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AD7986

Rev. B | Page 16 of 28

VOLTAGE REFERENCE INPUT

The AD7986 allows the choice of a very low temperature drift

internal voltage reference, an external reference, or an external

buffered reference.

The internal reference of the AD7986 provides excellent

performance and can be used in almost all applications.

Internal Reference, REF = 4.096V (PDREF = Low)

To use the internal reference, the PDREF input must be low.

This enables the on-chip band gap reference and buffer, result-

ing in a 4.096 V reference on the REF pin (1.2 V on REFIN).

The internal reference is temperature compensated to 4.096 V ±

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

10 ppm/°C.

The output resistance of REFIN is 6 kΩ when the internal

reference is enabled. It is necessary to decouple this pin with a

ceramic capacitor of at least 100 nF. The output resistance of

REFIN and the decoupling capacitor form an RC filter, which

helps to reduce noise.

Because the output impedance of REFIN is typically 6 kΩ, relative

humidity (among other industrial contaminants) can directly

affect the drift characteristics of the reference. A guard ring is

typically used to reduce the effects of drift under such circum-

stances. However, the fine pitch of the AD7986 makes this

difficult to implement. One solution, in these industrial and

other types of applications, is to use a conformal coating, such as

Dow Corning® 1-2577 or HumiSeal® 1B73.

External 1.2 V Reference and Internal Buffer (PDREF = High)

To use an external reference along with the internal buffer, PDREF

should be high. This powers down the internal reference and

allows the 1.2 V reference to be applied to REFIN, producing

4.096 V (typically) on the REF pin.

External Reference (PDREF = High, REFIN = Low)

To apply an external reference voltage directly to the REF pin,

PDREF should be tied high, and REFIN should be tied low.

BVDD should also be driven to the same potential as REF. For

example, if REF = 2.5 V, BVDD should be tied to 2.5 V.

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 larger reference voltage (5 V)

instead of a typical 4.096 V reference when the internal

reference is used. This is calculated by













096

log

SNR

•

The power savings when the internal reference is powered

down (PDREF high).

Reference Decoupling

The AD7986 voltage reference input, REF, has a dynamic input

impedance that requires careful decoupling between the REF

and REFGND pins. The Layout section describes how this can

be done.

When using an external reference, a very low impedance source

(for example, a reference buffer using the AD8031 or the

AD8605), and a 10 µF (X5R, 0805 size) ceramic chip capacitor

are appropriate for optimum performance.

If an unbuffered reference voltage is used, the decoupling value

depends on the reference used. For instance, a 22 µF (X5R,

1206 size) ceramic chip capacitor is appropriate for optimum

performance using a low temperature drift ADR43x reference.

If desired, a reference decoupling capacitor with values as small

as 2.2 µF can be used with minimal impact on performance,

especially DNL.

Regardless, there is no need for an additional lower value ceramic

decoupling capacitor (for example, 100 nF) between the REF

and REFGND pins.

POWER SUPPLY

The AD7986 uses four power supply pins: an analog supply

(AVDD), a buffer supply (BVDD), a digital supply (DVDD),

and a digital input/output interface supply (VIO). VIO allows

direct interface with any logic between 1.8 V and 2.7 V. To reduce

the number of supplies needed, VIO, DVDD, and AVDD can

be tied together. The AD7986 is independent of power supply

sequencing among all of its supplies. Additionally, it is very

insensitive to power supply variations over a wide frequency range.

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