Data Sheet

AD7694

AD7694

REF

GND

VDD

IN–

IN+

SCK

SDO

CNV

3-WIRE INTERFACE

100nF

2.7V TO 5.25V

2.2 TO 10

µF

(NOTE 2)

REF

33

Ω

2.7nF

(NOTE 3)

(NOTE 4)

(NOTE 1)

NOTES

1. SEE REFERENCE SECTION FOR REFERENCE SELECTION.

3. SEE DRIVER AMPLIFIER CHOICE SECTION.

4. OPTIONAL FILTER. SEE ANALOG INPUT SECTION.

Figure 20. Typical Application Diagram

TYPICAL CONNECTION DIAGRAM

Figure 20 shows an example of the recommended application

diagram for the AD7694.

ANALOG INPUT

Figure 21 shows an equivalent circuit of the AD7694 input

structure. 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 will cause 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 input buffer’s (U1) supplies 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 21. 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. For instance, by using IN− to sense a remote signal

ground, ground potential differences between the sensor and

the local ADC ground are eliminated. During the acquisition

phase, the impedance of the analog input IN+ can be modeled

component made up of some serial resistors and the on

the ADC sampling capacitor. During the conversion phase,

where 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

AD7694 can be driven directly. Large source impedances

significantly affect the ac performance, especially total

harmonic distortion (THD). The dc performances are less

sensitive to the input impedance.

DRIVER AMPLIFIER CHOICE

Although the AD7694 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 AD7694. Note that the AD7694

has a noise much lower than most of the other

16-bit ADCs and, therefore, can be driven by a noisier op

amp while preserving the same or better system perfor-

mance. The noise coming from the driver is filtered by the

AD7694 analog input circuit 1-pole, low-pass filter made

by R1 and C2 or by the external filter, if one is used.

•

For ac applications, the driver needs to have a THD

performance suitable to that of the AD7694. Figure 13

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

•

For multichannel, multiplexed applications, the driver

amplifier and the AD7694 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 amplifier’s data sheet, 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.

Rev. B | Page 13 of 16