AD7939BCP datasheet(28/32 Pages) AD

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AD7938/AD7939

Preliminary Technical Data

POWER MODES OF OPERATION

The AD7938/AD7939 have four different power modes of

operation. These modes are designed to provide flexible power

management options. Different options can be chosen to

optimize the power dissipation/throughput rate ratio for

differing applications. The mode of operation is selected by the

power management bits, PM1 and PM0, in the control register,

as detailed in

. When power is first applied to the

AD7938/AD7939 an on-chip, power-on reset circuit ensures

that the default power-up condition is normal mode.

Table 8

Note that, after power-on, the track-and-hold is in hold mode

and the first rising edge of CONVST places the track-and-hold

into track mode.

Normal Mode (PM1 = PM0 = 0)

This mode is intended for the fastest throughput rate

performance because the user does not have to worry about any

power-up times associated with the AD7938/AD7939 because it

remains fully powered up at all times. At power-on reset, this

mode is the default setting in the control register.

Auto-Shutdown (PM1 = 0; PM0 = 1)

In this mode of operation, the AD7938/AD7939 automatically

enters full shutdown at the end of each conversion, which is

shown at Point A in

. In shutdown mode, all internal

circuitry on the device is powered down. The part retains

information in the control register during shutdown. The track-

and-hold also goes into hold at this point and remains in hold

as long as the device is in shutdown. The AD7938/AD7939

remains in shutdown mode until the next rising edge of

CONVST (see Point B in

). In order to keep the device

in shutdown for as long as possible, CONVST should idle low

between conversions as shown in Error! Reference source not

found.

. On this rising edge, the part begins to power-up and

track-and-hold returns to track mode. The power-up time

required depends on whether the user is operating with the

internal or external reference. With the internal reference, the

power-up time is typically TBD, and with an external reference,

the power-up time is typically TBD. The user should ensure that

the power-up time has elapsed before initiating a conversion.

Figure 38

Auto-Standby (PM1 = 1; PM0 = 0)

In this mode of operation, the AD7938/AD7939 automatically

enters standby mode at the end of each conversion, which is

shown as Point A in

. When this mode is entered, all

circuitry on the AD7938/AD7939 is powered down except for

the reference and reference buffer. The track-and-hold goes into

hold at this point also and remains in hold as long as the device

is in standby. The part remains in standby until the next rising

edge of CONVST powers up the device, which takes at least

TBD. The user should ensure this power-up time has elapsed

before initiating another conversion as shown in Error!

Reference source not found.

. This rising edge of CONVST also

places track-and-hold back into track mode.

Full Shutdown Mode (PM1 =1; PM0 = 1)

When this mode is programmed, all circuitry on the

AD7938/AD7939 is powered down upon completion of the

write operation, i.e., on rising edge of WR. The part retains the

information in the control register while the part is in

shutdown. Also, track-and-hold goes into hold mode at this

point. The AD7938/AD7939 remains in full shutdown mode

and track-and-hold in hold mode, until the power management

bits (PM1 and PM0) in the control register are changed. If a

write to the control register occurs while the part is in full

shutdown mode, and the power management bits are changed

to PM0 = PM1 = 0, i.e., normal mode, the part begins to power-

up on the WR rising edge and the track-and-hold returns to

track. To ensure the part is fully powered up before a conversion

is initiated, the power-up time, TBD, should be allowed before

the next CONVST falling edge; otherwise, invalid data will be

read.

tPOWER-UP

BUSY

CLKIN

CONVST

Figure 42 Auto-Shutdown/Auto-Standby Mode

Rev. PrN | Page 28 of 32

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