Datasheet

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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

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

and

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 in Table 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 11. The selected input channel is decoded as shown in Table 10.

4, 3 MODE1,

MODE0

The two mode pins select the type of analog input on the eight V

pins. The AD7938/AD7939 can have either eight

single-ended inputs, four fully differential inputs, four pseudo differential inputs, or seven pseudo differential

inputs. See Table 10.

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 11.

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 11.

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 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

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 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 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/AD7939 enter this mode, all circuitry is powered down. The information in the control