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AD7653

Data Sheet

Rev. C | Page 18 of 26

Voltage Reference Input

The AD7653 allows the choice of either a very low temperature

drift internal voltage reference or an external 2.5 V reference.

Unlike many ADCs with internal references, the internal

reference of the AD7653 provides excellent performance and

can be used in almost all applications.

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.

The output impedance of REFBUFIN is 11 kΩ (minimum)

when the 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.

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.

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

and PDBUF should both be HIGH.

PDREF and PDBUF, respectively, power down the internal

reference and the internal reference. 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.

The internal reference is temperature compensated to 2.5 V ±

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

ppm/°C. This typical drift characteristic is shown in Figure 17.

For improved drift performance, an external reference such as

the AD780 can be used.

The AD7653 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 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

For applications that use multiple AD7653s, it is more effective

to use the internal buffer to buffer the reference voltage.

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.

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 TEMP pin, which measures the temperature of the

AD7653, can be used as shown in Figure 24. The output of the

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.

ADG779

AD8021

02966-0-024

ANALOG INPUT

(UNIPOLAR)

AD7653

TEMPERATURE

SENSOR

TEMP

Figure 24. Temperature Sensor Connection Diagram

Power Supply

The AD7653 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 Figure 22. The AD7653 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.

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