AD7699 datasheet(21/32 Pages) AD

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Data Sheet

AD7699

Rev. F | Page 21 of 32

External Reference

In any of the five voltage reference schemes, an external refer-

ence can be connected directly on the REF pin because the output

impedance of REF is >5 kΩ. To reduce power consumption, the

reference and buffer can be powered down independently or

together for the lowest power consumption. When only using the

external reference (and optional reference buffer as shown in

Figure 36), an internal buffer is disabled. Refer to Table 10 for

reference such as the ADR430/ADR431/ADR433/ADR434/

ADR435 or ADR440/ADR441/ADR443/ADR444/ADR445 is

recommended.

Reference Decoupling

Whether using an internal or external reference, the AD7699

voltage reference output/input, REF, has a dynamic input

impedance and should therefore be driven by a low impedance

source with efficient decoupling between the REF and GND

pins. This decoupling depends on the choice of the voltage

reference but usually consists of a low ESR capacitor connected

to REF and GND with minimum parasitic inductance. A 10 µF

(X5R, 1206 size) ceramic chip capacitor is appropriate when using

the internal reference, the ADR430/ADR431/ADR433/

ADR434/ADR435 or ADR440/ADR441/ADR443/ADR444/

ADR445 external 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 34.

10k

100

FREQUENCY (kHz)

Figure 34. 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 35. Operating Currents vs. Sampling Rate

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