to
read back the contents of the T est Register depending on the test mode in which the part has been placed.