AD7699 datasheet(18/28 Pages) AD

18 / 28 page

AD7699

Rev. 0 | Page 18 of 28

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 12 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 7. 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 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

performance and can be used in almost all applications. There

are five possible choices of voltage reference schemes briefly

described in Table 8 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 8. 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 AD7699

and is thus useful for performing a system calibration. Note

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

binary referenced from the AD7699 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 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 8 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.

External Reference

In any of the five voltage reference schemes, an external refer-

ence 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 applica-

tions requiring the use of the temperature sensor, the reference

must be active. Refer to Table 8 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 AD7699

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

AD7699 Datasheet(PDF) 18 Page - Analog Devices

AD7699 Datasheet(HTML) 18 Page - Analog Devices

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28

Datasheet Download

Link URL