AD7684

Rev. A | Page 13 of 16

AD7684

REF

GND

VDD

–IN

+IN

DCLOCK

CS

3-WIRE INTERFACE

100nF

2.7V TO 5.25V

2.2

μF TO 10μF

(NOTE 2)

REF

33

Ω

2.7nF

(NOTE 3)

(NOTE 4)

(NOTE 1)

33

Ω

2.7nF

(NOTE 3)

(NOTE 4)

NOTE 1: SEE VOLTAGE REFERENCE INPUT SECTION FOR REFERENCE SELECTION.

NOTE 3: SEE DRIVER AMPLIFIER CHOICE SECTION.

NOTE 4: OPTIONAL FILTER. SEE ANALOG INPUT SECTION.

NOTE 5: SEE DIGITAL INTERFACE FOR MOST CONVENIENT INTERFACE MODE.

Figure 22. Typical Application Diagram

TYPICAL CONNECTION DIAGRAM

Figure 22 shows an example of the recommended application

diagram for the AD7684.

ANALOG INPUTS

The analog inputs (+IN, −IN) need to be driven differentially

180° from each other, as shown in Figure 22. Holding either

input at GND or a fixed dc gives erroneous conversion results

because the AD7684 is intended for differential operation only.

For applications requiring –IN to be at GND (±100 mV), the

AD7683 should be used.

Figure 23 shows an equivalent circuit of the input structure of

the AD7684. The two diodes, D1 and D2, provide ESD protection

for the analog inputs, +IN and −IN. Care must be taken to

ensure that the analog input signal never exceeds the supply

rails by more than 0.3 V because this causes these diodes to

become forward-biased and start conducting current. However,

these diodes can handle a forward-biased current of 130 mA

maximum. For instance, these conditions could eventually

occur when the supplies of the input buffer (U1) are different

from VDD. In such a case, an input buffer with a short-circuit

current limitation can be used to protect the part.

D1

D2

+IN

OR –IN

GND

VDD

Figure 23. Equivalent Analog Input Circuit

This analog input structure allows the sampling of the differential

signal between +IN and −IN. By using this differential input, small

signals common to both inputs are rejected. During the acquisition

phase, the impedance of the analog inputs can be modeled as a

component made up of some serial resistors and the on-

the ADC sampling capacitor. During the conversion phase,

when the switches are opened, the input impedance is limited

undesirable aliasing effects and limits the noise.

When the source impedance of the driving circuit is low, the

AD7684 can be driven directly. Large source impedances

significantly affect the ac performance, especially THD. The dc

performances are less sensitive to the input impedance.

DRIVER AMPLIFIER CHOICE

Although the AD7684 is easy to drive, the driver amplifier

needs to meet the following requirements:

•

The noise generated by the driver amplifier needs to be

kept as low as possible to preserve the SNR and transition

noise performance of the AD7684. Note that the AD7684

has a noise level much lower than most other 16-bit ADCs

and, therefore, can be driven by a noisier op amp while

preserving the same or better system performance. The

noise coming from the driver is filtered by the AD7684

•

For ac applications, the driver needs to have a THD

performance commensurate with the AD7684. Figure 15

shows the THD vs. frequency that the driver should exceed.

•

For multichannel multiplexed applications, the driver

amplifier and the AD7684 analog input circuit must be

able to settle for a full-scale step of the capacitor array at a

16-bit level (0.0015%). In the data sheet of the amplifier,

settling at 0.1% to 0.01% is more commonly specified. This

could differ significantly from the settling time at a 16-bit

level and should be verified prior to driver selection.