AD7949 datasheet(19/32 Pages) AD

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Data Sheet

AD7949

Rev. F | Page 19 of 32

DRIVER AMPLIFIER CHOICE

Although the AD7949 is easy to drive, the driver amplifier must

meet the following requirements:

•

The noise generated by the driver amplifier must be kept as

low as possible to preserve the SNR and transition noise

performance of the AD7949. Note that the AD7949 has a

noise much lower than most of the other 14-bit ADCs and,

therefore, can be driven by a noisier amplifier to meet a

given system noise specification. The noise from the

amplifier is filtered by the AD7949 analog input circuit

low-pass filter made by RIN and CIN or by an external filter,

if one is used.

•

For ac applications, the driver should have a THD perfor-

mance commensurate with the AD7949. Figure 17 shows

THD vs. frequency for the AD7949.

•

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 amplifier data sheets,

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

ADA4841-1

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

performance 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 precision internal reference, suitable for most applications,

can be set for either a 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. Note that the voltage of REFIN

changes depending on the 2.5 V or 4.096 V internal reference.

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. For

applications requiring the use of the temperature sensor, the

internal reference must be active (internal buffer can be

disabled in this case). 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

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

±10 ppm/°C.

Connect the AD7949 as shown in Figure 30 for either a 2.5 V or

4.096 V internal reference.

REF

GND

TEMP

AD7949

10µF

100nF

REFIN

Figure 30. 2.5 V or 4.096 V Internal Reference Connection

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