AD7864

–13–

REV. A

CONVST

BUSY

EOC

RD

1 2 3 4567 8 9 10 11 12 13 14 1 2 3 4 5 6 7 8 9 10 11 12 13 14 1 213 14

CLK

FRSTDATA

FIRST CONVERSION

COMPLETE

LAST CONVERSION

COMPLETE

Figure 9. Using an External Clock

Standby Mode Operation

The AD7864 has a Standby Mode whereby the device can be

placed in a low current consumption mode (5

µA typ). The

AD7864 is placed in standby by bringing the logic input

STBY

low. The AD7864 can be powered up again for normal opera-

tion by bringing

STBY logic high. The output data buffers are

still operational while the AD7864 is in standby. This means

the user can still continue to access the conversion results while

the AD7864 is in standby. This feature can be used to reduce

the average power consumption in a system using low through-

put rates. To reduce the average power consumption the AD7864

can be placed in standby at the end of each conversion sequence,

i.e., when BUSY goes low and taken out of standby again prior

the start of the next conversion sequence. The time it takes the

AD7864 to come out of standby is called the “wake up” time.

This wake-up time will limit the maximum throughput rate at

which the AD7864 can be operated when powering down be-

tween conversion sequences. The AD7864 will wake-up in

approximately 2

µs when using an external reference. The

“wake up” time is also 2

µs when the standby time is less than 1

millisecond while using the internal reference. Figure 11 shows

the wake-up time of the AD7864 for standby times greater than

1 millisecond. Note when the AD7864 is left in standby for

periods of time greater than 1 millisecond the part will require

more than 2

µs to wake up. For example after initial power up,

using the internal reference the AD7864 takes 6 ms to power

up. The maximum throughput rate that can be achieved when

µs) = 100

kSPS, approximately. When operating the AD7864 in a

standby mode between conversions the power savings can be

significant. For example with a throughput rate of 10 kSPS the

µA) for 90 µs out of

every 100

µs. See Figure 10. Therefore the average power con-

sumption drops to (125/10) mW or 12.5 mW approximately.

STANDBY TIME – sec

1

0.9

0

0.0001

10

0.001

0.01

0.1

1

0.6

0.3

0.2

0.1

0.8

0.7

0.4

0.5

+105 C

+25 C

–40 C

Figure 11. Power-Up Time vs. Standby Time Using the

On-Chip Reference (Decoupled with 0.1

µF Capacitor)

Accessing the Output Data Registers

There are four Output Data Registers, one for each of the four

possible conversion results from a conversion sequence. The

result of the first conversion in a conversion sequence is placed

in Register 1 and the second result is placed in Register Number

2 and so on. For example if the conversion sequence VIN1,

VIN3 and VIN4 is selected (see Conversion Sequence Selec-

tion) then the results of the conversion on VIN1, VIN3 and

VIN4 are placed in Registers 1 to 3 respectively. The Output

Data register pointer is reset to point to Register 1 at the end of

the first conversion in the sequence, just prior to

EOC going

CONVST

BUSY

STBY

100 s

7 s

2 s

t

WAKEUP

Figure 10. Power-Down Between Conversion Sequences