AD7699 datasheet(20/32 Pages) AD

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AD7699

Data Sheet

Rev. F | Page 20 of 32

DRIVER AMPLIFIER CHOICE

Although the AD7699 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 AD7699. Note that the AD7699 has 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 AD7699 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 AD7699 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 AD7699

(14.7 MHz in full BW or 670 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 AD7699. Figure 13 shows

THD vs. frequency for the AD7699.

•

For multichannel, multiplexed applications on each input

or input pair, the driver amplifier and the AD7699 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

ADA4805-1

Low noise, small size, and low power

ADA4807-1

Very low noise and high frequency

AD8655

5 V single supply, low noise

ADA4627-1

Precision, low noise, and low input bias

ADA4522-1

Precision, zero drift, and EMI enhanced

ADA4500-2

Precision, rail-to-rail input/output, and zero input

crossover distortion

VOLTAGE REFERENCE OUTPUT/INPUT

The AD7699 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 AD7699 provides excellent per-

formance and can be used in almost all applications. There are

five 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 internal reference can be set for 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.

Enabling the internal reference also enables the internal

temperature sensor, which measures the internal temperature of

the AD7699 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 single-ended conversion, straight binary referenced

from the AD7699 GND pin. The AD7699 temperature sensor

voltage can be thought of as a normal analog input; therefore,

the code that represents it is calculated as temperature sensor

code = temperature sensor voltage × (reference voltage)/(216 − 1).

Its temperature sensor output voltage is typically 283 mV at

25°C. The internal reference is temperature-compensated to

within 10 mV. The reference is trimmed to provide a typical

drift of ±10 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 connected

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 10 for

limited to an input/output of 4.096 V.

The internal reference buffer is useful in multiconverter applica-

tions 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 AD7699.

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