AD7650 datasheet(21/32 Pages) AD

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AD7610

Rev. 0 | Page 21 of 32

Internal Reference (REF = 5 V)

(PDREF = Low, PDBUF = Low)

To use the internal reference, the PDREF and PDBUF inputs

must be low. This enables the on-chip band gap reference, buffer,

and TEMP sensor resulting in a 5.00 V reference on the REF pin.

The internal reference is temperature-compensated to 5.000 V

±35 mV. The reference is trimmed to provide a typical drift of

3 ppm/°C. This typical drift characteristic is shown in Figure 19.

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 ref-

erence buffer is useful in multiconverter applications since a

buffer is typically required in these applications.

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 ref-

erence such as the ADR445 or ADR435 is recommended.

Reference Decoupling

Whether using an internal or external reference, the AD7610

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 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 AD7610. 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 AD7610 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

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

Temperature Sensor

When the internal reference is enabled (PDREF = PDBUF =

low), the on-chip temperature sensor output (TEMP) is enabled

and can be use to measure the temperature of the AD7610. 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 30.

ADG779

ANALOG INPUT

AD7610

IN+

TEMPERATURE

SENSOR

TEMP

Figure 30. Use of the Temperature Sensor

POWER SUPPLIES

The AD7610 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 AD7610 analog and digital

cores respectively. Sufficient decoupling of these supplies is

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

each supply. The 100 nF capacitors should be placed as close as

possible to the AD7610. 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, VIN.

For example, if using the bipolar 10 V range, the supplies should

be ±12 V minimum. Sufficient decoupling of these supplies is

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

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 decou-

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

with the 100 nF placed as close as possible to the AD7610.

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