AD7476BRTZ-R23 datasheet(17/24 Pages) AD

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AD7476/AD7477/AD7478

Rev. F | Page 17 of 24

Power-Up Time

The power-up time of the AD7476/AD7477/AD7478 is typi-

cally 1 μs, which means that with any frequency of SCLK up to

20 MHz, one dummy cycle is always sufficient to allow the

device to power up. Once the dummy cycle is complete, the

ADC is fully powered up and the input signal is acquired

properly. The quiet time (tQUIET) must still be allowed from the

point at which the bus goes back into three-state (after the

dummy conversion), to the next falling edge of CS. When

running at 1 MSPS throughput rate, the AD7476/AD7477/

AD7478 powers up and acquires a signal within ±0.5 LSB in

one dummy cycle, such as 1 μs.

When powering up from the power-down mode with a dummy

cycle, as shown in Figure 21, the track-and-hold, that was in

hold mode while the part was powered down, returns to track

mode after the first SCLK edge the part receives after the falling

edge of CS. This is shown as Point A in

. Although at

any SCLK frequency, one dummy cycle is sufficient to power up

the device and acquire VIN, this does not necessarily mean that a

full dummy cycle of 16 SCLKs must always elapse to power up

the device and fully acquire VIN; 1 μs is sufficient to power up

the device and acquire the input signal. If, for example, a 5 MHz

SCLK frequency is applied to the ADC, the cycle time is 3.2 μs.

In one dummy cycle, 3.2 μs, the part is powered up and VIN is

fully acquired. However, after 1 μs with a 5 MHz SCLK, only

five SCLK cycles elapse. At this stage, the ADC is fully powered

up and the signal acquired. In this case, the

Figure 21

CS can be brought

high after the tenth SCLK falling edge and brought low again

after a time, tQUIET, to initiate the conversion.

When power supplies are first applied to the AD7476/AD7477/

AD7478, the ADC may power up in either power-down mode

or normal mode. Allow a dummy cycle to elapse to ensure the

part is fully powered up before attempting a valid conversion.

Likewise, to keep the part in the power-down mode while not

in use and then to power up the part in power-down mode, use

the dummy cycle to ensure the device is in power-down by

executing a cycle such as that shown in Figure 20. Once supplies

are applied to the AD7476/AD7477/AD7478, the power-up

time is the same when powering up from the power-down

mode. It takes approximately 1 μs to fully power up if the part

powers up in normal mode. It is not necessary to wait 1 μs

before executing a dummy cycle to ensure the desired mode of

operation. Instead, the dummy cycle can occur directly after

power is supplied to the ADC. If the first valid conversion is

then performed directly after the dummy conversion, ensure

that adequate acquisition time has been allowed.

When powering up from power-down mode, the part returns to

track upon the first SCLK edge applied after the falling edge of

CS. However, when the ADC powers up initially after supplies

are applied, the track-and-hold is already in track.

This means that if the ADC powers up in the desired mode of

operation, and a dummy cycle is not required to change mode,

then a dummy cycle is not required to place the track-and-hold

into track.

POWER VS. THROUGHPUT RATE

By using the power-down mode on the AD7476/AD7477/

AD7478 when not converting, the average power consumption

of the ADC decreases at lower throughput rates. Figure 22

shows that as the throughput rate reduces, the device remains in

its power-down state longer, and the average power

consumption over time drops accordingly.

For example, if the AD7476/AD7477/AD7478 operates in

continuous sampling mode with a throughput rate of 100 kSPS

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

the power-down mode between conversions, then the power

consumption is calculated as follows. The power dissipation

during normal operation is 17.5 mW (VDD = 5 V). If the power-

up time is one dummy cycle, such as 1 μs, and the remaining

conversion time is another cycle, such as 1 μs, then the part is

said to dissipate 17.5 mW for 2 μs during each conversion cycle.

If the throughput rate is 100 kSPS, the cycle time is 10 μs and

the average power dissipated during each cycle is

(2/10) × (17.5 mW) = 3.5 mW. If VDD = 3 V, SCLK = 20 MHz,

and the device is again in power-down mode between conver-

sions, the power dissipation during normal operation is

4.8 mW.

The AD7476/AD7477/AD7478 can now be said to dissipate

4.8 mW for 2 μs during each conversion cycle. With a through-

put rate of 100 kSPS, the average power dissipated during each

cycle is (2/10) × (4.8 mW) = 0.96 mW. Figure 22 shows the

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

between conversions with both 5 V and 3 V supplies.

100

0.01

0.1

350

300

250

200

150

100

THROUGHPUT RATE (kSPS)

VDD = 5V, SCLK = 20MHz

VDD = 3V, SCLK = 20MHz

Figure 22. Power vs. Throughput Rate

Power-down mode is intended for use with throughput rates of

approximately 333 kSPS and under. At higher sampling rates,

power is not saved by using power-down mode.

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