AD7682 View Datasheet(PDF) - Analog Devices

Part Name

Description

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AD7682

16-Bit, 4-Channel/8-Channel, 250 kSPS PulSAR ADC

Analog Devices 

AD7682/AD7689

⎜⎛

SNR LOSS

=

20 log

⎜

⎜

⎜

⎝

⎟⎞

35

⎟

352

+

π

2

f

−3dB

(

NeN

)2

⎟

⎟

⎠

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 is 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

performance 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 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 16-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 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 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

AD7682/AD7689 and is thus useful for performing a system

calibration. 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

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 AD7682/AD7689.

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 AD7682/

AD7689 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 AD7682/AD7689, 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.

Rev. 0 | Page 20 of 28

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