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AD7641ASTRL

18-Bit, 2 MSPS SAR ADC

Analog Devices 

However, because the AD7641 has a fine lead pitch, guarding

this node is not practical. Therefore, in these industrial and

other types of applications, it is recommended to use a conformal

coating, such as Dow Corning® 1-2577 or HumiSeal® 1B73.

External 1.2 V Reference and Internal Buffer (PDBUF =

Low, PDREF = High)

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 1.2 V reference to

be applied to REFBUFIN, producing 2.048 V (typically) on

the REF pin.

External 2.5 V Reference (PDBUF = High, PDREF = High)

To use an external 2.5 V 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:

• The 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

SNR

=

20log⎜⎝⎛

2.048

2.50

⎟⎠⎞

• The power savings when the internal reference is powered

down (PDREF high).

PDREF and PDBUF power down the internal reference and

the internal reference buffer, respectively. The input current

of PDREF and PDBUF should never exceed 20 mA. This can

occur when the driving voltage is above AVDD (for instance, at

power-up). In this case, a 125 Ω series resistor is recommended.

Reference Decoupling

Whether using an internal or external reference, the AD7641

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 the recommended reference voltages.

The placement of the reference decoupling is also important to

the performance of the AD7641. 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.

AD7641

For applications that use multiple AD7641 devices, it is more

effective to use an external reference with the internal reference

buffer to buffer the reference voltage. However, because the

reference buffers are not unity gain, ratiometric, simultaneously

sampled designs should use an external reference and external

buffer, such as the AD8031/AD8032; therefore, preserving the

same reference level for all converters.

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.

Note that VREF can be increased to AVDD + 0.1 V. Because the

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

increase the range to 0 V to 2.8 V with an AVDD = 2.7 V.

Temperature Sensor

The TEMP pin measures the temperature of the AD7641. 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 27.

ADG779

TEMP

ANALOG INPUT

(UNIPOLAR)

IN+

AD8021 CC

TEMPERATURE

SENSOR

AD7641

Figure 27. Use of the Temperature Sensor

Rev. 0 | Page 19 of 28

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