AD7938/AD7939

Preliminary Technical Data

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 below and the functions of the bits are described in

. At power up, the default bit settings in the control

register are all 0s.

Table 7

Table 7. Control Register Bit Function Description

Table 6. Control Register Bits

MSB

LSB

D11

D10

D9

D8

D7

D6

D5

D4

D3

D2

D1

D0

PM1

PM0

CODING

REF

ADD2

ADD1

ADD0

MODE1

MODE0

SHDW

SEQ

RANGE

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

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

.

4, 3

MODE1,

MODE0

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/AD7939. If it is set to 0, then the analog input range extends

the analog input remains within the supply rails. See

section for more information.

Table 9

Analog Inputs

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

Auto

Shutdown

When operating in auto-shutdown mode, the AD7938/AD7939 will enter full shutdown mode at the end of

each conversion. In this mode, all circuitry is powered down.

1

0

Auto

Standby

When the AD7938/AD7939 enter this mode, all circuitry is partially powered down. This mode is similar to

auto-shutdown mode, but it allows the part to power-up in 1 µsec.

1

1

Full

Shutdown

When the AD7938/AD7939 enter this mode, all circuitry is powered down. The information in the control

register is retained.

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