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AD7651

Voltage Reference Input

The AD7651 allows the choice of either a very low temperature

drift internal voltage reference or an external 2.5 V reference.

For applications that use multiple AD7651s, it is more effective

to use the internal buffer to buffer the reference voltage.

Unlike many ADCs with internal references, the internal

reference of the AD7651 provides excellent performance and

can be used in almost all applications.

Care should be taken with the voltage reference’s temperature

coefficient, which directly affects the full-scale accuracy if this

parameter matters. For instance, a ±15 ppm/°C temperature

coefficient of the reference changes full scale by ±1 LSB/°C.

To use the internal reference along with the internal buffer,

PDREF and PDBUF should both be LOW. This will produce a

1.207 V voltage on REFBUFIN which, amplified by the buffer,

will result in a 2.5 V reference on the REF pin.

Note that VREF can be increased to AVDD – 1.85 V. Since the

input range is defined in terms of VREF, this would essentially

increase the range to 0 V to 3 V with an AVDD above 4.85 V.

The AD780 can be selected with a 3 V reference voltage.

The output impedance of REFBUFIN is 11 kΩ (minimum) when

the internal reference is enabled.

It is useful to decouple

REFBUFIN with a 100 nF ceramic capacitor. Thus, the 100 nF

capacitor provides an RC filter for noise reduction.

The TEMP pin, which measures the temperature of the AD7651,

can be used as shown in

. The output of TEMP pin is

applied to one of the inputs of the analog switch (e.g., ADG779),

and the ADC itself is used to measure its own temperature. This

configuration is very useful for improving the calibration

accuracy over the temperature range.

Figure 24

Figure 24. Temperature Sensor Connection Diagram

To use an external reference along 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.

ADG779

AD8021

02964-0-024

ANALOG INPUT

(UNIPOLAR)

AD7651

TEMPERATURE

SENSOR

TEMP

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

PDBUF should both be HIGH.

PDREF and PDBUF respectively power down the internal

reference and the internal reference buffer. Note that the PDREF

and PDBUF input current should never exceed 20 mA. This

could eventually occur when input voltage is above AVDD (for

instance at power up). In this case, a 100 Ω series resistor is

recommended.

Power Supply

The AD7651 uses three power supply pins: an analog 5 V supply

AVDD, a digital 5 V core supply DVDD, and a digital input/

output interface supply OVDD. OVDD allows direct interface

with any logic between 2.7 V and DVDD + 0.3 V. To reduce the

supplies needed, the digital core (DVDD) can be supplied

through a simple RC filter from the analog supply, as shown in

. The AD7651 is independent of power supply

sequencing once OVDD does not exceed DVDD by more than

0.3 V, and is thus free of supply voltage induced latch-up.

The internal reference is temperature compensated to 2.5 V ±

20 mV. The reference is trimmed to provide a typical drift of 7

. This typical drift characteristic is shown in

For improved drift performance, an external reference such as

the AD780 can be used.

ppm/°C

Figure 17

Figure 22

The AD7651 voltage reference input REF has a dynamic input

impedance; it should therefore 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 tantalum 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 ADR421 and AD780

•

The low power ADR291

•

The low cost AD1582

Rev. 0 | Page 18 of 28

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