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AD7622

Rev. 0 | Page 19 of 28

VOLTAGE REFERENCE INPUT

The AD7622 allows the choice of either a very low temperature

drift internal voltage reference, an external 1.2 V reference that

can be buffered using the internal reference buffer, or an

external reference.

Unlike many ADCs with internal references, the internal

reference of the AD7622 provides excellent performance and

can be used in almost all applications.

Internal Reference (PDBUF = Low, PDREF = Low)

To use the internal reference, the PDREF and PDBUF inputs

must both be low. This produces a 1.2 V band gap output on

REFBUFIN, which is amplified by the internal buffer and

results in a 2.048 V reference on the REF pin.

The internal reference is temperature compensated to 2.048 V ±

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

8 ppm/°C. This typical drift characteristic is shown in Figure 8.

The output resistance of REFBUFIN is 6.33 kΩ (minimum)

when the internal reference is enabled. It is necessary to

decouple this with a ceramic capacitor greater than 100 nF.

Therefore, the capacitor provides an RC filter for noise reduction.

Because the output impedance of REFBUFIN is typically

6.33 kΩ, relative humidity (among other industrial contaminates)

can directly affect the drift characteristics of the reference.

Typically, a guard ring is used to reduce the effects of drift

under such circumstances. However, because the AD7622 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 an external 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, ADR421, ADR431, or ADR441, 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

⎟

⎠

⎞

⎜

⎝

⎛

048

log

SNR

•

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 AD7622

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 AD7622. 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.

For applications that use multiple AD7622 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 ±15 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.

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