AD7699 datasheet(21/28 Pages) AD

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AD7682/AD7689

Rev. 0 | Page 21 of 28

If desired, smaller reference decoupling capacitor values down

to 2.2 μF can be used with a minimal impact on performance,

especially on DNL.

Regardless, there is no need for an additional lower value

ceramic decoupling capacitor (for example, 100 nF) between the

REF and GND pins.

For applications that use multiple AD7682/AD7689s or other

PulSAR devices, it is more effective to use the internal reference

buffer to buffer the external reference voltage, thus reducing

SAR conversion crosstalk.

The voltage reference temperature coefficient (TC) directly impacts

full scale; therefore, in applications where full-scale accuracy

matters, care must be taken with the TC. For instance, a

±15 ppm/°C TC of the reference changes full scale by ±1 LSB/°C.

POWER SUPPLY

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

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

interface 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. The only restriction is that CNV must

be low when powering up the AD7682/AD7689. Additionally, it

is very insensitive to power supply variations over a wide

frequency range, as shown in Figure 31.

10k

100

FREQUENCY (kHz)

Figure 31. 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.

10000

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 32. Operating Currents vs. Sampling Rate

SUPPLYING THE ADC FROM THE REFERENCE

For simplified applications, the AD7682/AD7689, with their

low operating current, can be supplied directly using the

reference circuit as shown in Figure 33. 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 33

AD8605

AD7689

VIO

REF

VDD

10µF

1µF

0.1µF

10Ω

10kΩ

1µF

1OPTIONAL REFERENCE BUFFER AND FILTER.

0.1µF

Figure 33. Example of an Application Circuit

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