AD7265BCP datasheet(21/28 Pages) AD

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AD7265

Rev. A | Page 21 of 28

SCLK

DOUTA

DOUTB

INVALID DATA

VALID DATA

THE PART BEGINS

TO POWER UP.

THE PART IS FULLY POWERED UP,

SEE POWER-UP TIMES SECTION.

tPOWER-UP2

Figure 38. Exiting Full Power-Down Mode

POWER-UP TIMES

As described in detail, the AD7265 has two power-down

modes, partial power-down and full power-down. This section

deals with the power-up time required when coming out of

either of these modes. It should be noted that the power-up

times, as explained in this section, apply with the recommended

capacitors in place on the DCAPA and DCAPB pins.

To power up from full power-down (whether using an internal

or external reference), approximately 1.5 ms should be allowed

from the falling edge of CS, shown as tPOWER-UP2 in Figure 38.

Powering up from partial power-down requires much less time.

The power-up time from partial power-down is typically 1 μs;

however, if using the internal reference, then the AD7265 must

be in partial power-down for at least 67 μs in order for this

power-up time to apply.

When power supplies are first applied to the AD7265, the ADC

may power up in either of the power-down modes or normal

mode. Because of this, it is best to allow a dummy cycle to

elapse to ensure the part is fully powered up before attempting a

valid conversion. Likewise, if it is intended to keep the part in

the partial power-down mode immediately after the supplies are

applied, then two dummy cycles must be initiated. The first

dummy cycle must hold CS low until after the 10th SCLK falling

edge (see Figure 34); in the second cycle, CS must be brought

high before the 10th SCLK edge but after the second SCLK

falling edge (see Figure 35). Alternatively, if it is intended to

place the part in full power-down mode when the supplies are

applied, then three dummy cycles must be initiated. The first

dummy cycle must hold CS low until after the 10

th SCLK falling

edge (see Figure 34); the second and third dummy cycles place

the part in full power-down (see Figure 37).

Once supplies are applied to the AD7265, enough time must be

allowed for any external reference to power up and charge the

various reference buffer decoupling capacitors to their final values.

POWER vs. THROUGHPUT RATE

The power consumption of the AD7265 varies with throughput

rate. When using very slow throughput rates and as fast an

SCLK frequency as possible, the various power-down options

can be used to make significant power savings. However, the

AD7265 quiescent current is low enough that even without

using the power-down options, there is a noticeable variation in

power consumption with sampling rate. This is true whether a

fixed SCLK value is used or if it is scaled with the sampling rate.

Figure 39 and Figure 40 show plots of power vs. the throughput

rate when operating in normal mode for a fixed maximum

SCLK frequency, and an SCLK frequency that scales with the

sampling rate with VDD = 3 V and VDD = 5 V, respectively. In all

cases, the internal reference was used.

THROUGHPUT (kSPS)

1000

100

200

300

500

400

700

800

900

600

10.0

9.0

8.5

9.5

8.0

7.0

6.5

7.5

6.0

5.5

5.0

16MHz SCLK

VARIABLE SCLK

TA = 25°C

Figure 39. Power vs. Throughput in Normal Mode with VDD = 3 V

THROUGHPUT (kSPS)

1000

200

300

400

100

500

600

700

800

900

VARIABLE SCLK

TA = 25°C

16MHz SCLK

Figure 40. Power vs. Throughput in Normal Mode with VDD = 5 V

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