AD7484BSTZ datasheet(13/20 Pages) AD

13 / 20 page

AD7484

Rev. C | Page 13 of 20

For higher input bandwidth applications, the AD8021 op amp

(also available as a dual AD8022 op amp) is the recommended

choice to drive the AD7484. Figure 15 shows the analog input

circuit used to obtain the data for the FFT plot shown in Figure 4.

A bipolar analog signal is applied to the terminal and biased

up with a stable, low noise dc voltage connected, as shown in

Figure 12. A 10 pF compensation capacitor is connected between

Pin 5 of the AD8021 and the negative supply. The AD8021 is

supplied with +12 V and −12 V supplies. The supply pins are

decoupled as close to the device as possible, with both a 0.1 µF

and a 10 µF capacitor connected to each pin. In each case, the

0.1 µF capacitor should be the closer of the two caps to the

device. The AD8021 logic reference pin is tied to analog ground,

and the DISABLE

ADC TRANSFER FUNCTION

pin is tied to the positive supply. Detailed

information on the AD8021 is available at www.analog.com.

The output coding of the AD7484 is straight binary. The designed

code transitions occur midway between the successive integer

LSB values, that is, 1/2 LSB, 3/2 LSB, and so on. The LSB size is

VREF/16,384. The nominal transfer characteristic for the AD7484 is

shown in Figure 16. This transfer characteristic may be shifted

as detailed in the Offset/Overrange section.

000...000

ANALOG INPUT

111...111

000...001

000...010

111...110

111...000

011...111

0.5LSB

+VREF – 1.5LSB

1LSB = VREF/16384

Figure 16. AD7484 Transfer Characteristic

POWER SAVING

The AD7484 uses advanced design techniques to achieve very

low power dissipation at high throughput rates. In addition, the

AD7484 features two power saving modes, nap and standby.

These modes are selected by bringing either the NAP pin or the

STBY pin to a logic high, respectively.

When operating the AD7484 in normal fully powered mode,

the current consumption is 18 mA during conversion and the

quiescent current is 12 mA. Operating at a throughput rate of

1 MSPS, the conversion time of 300 ns contributes 27 mW to

the overall power dissipation.

(300 ns/1 μs) × (5 V × 18 mA) = 27 mW

For the remaining 700 ns of the cycle, the AD7484 dissipates

42 mW of power.

(700 ns/1 μs) × (5 V × 12 mA) = 42 mW

Therefore, the power dissipated during each cycle is

27 mW + 42 mW = 69 mW

Figure 17 shows the AD7484 conversion sequence operating in

normal mode.

CONVST

BUSY

300ns

1µs

700ns

Figure 17. Normal Mode Power Dissipation

In nap mode, almost all of the internal circuitry is powered down.

In this mode, the power dissipation is reduced to 2.5 mW. When

using an external reference, there must be a minimum of 300 ns

from exiting nap mode to initiating a conversion. This is necessary

to allow the internal circuitry to settle after power-up and for

the track-and-hold to properly acquire the analog input signal.

The internal reference cannot be used in conjunction with the

nap mode.

If the AD7484 is put into nap mode after each conversion, the

average power dissipation is reduced, but the throughput rate is

limited by the power-up time. Using the AD7484 with a through-

put rate of 500 kSPS while placing the part in nap mode after

each conversion results in average power dissipation as follows:

The power-up phase contributes

(300 ns/2 μs) × (5 V × 12 mA) = 9 mW

The conversion phase contributes

(300 ns/2 μs) × (5 V × 18 mA) = 13.5 mW

While in nap mode for the rest of the cycle, the AD7484

dissipates only 1.75 mW of power.

(1400 ns/2 μs) × (5 V × 0.5 mA) = 1.75 mW

Therefore, the power dissipated during each cycle is

9 mW + 13.5 mW + 1.75 mW = 24.25 mW

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