AD7682BCPZ datasheet(24/35 Pages) AD

24 / 35 page

AD7682/AD7689

Data Sheet

Rev. H | Page 24 of 35

External Reference and Internal Buffer

For improved drift performance, an external reference can be

used with the internal buffer, as shown in Figure 34. The

external source is connected to REFIN, the input to the on-chip

unity-gain buffer, and the output is produced on the REF pin.

An external reference can be used with the internal buffer with

or without the temperature sensor enabled. Refer to Table 11 for

limited to an input/output of VDD = −0.2 V; however, the

maximum voltage allowable must be ≤(VDD − 0.5 V).

The internal reference buffer is useful in multiconverter

applications because a buffer is typically required in these

applications. In addition, a low power reference can be used

because the internal buffer provides the necessary performance

to drive the SAR architecture of the AD7682/AD7689.

REF

GND

TEMP

AD7682/

AD7689

10µF

100nF

REFIN

REF SOURCE

≤ (VDD – 0.5V)

Figure 34. External Reference Using Internal Buffer

External Reference

In any of the six voltage reference schemes, an external ref-

erence can be connected directly on the REF pin as shown in

Figure 35 because the output impedance of REF is >5 kΩ. To

reduce power consumption, power down the reference and

buffer. Refer to Table 11 for register details. For improved drift

performance, an external reference from the family of devices

that includes the ADR430, ADR431, ADR433, ADR434, and

ADR435, or the family of devices that includes the ADR440,

ADR441, ADR443, ADR444, and ADR445 is recommended.

REF

GND

TEMP

10µF

REFIN

REF SOURCE

0.5V < REF < (VDD + 0.3V)

NO CONNECTION

REQUIRED

AD7682/

AD7689

Figure 35.External Reference

Note that the best SNR is achieved with a 5 V external reference

as the internal reference is limited to 4.096 V. The SNR

degradation is as follows:

096

log

LOSS

SNR

Reference Decoupling

Whether using an internal or external reference, the AD7682/

AD7689 voltage reference output/input, REF, has a dynamic

input impedance and must be driven by a low impedance source

with efficient decoupling between the REF and GND pins. This

decoupling depends on the choice of the voltage reference but

usually consists of a low ESR capacitor connected to REF and

GND with minimum parasitic inductance. A 10 µF (X5R,

1206 size) ceramic chip capacitor is appropriate when using

the internal reference, a member of the ADR430, ADR431,

ADR433, ADR434, and ADR435 family of external references,

a member of the ADR440, ADR441, ADR443, ADR444, and

ADR445 family of external references, or a low impedance buffer

such as the AD8031 or the AD8605.

The placement of the reference decoupling capacitor is also

important to the performance of the AD7682/AD7689, as

explained in the Layout section. Mount the decoupling capacitor

with a thick PCB trace on the same side as the ADC at the REF pin.

The GND must also connect to the reference decoupling capacitor

with the shortest distance and to the analog ground plane with

several vias.

If desired, smaller reference decoupling capacitor values down

to 2.2 µF can be used with a minimal impact on performance,

especially on DNL.

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

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

and GND pins.

For applications that use multiple AD7682/AD7689 devices or

other PulSAR devices, it is more effective to use the internal

reference buffer to buffer the external reference voltage, thus

reducing SAR conversion crosstalk.

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 ±10 ppm/°C TC of the reference changes full scale by

±1 LSB/°C.

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