AD7946

Rev. A | Page 13 of 24

TYPICAL CONNECTION DIAGRAM

Figure 25 shows an example of the recommended connection diagram for the AD7946 when multiple supplies are available.

AD7946

REF

GND

VDD

IN–

IN+

VIO

SDI

SCK

SDO

CNV

3- OR 4-WIRE INTERFACE

(NOTE 5)

100nF

100nF

5V

(NOTE 2)

V+

V+

V–

1.8V TO VDD

REF

0V TO REF

2.7nF

NOTES

1. SEE REFERENCE SECTION FOR REFERENCE SELECTION.

3. SEE DRIVER AMPLIFIER CHOICE SECTION.

4. OPTIONAL FILTER. SEE ANALOG INPUT SECTION.

5. SEE DIGITAL INTERFACE FOR MOST CONVENIENT INTERFACE MODE.

(NOTE 1)

(ADA4841

OR NOTE 3)

(NOTE 4)

Figure 25. Typical Application Diagram with Multiple Supplies

ANALOG INPUT

Figure 26 shows an equivalent circuit of the input structure of

the AD7946.

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 begin to forward-

bias and start conducting current. These diodes can handle a

maximum forward-biased current of 130 mA. 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 26. Equivalent Analog Input Circuit

The 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, as shown in Figure 27, which represents the typical

CMRR over frequency. For instance, by using IN− to sense a

remote signal ground, ground potential differences between the

sensor and the local ADC ground are eliminated.

FREQUENCY (kHz)

50

40

60

70

10

1

100

1k

10k

VDD = 5V

Figure 27. Analog Input CMRR vs. Frequency

During the acquisition phase, the impedance of the analog

inputs (IN+ or IN−) can be modeled as a parallel combination

typically 600 Ω and is a lumped component made up of some

typically 30 pF and is mainly the ADC sampling capacitor.

During the conversion phase, when the switches are opened, the

low-pass filter, which reduces undesirable aliasing effects and

limits the noise.

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

AD7946 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. The maximum source

impedance depends on the amount of THD that can be