AD7960BCPZ datasheet(15/24 Pages) AD

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Data Sheet

AD7960

Rev. 0 | Page 15 of 24

TRANSFER FUNCTION

The AD7960 uses a 5 V or a 4.096 V reference. The AD7960

converts the differential voltage of the antiphase analog inputs

(IN+ and IN−) into a digital output. IN+ and IN− require a

REF/2 V common-mode voltage.

The 18-bit conversion result is in MSB first, twos complement

format. The ideal transfer functions for the AD7960 are shown

in Figure 29 and Table 7.

Figure 29. ADC Ideal Transfer Functions (FSR = Full-Scale Range)

ANALOG INPUTS

The analog inputs applied to the AD7960, IN+ and IN−, must be

180° out of phase with each other. Figure 30 shows an equivalent

circuit of the input structure of the AD7960.

The two diodes provide ESD protection for IN+ and IN−. Care

must be taken to ensure that the analog input signals do not

exceed the supply rails of the AD7960 by more than 0.3 V (VDD1

and GND). If the analog input signals exceed this level, the

diodes become forward-biased and start conducting current.

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

maximum. However, if the supplies of the input buffer amplifier

are different from the VDD1/GND supply, the analog input

signal may eventually exceed the supply rails by more than 0.3 V.

In such a case (for example, an input buffer with a short circuit),

the current limitation can be used to protect the part.

Figure 30. Equivalent Analog Input Circuit

The analog input structure allows the sampling of the true

differential signal between IN+ and IN−. By using these differ-

ential inputs, signals common to both inputs are rejected. The

AD7960 shows some degradation in THD with higher analog

input frequencies.

Figure 31. Analog Input CMRR vs. Frequency

Table 7. Output Codes and Ideal Input Voltages

Description

Analog Input (IN+ − IN−),

REF = 5 V

Analog Input (IN+ − IN−),

REF = 4.096 V

Digital Output Code, Twos Complement (Hex)

FSR − 1 LSB

+4.999962 V

+4.095969 V

0x1FFFF

Midscale + 1 LSB

+38.15 μV

+31.25 μV

0x00001

Midscale

0 V

0x00000

Midscale − 1 LSB

−38.15 μV

−31.25 μV

0x3FFFF

−FSR + 1 LSB

−4.999962 V

−4.095969 V

0x20001

−FSR

−5 V

−4.096 V

0x20000

100 ... 000

100 ... 001

100 ... 010

011 ... 101

011 ... 110

011 ... 111

ANALOG INPUT

+FSR – 1.5LSB

+FSR – 1LSB

–FSR + 1LSB

–FSR

–FSR + 0.5LSB

185Ω

26pF

VDD1

IN+

OR IN–

100

10k

100k

FREQUENCY (Hz)

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