AD7921 Analog Devices, AD7921 Datasheet - Page 20

• Manufacturer Part Number: AD7921
• Description: 2-Channel, 2.35 V to 5.25 V, 250 kSPS, 12-Bit A/D Converter
• Manufacturer: Analog Devices

Specifications of AD7921

• Resolution (bits): 12bit
• # Chan: 2
• Sample Rate: 250kSPS
• Interface: Ser,SPI
• Analog Input Type: SE-Uni
• Ain Range: Uni Vdd
• Adc Architecture: SAR
• Pkg Type: SOP,SOT

AD7911/AD7921
POWER VS. THROUGHPUT RATE
By using the power-down mode on the AD7911/AD7921 when
not converting, the average power consumption of the ADC
decreases at lower throughput rates. Figure 29 shows how, as the
throughput rate is reduced, the device remains in its power-
down state longer and the average power consumption over
time drops accordingly.
For example, if the AD7911/AD7921 are operating in a
continuous sampling mode with a throughput rate of 50 kSPS
and a SCLK of 5 MHz (V
power-down mode between conversions, then the power
consumption is calculated as follows. The power dissipation
during normal operation is 20 mW (V
cycle powers up the part between conversions (3.2 μs), and the
remaining conversion time is another cycle (3.2 μs), then the
AD7911/AD7921 dissipate 20 mW for 6.4 μs during each
conversion cycle. If the throughput rate is 50 kSPS and the cycle
time is 20 μs, then the average power dissipated during each
cycle is
If V
down mode between conversions, then the power dissipation
during normal operation is 6 mW. The AD7911/AD7921 now
dissipate 6 mW for 6.4 μs during each conversion cycle. With a
throughput rate of 50 kSPS, the average power dissipated during
each cycle is
DD
(6.4/20) × (20 mW) = 6.4 mW
(6.4/20) × (6 mW) = 1.92 mW
= 3 V, SCLK= 5 MHz, and the device is again in power-
DD
= 5 V) and the devices are placed in
DD
= 5 V). If one dummy
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In the previous examples, the power dissipation when the part
is in power-down mode has not been taken into account,
because the shutdown current is so low that it does not have any
effect on the overall power dissipation value. Figure 29 shows
the power consumption versus throughput rate when using the
power-down mode between conversions with both 5 V and 3 V
supplies.
Power-down mode is intended for use with throughput rates of
approximately 120 kSPS and under, because higher sampling
rates do not have a power saving in power-down mode.
0.01
100
0.1
10
1
0
Figure 29. Power Consumption vs. Throughput Rate
15
30
V
DD
= 5V, SCLK = 5MHz
45
THROUGHPUT (kSPS)
V
DD
60
= 3V, SCLK = 5MHz
75
90
105
120
135