AD7682BCPZ datasheet(21/32 Pages) AD

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Data Sheet

AD7682/AD7689

Rev. D | Page 21 of 32

DRIVER AMPLIFIER CHOICE

Although the AD7682/AD7689 are 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 AD7682/AD7689. Note that the AD7682/

AD7689have a noise much lower than most of the other

16-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 AD7682/AD7689

analog input circuit low-pass filter made by RIN and CIN or

by an external filter, if one is used. Because the typical noise

of the AD7682/AD7689 is 35 μV rms (with VREF = 5 V), the

SNR degradation due to the amplifier is

⎟⎟

⎟

⎠

⎞

⎜⎜

⎜

⎝

⎛

−

)

(

log

3dB

LOSS

SNR

where:

f–3dB is the input bandwidth in megahertz of the AD7682/

AD7689 (1.7 MHz in full BW or 425 kHz in ¼ BW) or the

cutoff frequency of an input filter, if one is used.

N is the noise gain of the amplifier (for example, 1 in buffer

configuration).

eN is the equivalent input noise voltage of the op amp, in

nV/√Hz.

•

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

mance commensurate with the AD7682/AD7689. Figure 18

shows THD vs. frequency for the AD7682/AD7689.

•

For multichannel, multiplexed applications on each input

or input pair, the driver amplifier and the AD7682/AD7689

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

capacitor array at a 16-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 16-bit level and should be verified prior to driver

selection.

Table 9. 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 AD7682/AD7689 allow the choice of a very low temper-

ature drift internal voltage reference, an external reference, or an

external buffered reference.

The internal reference of the AD7682/AD7689 provide excel-

lent performance and can be used in almost all applications.

There are six possible choices of voltage reference schemes

briefly described in Table 10, 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 10. 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 AD7682/AD7689,

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 AD7682/AD7689 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 AD7682/AD7689 as shown in Figure 31 for either

a 2.5 V or 4.096 V internal reference.

REF

GND

TEMP

AD7682/

AD7689

10µF

100nF

REFIN

Figure 31. 2.5 V or 4.096 V Internal Reference Connection

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