Datasheet

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-
mal operation is 10.5 mW, V
DD
= 3 V. If the power-up time is
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
05010
0.1
20 30 405 15253545
POWER – mV
Figure 12. AD7811/AD7812 Power vs. Throughput
FREQUENCY – kHz
0
–10
–100
0 12217 87 105
–40
–70
–80
–90
–20
–30
–60
–50
35 52 70 140 157 174
AD7811/12
2048 POINT FFT
SAMPLING 357.142kHz
f
IN
= 30.168kHz
dBs
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
REF
pin can be
connected to V
DD
. The CONVST signal is connected to TFS
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
V
REF
DIN
SCLK
CONVST
V
DD
C
REF
V
IN1
AGND
TFS
RFS
DOUT
A0
DGND
0.1␮F
V
IN2
AD7812
0V TO V
DD
INPUT
V
IN7
V
IN8
SUPPLY
V
DD
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
up to use external reference (i.e., V
DD
) and the analog input
V
IN1
is selected. The data read from the part during this read/
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.