AD7699BCPZ datasheet(19/28 Pages) AD

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AD7699

Rev. 0 | Page 19 of 28

reference, or a low impedance buffer such as the AD8031 or the

AD8605.

The placement of the reference decoupling capacitor is also

important to the performance of the AD7699, as explained in the

Layout section. Mount the decoupling capacitor on the same side

as the ADC at the REF pin with a thick PCB trace. The GND

should also connect to the reference decoupling capacitor with

the shortest distance and to the analog ground plane with

several vias.

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 AD7699s 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 AD7699 uses 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 AD7699 is independent of power

supply sequencing between VIO and VDD. The only restriction

is that CNV must be low when powering up the AD7699.

Additionally, it is very insensitive to power supply variations

over a wide frequency range, as shown in Figure 32.

10k

100

FREQUENCY (kHz)

Figure 32. PSRR vs. Frequency

The AD7699 powers 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

VIO

Figure 33. Operating Currents vs. Sampling Rate

SUPPLYING THE ADC FROM THE REFERENCE

For simplified applications, the AD7699, with its low operating

current, can be supplied directly using the reference circuit, as

shown in Figure 34. 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 34

AD8605

AD7699

VIO

REF

VDD

10µF

1µF

0.1µF

Ω

10k

Ω

1µF

1OPTIONAL REFERENCE BUFFER AND FILTER.

0.1µF

Figure 34. Example of an Application Circuit

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