REV.

–10–

AD7485

Figure 7 shows the AD7485 conversion sequence operating in

normal mode.

2 s

960ns

1.04 s

READ DATA

CONVERSION

FINISHED

CONVST

TFS

Figure 7. Normal Mode Power Dissipation

In NAP mode, all the internal circuitry except for the internal

reference is powered down. In this mode, the power dissipation

of the AD7485 is reduced to 3 mW. When exiting NAP mode,

a minimum of 300 ns when using an external reference must be

waited before initiating a conversion. This is necessary to allow

the internal circuitry to settle after power-up and for the track/hold

to properly acquire the analog input signal.

If the AD7485 is put into NAP mode after each conversion, the

average power dissipation will be reduced but the throughput

rate will be limited by the power-up time. Using the AD7485 with

a throughput rate of 100 kSPS while placing the part in NAP

mode after each conversion would result in average power dissi-

pation as follows:

The power-up phase contributes:

300

10

5

12

1 8

ns

s

V

mA

mW

/.

The conversion phase contributes:

960

10

5

16

7 68

ns

s

V

mA

mW

/.

While in NAP mode for the rest of the cycle, the AD7485 dissipates

only 2.185 mW of power.

874

10

5

06

2622

./

.

.

ssV

mA

mW

Thus the power dissipated during each cycle is:

18

768

2 622

12 1

..

.

.

mW

mW

mW

mW

++

+

Figure 8 shows the AD7485 conversion sequence if putting the

part into NAP mode after each conversion.

1.26 s

10 s

8.74 s

300ns

CONVST

NAP

TFS

Figure 8. NAP Mode Power Dissipation

Figures 9 and 10 show a typical graphical representation of

power versus throughput for the AD7485 when in normal and

NAP modes, respectively.

THROUGHPUT – kSPS

80

76

0

100

72

68

64

60

200

400

300

500

600

700

800

900

1000

78

74

70

66

62

Figure 9. Normal Mode, Power vs. Throughput

THROUGHPUT – kSPS

50

40

050

30

20

10

0

100

200

150

250

300

350

400

450

500

45

35

25

15

5

Figure 10. NAP Mode, Power vs. Throughput

In STANDBY mode, all the internal circuitry is powered down

and the power consumption of the AD7485 is reduced to 10

µW.

Because the internal reference has been powered down, the

power-up time necessary before a conversion can be initiated is

longer. If using the internal reference of the AD7485, the ADC

must be brought out of STANDBY mode 500 ms before a conver-

sion is initiated. Initiating a conversion before the required

power-up time has elapsed will result in incorrect conversion

data. If an external reference source is used and kept powered up

while the AD7485 is in STANDBY mode, the power-up time

required will be reduced to 80

µs.

A