AD7475ARMZ datasheet(19/24 Pages) AD

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Data Sheet

AD7475/AD7495

Rev. D | Page 19 of 24

POWER VS. THROUGHPUT RATE

By using the partial power-down mode on the AD7475/AD7495

when not converting, the average power consumption of the ADC

decreases at lower throughput rates. Figure 24 shows how, as the

throughput rate is reduced, the device remains in its partial power-

down state longer and the average power consumption over

time drops accordingly.

THROUGHPUT (kSPS)

100

0.001

100

0.01

0.1

150

200

250

300

350

AD7495 5V

SCLK = 20MHz

AD7495 3V

SCLK = 20MHz

AD7475 5V

SCLK = 20MHz

AD7475 3V

SCLK = 20MHz

Figure 24. Power vs. Throughput for Partial Power Down

For example, if the AD7495 is operated in a continuous

sampling mode with a throughput rate of 100 kSPS and an

SCLK of 20 MHz (VDD = 5 V), and the device is placed in partial

power-down mode between conversions, then the power

consumption is calculated as follows. The maximum power

dissipation during normal operation is 13 mW (VDD = 5 V). If

the power-up time from partial power-down is one dummy

cycle, that is, 1 µs, and the remaining conversion time is another

cycle, that is, 1 µs, then the AD7495 can be said to dissipate

13 mW for 2 µs during each conversion cycle. For the remainder of

the conversion cycle, 8 µs, the device remains in partial power-

down mode. The AD7495 dissipates 1.15 mW for the remaining

8 µs of the conversion cycle. If the throughput rate is 100 kSPS,

and the cycle time is 10 µs, the average power dissipated during

each cycle is (2/10) × (13 mW) + (8/10) × (1.15 mW) = 3.52 mW. If

VDD = 3 V, SCLK = 20 MHz and the device is again in partial

power-down mode between conversions, the power dissipated

during normal operation is 6 mW.

The AD7495 dissipates 6 mW for 2 µs during each conversion

cycle and 0.69 mW for the remaining 8 µs where the device is in

partial power-down. With a throughput rate of 100 kSPS, the

average power dissipated during each conversion cycle is (2/10) ×

(6 mW) + (8/10) × (0.69 mW) = 1.752 mW. Figure 24 shows the

power vs. throughput rate when using partial power-down mode

between conversions with both 5 V and 3 V supplies for both the

AD7475 and AD7495. For the AD7475, partial power-down

current is lower than that of the AD7495.

Full power-down mode is intended for use in applications with

slower throughput rates than required for partial power-down

mode. It is necessary to leave 650 µs for the AD7495 to be fully

powered up from full power-down before initiating a conversion.

Current consumptions between conversions is typically less

than 1 µA.

Figure 25 shows a typical graph of current vs. throughput for

the AD7495 while operating in different modes. At slower

throughput rates, for example, 10 SPS to 1 kSPS, the AD7495

was operated in full power-down mode. As the throughput rate

increased, up to 100 kSPS, the AD7495 was operated in partial

power-down mode, with the device being powered down between

conversions. With throughput rates from 100 kSPS to 1 MSPS,

the device operated in normal mode, remaining fully powered

up at all times.

THROUGHPUT (SPS)

2.0

1.8

1.6

1.4

1.2

1.0

0.8

0.6

0.4

0.2

100

10k

100k

VDD = 5V

FULL

POWER-DOWN

PARTIAL

POWER-DOWN

NORMAL

Figure 25. Typical AD7495 Current vs. Throughput

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