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AD7625

Data Sheet

Rev. A | Page 14 of 24

TRANSFER FUNCTIONS

The AD7625 uses a 4.096 V reference. The AD7625 converts

the differential voltage of the antiphase analog inputs (IN+ and

IN−) into a digital output. The analog inputs, IN+ and IN−,

require a 2.048 V common-mode voltage (REF/2).

The 16-bit conversion result is in MSB first, twos complement

format.

The ideal transfer functions for the AD7625 are shown in

Figure 19 and Table 7.

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

Figure 19. ADC Ideal Transfer Functions (FSR = Full-Scale Range)

Table 7. Output Codes and Ideal Input Voltages

Description

Analog Input

(IN+ − IN−)

REF = 4.096 V

Digital Output Code

Twos Complement (Hex)

FSR − 1 LSB

+4.0959375 V

0x1FFF

Midscale + 1 LSB

+62.5 μV

0x0001

Midscale

0 V

0x0000

Midscale − 1 LSB

−62.5 μV

0xFFFF

−FSR + 1 LSB

−4.0959375 V

0x1001

−FSR

−4.096 V

0x1000

ANALOG INPUTS

The analog inputs, IN+ and IN−, applied to the AD7625 must be

180° out of phase with each other. Figure 20 shows an equivalent

circuit of the input structure of the AD7625.

The two diodes provide ESD protection for the analog inputs,

IN+ and IN−. Care must be taken to ensure that the analog input

signal does not exceed the reference voltage by more than 0.3 V.

If the analog input signal exceeds 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 (for example, the supplies of

the ADA4899-1 in Figure 24) are different from those of the

reference, 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.

IN+

IN–

CNV

25pF

250Ω

VDD1

Figure 20. 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

AD7625 shows some degradation in THD with higher analog

input frequencies.

100

10k

100k

10M

INPUT COMMON-MODE FREQUENCY (Hz)

Figure 21. Analog Input CMRR vs. Frequency

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