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AD7949

Rev. A | Page 19 of 28

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For multichannel, multiplexed applications on each input

or input pair, the driver amplifier and the AD7949 analog

input circuit must settle a full-scale step onto the capacitor

array at a 14-bit level (0.0015%). In the amplifier data

sheet, settling at 0.1% to 0.01% is more commonly

specified. This may differ significantly from the settling

time at a 14-bit level and should be verified prior to driver

selection.

Table 8. Recommended Driver Amplifiers

Amplifier

Typical Application

ADA4841-x

Very low noise, small, and low power

AD8655

5 V single supply, low noise

AD8021

Very low noise and high frequency

AD8022

Low noise and high frequency

OP184

Low power, low noise, and low frequency

AD8605, AD8615

5 V single supply, low power

VOLTAGE REFERENCE OUTPUT/INPUT

The AD7949 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 AD7949 provides excellent perfor-

mance and can be used in almost all applications. There are six

possible choices of voltage reference schemes briefly described

in Table 9 with more details in each of the following sections.

Internal Reference/Temperature Sensor

The internal reference can be set for either 2.5 V or a 4.096 V

output as detailed in Table 9. With the internal reference

enabled, the band gap voltage is also present on the REFIN pin,

which requires an external 0.1 μF capacitor. Because the current

output of REFIN is limited, it can be used as a source if followed

by a suitable buffer, such as the AD8605.

Enabling the reference also enables the internal temperature

sensor, which measures the internal temperature of the AD7949

and is thus useful for performing a system calibration. Note

that, when using the temperature sensor, the output is straight

binary referenced from the AD7949 GND pin.

The internal reference is temperature-compensated to within

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

3 ppm/°C.

External Reference and Internal Buffer

For improved drift performance, an external reference can be

used with the internal buffer. The external reference is con-

nected to REFIN, 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

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

limited to an input/output of 4.096 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.

External Reference

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

can be connected directly on the REF pin because the output

impedance of REF is >5 kΩ. To reduce power consumption, the

reference and buffer can be powered down independently or

together for the lowest power consumption. However, for

applications requiring the use of the temperature sensor, the

reference must be active. Refer to Table 9 for register details.

For improved drift performance, an external reference such as

the ADR43x or ADR44x is recommended.

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 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

important to the performance of the AD7949, as explained in the

Layout section. The decoupling capacitor should be mounted 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 AD7949s 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

±61 ppm/°C TC of the reference changes full scale by ±1 LSB/°C.

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