Datasheet

Data Sheet AD7938-6
Rev. C | Page 13 of 32
ON-CHIP REGISTERS
The AD7938-6 has 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 AD7938-6 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 and the functions
of the bits are described in . At power-up, the default bit
settings in the control register are all 0s. When writing to the
control register between conversions, ensure that
Table 6
Table 7
CONVST
returns high before the write is performed.
Table 6. 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 7. 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 in Table 8.
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 V
REF
pin. If this bit is Logic 1, the internal reference is selected. See the
Reference section.
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
described in Table 10. The selected input channel is decoded as shown in Table 9.
4, 3
MODE1,
MODE0
The two mode pins select the type of analog input on the eight V
IN
pins. The AD7938-6 can have either eight single-
ended inputs, four fully differential inputs, four pseudo differential inputs, or seven pseudo differential inputs. See
Table 9.
2 SHDW
The SHDW bit in the control register is used in conjunction with the SEQ bit to control the sequencer function and
access the SHDW register. See Table 10.
1 SEQ
The SEQ bit in the control register is used in conjunction with the SHDW bit to control the sequencer function and
access the SHDW register. See Table 10.
0 RANGE
This bit selects the analog input range of the AD7938-6. If it is set to 0, then the analog input range extends from
0 V to V
REF
. If it is set to 1, the analog input range extends from 0 V to 2 × V
REF
. When this range is selected, V
DD
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 8. Power Mode Selection using the Power Management Bits in the Control Register
PM1 PM0 Mode Description
0 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 AD7938-6 enters full shutdown mode at the end of each
conversion. In this mode, all circuitry is powered down.
1 0 Autostandby
When the AD7938-6 enters 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 μs (or 600 ns if
an external reference is used). See the Power Modes of Operation section for more information.
1 1 Full Shutdown
When the AD7938-6 enters this mode, all circuitry is powered down. The information in the control register
is retained.