AD7682BCPZ datasheet(23/32 Pages) AD

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Data Sheet

AD7682/AD7689

Rev. D | Page 23 of 32

POWER SUPPLY

SUPPLYING THE ADC FROM THE REFERENCE

The AD7682/AD7689 use two power supply pins: an analog

and digital core supply (VDD) and a digital input/output inter-

face supply (VIO). VIO allows direct interface with any logic

between 1.8 V and VDD. To reduce the supplies needed, the

VIO and VDD pins can be tied together. The AD7682/AD7689

are independent of power supply sequencing between VIO and

VDD. Additionally, it is very insensitive to power supply varia-

tions over a wide frequency range, as shown in Figure 34.

For simplified applications, the AD7682/AD7689, with their

low operating current, can be supplied directly using an

external reference circuit like the one shown in Figure 36. The

reference line can be driven by:

•

The system power supply directly

•

A reference voltage with enough current output capability,

such as the ADR43x/ADR44x

•

A reference buffer, such as the AD8605, which can also

filter the system power supply, as shown in Figure 36

10k

100

FREQUENCY (kHz)

AD8605

AD7689

VIO

REF

VDD

10µF

1µF

0.1µF

10Ω

10kΩ

1µF

1OPTIONAL REFERENCE BUFFER AND FILTER.

0.1µF

Figure 36. Example of an Application Circuit

Figure 34. PSRR vs. Frequency

The AD7682/AD7689 power down automatically at the end of

each conversion phase; therefore, the operating currents and

power scale linearly with the sampling rate. This makes the part

ideal for low sampling rates (even of a few hertz) and low

battery-powered applications.

10,000

1000

100

0.1

0.010

0.001

100

10k

100k

SAMPLING RATE (SPS)

VDD = 5V, INTERNAL REF

VDD = 5V, EXTERNAL REF

VDD = 2.5V, EXTERNAL REF

VIO

Figure 35. Operating Currents vs. Sampling Rate

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