AD7939BCPZ datasheet(28/36 Pages) AD

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AD7938/AD7939

Data Sheet

Rev. C | Page 28 of 36

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 Table 9. 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.

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. It

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

mode is the default setting in the control register.

Autoshutdown (PM1 = 0; PM0 = 1)

In this mode of operation, the AD7938/AD7939 automatically

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

shown at Point A in Figure 36 and Figure 40. In shutdown

mode, all internal circuitry on the device is powered down.

The parts retain 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 Figure 36 or Figure 40).

In order to keep the device in shutdown for as long as possible,

CONVST should idle low between conversions, as shown in

Figure 40. On this rising edge, the part begins to power-up and

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

required is 10 ms minimum regardless of whether the user is

operating with the internal or external reference. The user

should ensure that the power-up time has elapsed before

initiating a conversion.

Autostandby (PM1 = 1; PM0 = 0)

In this mode of operation, the AD7938/AD7939 automatically

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

shown as Point A in Figure 36. 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 parts remain in standby until the next rising

edge of CONVST powers up the device. The power-up time

required depends on whether the internal or external reference

is used. With an external reference, the power-up time required

is a minimum of 600 ns, while when using the internal

reference, the power-up time required is a minimum of 7 μs.

The user should ensure this power-up time has elapsed before

initiating another conversion as shown in Figure 40. This

rising edge of CONVST also places the 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, that is, on the rising edge of WR. The track-

and-hold enters hold mode at this point. The parts retain

the information in the control register while the part is in

shutdown. The AD7938/AD7939 remain in full shutdown

mode, with the 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 (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 of 10 ms minimum should be

allowed before the next CONVST falling edge; otherwise,

invalid data is read.

Note that all power-up times quoted apply with a 470 nF

capacitor on the VREFIN pin.

tPOWER-UP

BUSY

CLKIN

CONVST

Figure 40. Autoshutdown/Autostandby Mode

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