Datasheet

Data Sheet AD7091R
Rev. 0 | Page 15 of 20
Normal and Power-Down Mode Combination
A combination of normal mode and power-down mode
achieves the optimum power performance.
The internal circuitry of the AD7091R requires 100 μs to power
up from power-down mode. Power-down mode can therefore
be performed at sampling rates of less than 10 kSPS.
Recharging the reference capacitor should also be considered
when using the on-chip reference. The AD7091R can fully
charge a 2.2 µF reference capacitor in typically 50 ms. However,
the time to charge the reference capacitor is dependent on the
amount of charge remaining on the capacitor when exiting
power-down mode. The reference capacitor loses charge very
slowly, resulting in much faster recharge times.
Figure 25 shows the AD7091R conversion sequence with a
combination of normal mode and power-down mode with a
throughput of 5 kSPS when using an external reference. With a V
DD
supply voltage of 3 V, the static current is 21.6 μA. The dynamic
current is 327.4 μA at 1 MSPS. The current consumption during
power-down mode is 264 nA. A conversion requires 650 ns to
complete, and the AD7091R requires 100 μs to power up from
power-down mode when using an external reference.
The dynamic conversion time contributes 4.9 μW to the overall
power dissipation as follows:
((5 kSPS/1 MSPS) × 327.4 μA) × 3 V = 4.9 μW
The contribution to the total power dissipated by the normal
mode static operation and power-down mode is
((100.6 μs/200 μs) × 21.6 μA) × 3 V +
((99.4 μs/200 μs) × 264 nA) × 3 V = 33 μW
The conversion time of 650 ns is included in the static opera-
tion time.
The total power dissipated at 5 kSPS is
4.9 μW + 33 μW = 37.9 μW
Figure 23 and Figure 24 show the typical power vs. throughput
rate for the AD7091R at 3 V for the V
DD
supply and for the
V
DRIVE
supply. Power consumption for the V
DRIVE
supply can be
calculated by the same principles as those for the V
DD
supply.
Additionally,
Figure 24 shows the reduction in power consumption
that can be achieved when power-down mode is used compared
with using only normal mode at lower throughput rates.
Figure 23. Power Dissipation vs. Throughput Rate (Full Range)
Figure 24. Power Dissipation vs. Throughput Rate (Lower Range)
Figure 25. 10 SPS with Normal and Power-Down Mode
0
100
200
300
400
500
600
700
800
900
1000
1100
0 200 400 600 800 1000
POWER (μW)
THROUGHPUT RATE (kSPS)
V
DD
V
DRIVE
V
DD
= V
DRIVE
= 3V
V
DD
= V
DRIVE
= 3V
10494-016
0.001
0.01
0.1
1
10
1000
100
0.01 0.1 1 10010
POWER (μW)
THROUGHPUT RATE (kSPS)
10494-117
V
DD
= V
DRIVE
= 3V
V
IN
= 0V
EXTERNAL REFERENCE
V
DD
(NO PD)
V
DRIVE
(NO PD)
V
DD
V
DRIVE
99µs
POWER-DOWN
100µs
POWER-UP
650ns
CONVERSION
CONVST
EOC
CS
SDO DATA
200µs
10494-022