AD7631BSTZ datasheet(22/32 Pages) AD

22 / 32 page

AD7631

Rev. A | Page 22 of 32

External 2.5 V Reference and Internal Buffer (REF = 5 V)

(PDREF = High, PDBUF = Low)

To use an external reference with the internal buffer, PDREF

should be high and PDBUF should be low. This powers down

the internal reference and allows the 2.5 V reference to be applied

to REFBUFIN producing 5 V on the REF pin. The internal

reference buffer is useful in multiconverter applications because

a buffer is typically required in these applications to avoid

reference coupling amongst the different converters.

External 5 V Reference (PDREF = High, PDBUF = High)

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

and PDBUF should both be high. PDREF and PDBUF power

down the internal reference and the internal reference buffer,

respectively. For improved drift performance, an external

reference, such as the ADR445 or ADR435, is recommended.

Reference Decoupling

Whether using an internal or external reference, the AD7631

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 22 μF (X5R,

1206 size) ceramic chip capacitor (or 47 μF low ESR tantalum

capacitor) is appropriate when using either the internal

reference or the ADR445/ADR435 external reference.

The placement of the reference decoupling is also important to

the performance of the AD7631. 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

and to the analog ground plane with several vias.

For applications that use multiple AD7631s or other PulSAR

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

to buffer the external 2.5 V 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 ±4 ppm/°C TC of the reference changes full scale by ±1 LSB/°C.

Temperature Sensor

The TEMP pin measures the temperature of the AD7631. 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 32.

ADG779

ANALOG INPUT

AD7631

IN+

TEMPERATURE

SENSOR

TEMP

Figure 32. Use of the Temperature Sensor

POWER SUPPLIES

The AD7631 uses five sets of power supply pins:

•

AVDD: analog 5 V core supply

•

VCC: analog high voltage positive supply

•

VEE: high voltage negative supply

•

DVDD: digital 5 V core supply

•

OVDD: digital input/output interface supply

Core Supplies

The AVDD and DVDD supply the AD7631 analog and digital

cores, respectively. Sufficient decoupling of these supplies is

required consisting of at least a 10 μF capacitor and a 100 nF

capacitor on each supply. The 100 nF capacitors should be

placed as close as possible to the AD7631. To reduce the number

of supplies needed, the DVDD can be supplied through a simple

RC filter from the analog supply, as shown in Figure 27.

High Voltage Supplies

The high voltage bipolar supplies, VCC and VEE, are required

and must be at least 2 V larger than the maximum input voltage.

For example, if using the ±10 V range, the supplies should be

±12 V minimum. This allows for 40 V p-p fully differential

input (±10 V on each input IN+ and IN−). Sufficient decoupling of

these supplies is also required consisting of at least a 10 μF

capacitor and a 100 nF capacitor on each supply. For unipolar

operation, the VEE supply can be grounded with some slight

THD performance degradation.

Digital Output Supply

The OVDD supplies the digital outputs and allows direct interface

with any logic working between 2.3 V and 5.25 V. OVDD should

be set to the same level as the system interface. Sufficient

decoupling is required consisting of at least a 10 μF capacitor and

a 100 nF capacitor with the 100 nF placed as close as possible

to the AD7631.

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