AD7621 datasheet(19/32 Pages) AD

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AD7621

Rev. 0 | Page 19 of 32

External Reference (PDBUF = High, PRBUF = High)

To use an external reference directly on the REF pin, PDREF

and PDBUF should both be high.

For improved drift performance, an external reference, such as

the AD780 or ADR431, can be used. The advantages of directly

using the external voltage reference are:

•

SNR and dynamic range improvement (about 1.7 dB)

resulting from the use of a reference voltage very close to

the supply (2.5 V) instead of a typical 2.048 V reference

when the internal reference is used. This is calculated by

⎟

⎠

⎞

⎜

⎝

⎛

048

log

SNR

•

Power savings when the internal reference is powered

down (PBREF = PDBUF = high).

PDREF and PDBUF power down the internal reference and the

internal reference buffer, respectively.

Reference Decoupling

Whether using an internal or external reference, the AD7621

voltage reference input (REF) has a dynamic input impedance;

therefore, it should be driven by a low impedance source with

efficient decoupling between the REF and REFGND inputs.

This decoupling depends on the choice of the voltage reference,

but usually consists of a low ESR capacitor connected to REF

and REFGND with minimum parasitic inductance. A 10 μF

(X5R, 1206 size) ceramic chip capacitor (or 47 μF tantalum

capacitor) is appropriate when using either the internal

reference or one of these recommended reference voltages:

•

The low noise, low temperature drift ADR431 and AD780

•

The low power ADR291

•

The low cost AD1582

The placement of the reference decoupling is also important to

the performance of the AD7621. The decoupling capacitor

should be mounted on the same side as the ADC right at the

REF pin with a thick PCB trace. The REFGND should also

connect to the reference decoupling capacitor with the shortest

distance.

For applications that use multiple AD7621 devices, it is more

effective to use the internal reference buffer in order to buffer

the reference voltage.

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.

Temperature Sensor

The TEMP pin measures the temperature of the AD7621. To

improve the calibration accuracy over the temperature range,

the output of the TEMP pin is applied to one of the inputs of the

analog switch (such as, ADG779), and the ADC itself is used to

measure its own temperature. This configuration is shown in

Figure 28.

ADG779

AD8021

ANALOG INPUT

(UNIPOLAR)

AD7621

IN+

TEMPERATURE

SENSOR

TEMP

Figure 28. Use of the Temperature Sensor

POWER SUPPLY

The AD7621 uses three sets of power supply pins: an analog

2.5 V supply AVDD, a digital 2.5 V core supply DVDD, and a

digital input/output interface supply OVDD. The OVDD supply

allows direct interface with any logic working between 2.3 V

and 5.25 V. To reduce the number of supplies needed, the digital

core (DVDD) can be supplied through a simple RC filter from

the analog supply as shown in Figure 23.

Power Sequencing

The AD7621 is independent of power supply sequencing once

OVDD does not exceed DVDD by more than 0.3 V until

DVDD = 2.3 V during any time; for instance, at power-up or

power-down (see the Absolute Maximum Ratings section).

Additionally, it is very insensitive to power supply variations

over a wide frequency range as shown in Figure 29.

FREQUENCY (kHz)

100

10k

EXT REF

INT REF

Figure 29. PSRR vs. Frequency

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