AD7811/AD7812

–13–

REV. B

t

POWER-UP

1.5 s

CONVST

t

CONVERT

2.3 s

100 s @ 10kSPS

POWER-DOWN

t

CYCLE

Figure 11. Automatic Power-Down

For example, if the AD7811 is operated in a continuous sam-

pling mode with a throughput rate of 10 kSPS, PD1 = 1,

PD0 = 0 and using the on chip reference the power consump-

tion is calculated as follows. The power dissipation during nor-

1.5

µs and the conversion time is 2.3 µs, the AD7811 can be

said to dissipate 10.5 mW for 3.8

µs (worst-case) during each

conversion cycle. If the throughput rate is 10 kSPS, the cycle

time is 100

µs and the average power dissipated during each

cycle is (3.8/100)

× (10.5 mW) = 400 µW.

Figure 12 shows the Power vs. Throughput Rate for automatic

full power-down.

THROUGHPUT – kSPS

10

1

0.01

050

10

0.1

20

30

40

5

15

253545

Figure 12. AD7811/AD7812 Power vs. Throughput

FREQUENCY – kHz

0

–10

–100

0

122

17

87

105

–40

–70

–80

–90

–20

–30

–60

–50

35

52

70

140

157

174

AD7811/12

2048 POINT FFT

SAMPLING 357.142kHz

Figure 13. AD7811/AD7812 SNR

QUICK EVALUATION SETUP

The schematic shown in Figure 14 shows a suggested configura-

tion of the AD7812 for a first look evaluation of the part. No

external reference circuit is needed as the V

connected to V

and RFS to enable the serial port. Also by selecting Mode 2

operation (see Operating Modes section) the power performance

of the AD7812 can be evaluated.

10 F

10nF

DIN

SCLK

CONVST

AGND

TFS

RFS

DOUT

A0

DGND

0.1 F

AD7812

INPUT

SUPPLY

Figure 14. Evaluation Quick Setup

The setup uses a full duplex, 16-bit, serial interface protocol,

e.g., SPI. It is possible to use 8-bit transfers by carrying out two

consecutive read/write operations. The MSB of data is trans-

ferred first.

1. When power is first connected to the device it is in a powered

down mode of operation and is consuming only 1

µA. The

AD7812 must first be configured by carrying out a serial

write operation.

2. The

CONVST signal is first pulsed to enable the serial port

(rising and falling edge on RFS and TFS respectively—see

Serial Interface section).

3. Next, a 16-bit serial read/write operation is carried out. By

writing 6040 Hex to the AD7812 the part is powered up, set

write operation is invalid.

4. It is necessary to wait approximately 1.5

µs before pulsing

CONVST again and initiating a conversion. The 1.5

µs is to

allow the AD7812 to power up correctly—see Power-Up

Times section.

5. Approximately 2.3

µs after the falling edge of CONVST, i.e.,

after the end of the conversion, a serial read/write can take

place. This time 4040 Hex is written to the AD7812 and the

data read from the part is the result of the conversion. The

output code is in a straight binary format and will be left

justified in the 16-bit serial register (MSB clocked out first).

6. By idling the

CONVST signal high or low it is possible to

operate the AD7812 in Mode 1 and Mode 2 respectively.