Data Sheet
AD7938/AD7939
Rev. C | Page 15 of 36
ON-CHIP REGISTERS
The AD7938/AD7939 have two on-chip registers that are
necessary for the operation of the device. These are the control
register, which is used to set up different operating conditions,
and the shadow register, which is used to program the analog
input channels to be converted.
CONTROL REGISTER
The control register on the AD7938/AD7939 is a 12-bit, write-
only register. Data is written to this register using the CS and
WR pins. The control register is shown in
Table 7 and the
functions of the bits are described
in Table 8. At power up, the
default bit settings in the control register are all 0s.
Table 7. Control Register Bits
MSB
LSB
DB11
DB10
DB9
DB8
DB7
DB6
DB5
DB4
DB3
DB2
DB1
DB0
PM1
PM0
CODING
REF
ADD2
ADD1
ADD0
MODE1
MODE0
SHDW
SEQ
RANGE
Table 8. Control Register Bit Function Description
Bit No.
Mnemonic
Description
11, 10
PM1, PM0
Power Management Bits. These two bits are used to select the power mode of operation. The user can choose
between either normal mode or various power-down modes of operation, as shown i
n Table 9.9
CODING
This bit selects the output coding of the conversion result. If this bit is set to 0, the output coding is straight
(natural) binary. If this bit is set to 1, the output coding is twos complement.
8
REF
This bit selects whether the internal or external reference is used to perform the conversion. If this bit is Logic 0, an
external reference should be applied to the VREF pin. If this bit is Logic 1, the internal reference is selected. See the
7 to 5
ADD2 to
ADD0
These three address bits are used to either select which analog input channel is converted in the next conversion if
the sequencer is not used, or to select the final channel in a consecutive sequence when the sequencer is used, as
4, 3
MODE1,
MODE0
The two mode pins select the type of analog input on the eight VIN pins. The AD7938/AD7939 can have either eight
single-ended inputs, four fully differential inputs, four pseudo differential inputs, or seven pseudo differential
2
SHDW
The SHDW bit in the control register is used in conjunction with the SEQ bit to control the sequencer function and
1
SEQ
The SEQ bit in the control register is used in conjunction with the SHDW bit to control the sequencer function and
0
RANGE
This bit selects the analog input range of the AD7938/AD7939. If it is set to 0, the analog input range extends from
0 V to VREF. If it is set to 1, the analog input range extends from 0 V to 2 × VREF. When this range is selected, VDD must
be 4.75 V to 5.25 V if a 2.5 V reference is used; otherwise, care must be taken to ensure that the analog input remains
within the supply rails. See the
Analog Inputs section for more information.
Table 9. Power Mode Selection Using the Power Management Bits in the Control Register
PM1
PM0
Mode
Description
0
Normal Mode
When operating in normal mode, all circuitry is fully powered up at all times.
0
1
Autoshutdown
When operating in autoshutdown mode, the AD7938/AD7939 enter full shutdown mode at the end of
each conversion. In this mode, all circuitry is powered down.
1
0
Autostandby
When the AD7938/AD7939 enter this mode, all circuitry is powered down except for the reference and
reference buffer. This mode is similar to autoshutdown mode, but it allows the part to power up in 7 s (or
1
Full Shutdown
When the AD7938/AD7939 enter this mode, all circuitry is powered down. The information in the control
register is retained.