Data Sheet

AD7683

Rev. B | Page 13 of 16

AD7683

REF

GND

VDD

–IN

+IN

DCLOCK

CS

3-WIRE INTERFACE

100nF

2.7V TO 5.25V

2.2µF TO 10µF

(NOTE 2)

REF

33Ω

2.7nF

(NOTE 3)

(NOTE 4)

(NOTE 1)

NOTES

1. SEE VOLTAGE REFERENCE INPUT SECTION FOR REFERENCE SELECTION.

3. SEE DRIVER AMPLIFIER CHOICE SECTION.

4. OPTIONAL FILTER. SEE ANALOG INPUT SECTION.

Figure 23. Typical Application Diagram

TYPICAL CONNECTION DIAGRAM

Figure 23 shows an example of the recommended application

diagram for the AD7683.

ANALOG INPUT

Figure 24 shows an equivalent circuit of the input structure of

the AD7683. The two diodes, D1 and D2, provide ESD protec-

tion 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 can eventually occur when the

input buffer (U1) supplies are different from VDD. In such a

case, use an input buffer with a short-circuit current limitation

to protect the part.

D1

D2

+IN

OR –IN

GND

VDD

Figure 24. Equivalent Analog Input Circuit

This analog input structure allows the sampling of the differen-

tial 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 as a parallel combination of

600 Ω and is a lumped component consisting of some serial

30 pF and is mainly the ADC sampling capacitor. During the

conversion phase, when the switches are opened, the input

pass filter that reduces undesirable aliasing effects and limits

the noise.

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

AD7683 can be driven directly. Large source impedances signi-

ficantly affect the ac performance, especially THD. The dc

performances are less sensitive to the input impedance.

DRIVER AMPLIFIER CHOICE

Although the AD7683 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 AD7683. Note that the AD7683

has a noise figure 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 AD7683



For ac applications, the driver needs to have a THD

performance suitable to that of the AD7683. Figure 16 shows

the THD vs. frequency that the driver should exceed.



For multichannel multiplexed applications, the driver

amplifier and the AD7683 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 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.

Table 11. Recommended Driver Amplifiers

Amplifier

Typical Application

ADA4841-1

Very low noise and low power

OP184

Low power, low noise, and low frequency

AD8605, AD8615

5 V single-supply, low power

AD8519

Low power and low frequency

AD8031

High frequency and low power