AD7682BCPZ datasheet(22/32 Pages) AD

22 / 32 page

AD7682/AD7689

Data Sheet

Rev. D | Page 22 of 32

External Reference and Internal Buffer

For improved drift performance, an external reference can be

used with the internal buffer, as shown in Figure 32. 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 10 for

is 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 appli-

cations 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 32. External Reference Using Internal Buffer

External Reference

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

can be connected directly on the REF pin as shown in Figure 33

because the output impedance of REF is >5 kΩ. To reduce power

consumption, the reference and buffer should be powered down.

Refer to Table 10 for register details. For improved drift

performance, an external reference such as the ADR43x or

ADR44x is recommended.

REF

GND

TEMP

10µF

REFIN

REF SOURCE

0.5V < REF < (VDD + 0.3V)

NO CONNECTION

REQUIRED

AD7682/

AD7689

Figure 33.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 should therefore 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, the ADR43x/ADR44x

external reference, or a low impedance buffer such as the

AD8031 or the AD8605.

The placement of the reference decoupling capacitor is also impor-

tant to the performance of the AD7682/AD7689, as explained in

the Layout section. Mount the decoupling capacitor on the same

side as the ADC at the REF pin with a thick PCB trace. The GND

should 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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