AD7721

REV. A

–8–

TERMINOLOGY

Integral Nonlinearity

This is the maximum deviation of any code from a straight line

passing through the endpoints of the transfer function. The end-

points of the transfer function are zero scale (not to be con-

fused with bipolar zero), a point 0.5 LSB below the first code

transition (100 . . . 00 to 100 . . . 01 in bipolar mode and

000 . . . 00 to 000 . . . 01 in unipolar mode) and full scale, a point

0.5 LSB above the last code transition (011 . . . 10 to 011 . . . 11 in

bipolar mode and 111 . . . 10 to 111 . . . 11 in unipolar mode). The

error is expressed in LSBs.

Differential Nonlinearity

This is the difference between the measured and the ideal 1 LSB

change between two adjacent codes in the ADC.

Common Mode Rejection Ratio

The ability of a device to reject the effect of a voltage applied to

both input terminals simultaneously—often through variation of

a ground level—is specified as a common-mode rejection ratio.

CMRR is the ratio of gain for the differential signal to the gain

for the common-mode signal.

Unipolar Offset Error

Unipolar offset error is the deviation of the first code transition

from the ideal VIN1 voltage which is (VIN2 + 0.5 LSB) when

operating in the unipolar mode.

Bipolar Offset Error

This is the deviation of the midscale transition (111 . . . 11

to 000 . . . 00) from the ideal VIN1 voltage which is (VIN2 –

0.5 LSB) when operating in the bipolar mode.

Unipolar Full-Scale Error

Unipolar full-scale error is the deviation of the last code transition

(111 . . . 10 to 111 . . . 11) from the ideal VIN1 voltage which is

Bipolar Full-Scale Error

The bipolar full-scale error refers to the positive full-scale error and

the negative full-scale error. The positive full-scale error is the

deviation of the last code transition (011 . . . 10 to 011 . . . 11) from

The negative full-scale error is the deviation of the first code transi-

tion (100 . . . 00 to 100 . . . 01) from the ideal VIN1 voltage which

Signal to (Noise + Distortion)

Signal to (Noise + Distortion) is measured signal to noise at the

output of the ADC. The signal is the rms magnitude of the funda-

mental. Noise is the rms sum of all the nonfundamental signals up

ponent. Signal to (Noise + Distortion) is dependent on the num-

ber of quantization levels used in the digitization process; the more

levels, the smaller the quantization noise. The theoretical Signal to

(Noise + Distortion) ratio for a sine wave input is given by

Signal to (Noise + Distortion) = (6.02 N + 1.76) dB

where N is the number of bits. Thus, for an ideal 12-bit converter,

Signal to (Noise + Distortion) = 74 dB.

Total Harmonic Distortion

Total Harmonic Distortion (THD) is the ratio of the rms sum

of harmonics to the rms value of the fundamental. For the

AD7721, THD is defined as

THD

= 20 log

(V

2

3

4

5

6

2

)

V

1

sixth harmonic.

USING THE AD7721

ADC Differential Inputs

The AD7721 uses differential inputs to provide common-mode

noise rejection. In the bipolar mode configuration, the analog

input range is

±1.25 V. The designed code transitions occur

midway between successive integer LSB values. The output

code is 2s complement binary with 1 LSB = 0.61 mV in paral-

lel mode and 38

µV in serial mode. The ideal input/output

transfer function is illustrated in Figure 2.

In the unipolar mode, the analog input range is 0 V to 2.5 V.

Again, the designed code transitions occur midway between suc-

cessive integer LSB values. The output code is straight binary with

1 LSB = 0.61 mV in parallel mode and 38

µV in serial mode. The

ideal input/output transfer function is shown in Figure 3.

0V

100...000

011...111

000...000

011...110

000...010

000...001

111...111

111...110

100...001

DIFFERENTIAL INPUT VOLTAGE (VIN1–VIN2)

OUTPUT

CODE

AD7721

–REF IN/2

+REF IN/2–1LSB

Figure 2. AD7721 Bipolar Mode Transfer Function

000...000

000...010

000...001

111...111

111...110

DIFFERENTIAL INPUT VOLTAGE (VIN1–VIN2)

OUTPUT

CODE

AD7721

111...101

111...100

000...011

0V

REF IN–1LSB

Figure 3. AD7721 Unipolar Mode Transfer Function