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AD7949

14-Bit, 8-Channel, 250 kSPS PulSAR ADC

Analog Devices 

AD7949

External Reference and Internal Buffer

For improved drift performance, an external reference can be

used with the internal buffer, as shown in Figure 31. 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 9 for

register details. With the buffer enabled, the gain is unity and 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 AD7949.

REF SOURCE

≤ (VDD – 0.5V)

10µF

100nF

REF REFIN

AD7949

TEMP

GND

Figure 31. 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 32

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

consumption, the reference and buffer should be powered down.

When using only the external reference (and optional reference

buffer as shown in Figure 35), the internal buffer is disabled.

Refer to Table 9 for register details. For improved drift perfor-

mance, an external reference such as the ADR430/ADR431/

ADR433/ADR434/ADR435 or ADR440/ADR441/ADR443/

ADR444/ADR445 is recommended.

10µF

REF SOURCE

0.5V < REF < (VDD + 0.3V)

NO CONNECTION

REQUIRED

REF REFIN

AD7949

TEMP

GND

Figure 32. 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:

SNR LOSS

= 20 log

4.096

5

Data Sheet

Reference Decoupling

Whether using an internal or external reference, the AD7949

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 ADR430/ADR431/ADR433/

ADR434/ADR435 or ADR440/ADR441/ADR443/ADR444/

ADR445 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 AD7949, 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 be connected 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 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 AD7949 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.

Rev. F | Page 20 of 32

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