Datasheet AD7721 (Analog Devices) - 9

AD7721 TERMINOLOGY
where V1 is the rms amplitude of the fundamental and V2, V3,
Integral Nonlinearity
V4, V5 and V6 are the rms amplitudes of the second through the This is the maximum deviation of any code from a straight line sixth harmonic. passing through the endpoints of the transfer function. The end- points of the transfer function are zero scale (not to be con-
USING THE AD7721
fused with bipolar zero), a point 0.5 LSB below the first code
ADC Differential Inputs
transition (100 . 00 to 100 . 01 in bipolar mode and The AD7721 uses differential inputs to provide common-mode 000 . 00 to 000 . 01 in unipolar mode) and full scale, a point noise rejection. In the bipolar mode configuration, the analog 0.5 LSB above the last code transition (011 . 10 to 011 . 11 in input range is ± 1.25 V. The designed code transitions occur bipolar mode and 111 . 10 to 111 . 11 in unipolar mode). The midway between successive integer LSB values. The output error is expressed in LSBs. code is 2s complement binary with 1 LSB = 0.61 mV in paral-
Differential Nonlinearity
lel mode and 38 µV in serial mode. The ideal input/output This is the difference between the measured and the ideal 1 LSB transfer function is illustrated in Figure 2. change between two adjacent codes in the ADC. In the unipolar mode, the analog input range is 0 V to 2.5 V.
Common Mode Rejection Ratio
Again, the designed code transitions occur midway between suc- The ability of a device to reject the effect of a voltage applied to cessive integer LSB values. The output code is straight binary with both input terminals simultaneously—often through variation of 1 LSB = 0.61 mV in parallel mode and 38 µV in serial mode. The a ground level—is specified as a common-mode rejection ratio. ideal input/output transfer function is shown in Figure 3. CMRR is the ratio of gain for the differential signal to the gain for the common-mode signal.
OUTPUT CODE Unipolar Offset Error 011...111
Unipolar offset error is the deviation of the first code transition
AD7721
from the ideal VIN1 voltage which is (VIN2 + 0.5 LSB) when
011...110
operating in the unipolar mode.
Bipolar Offset Error 000...010
This is the deviation of the midscale transition (111 . 11
000...001
to 000 . 00) from the ideal VIN1 voltage which is (VIN2 –
–REF IN/2
0.5 LSB) when operating in the bipolar mode.
000...000 +REF IN/2–1LSB Unipolar Full-Scale Error 111...111
Unipolar full-scale error is the deviation of the last code transition
111...110
(111 . 10 to 111 . 11) from the ideal VIN1 voltage which is (VIN2 + VREFIN – 3/2 LSBs).
100...001 Bipolar Full-Scale Error
The bipolar full-scale error refers to the positive full-scale error and
100...000
the negative full-scale error. The positive full-scale error is the
0V
deviation of the last code transition (011 . 10 to 011 . 11) from
DIFFERENTIAL INPUT VOLTAGE (VIN1–VIN2)
the ideal VIN1 voltage which is (VIN2 + VREFIN/2 – 3/2 LSB). The negative full-scale error is the deviation of the first code transi- Figure 2. AD7721 Bipolar Mode Transfer Function tion (100 . 00 to 100 . 01) from the ideal VIN1 voltage which is (VIN2 – VREFIN/2 + 0.5 LSB).
OUTPUT CODE Signal to (Noise + Distortion)
Signal to (Noise + Distortion) is measured signal to noise at the
111...111
output of the ADC. The signal is the rms magnitude of the funda-
111...110 AD7721
mental. Noise is the rms sum of all the nonfundamental signals up to half the sampling frequency (f
111...101
CLK/2) but excluding the dc com- ponent. Signal to (Noise + Distortion) is dependent on the num-
111...100
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
000...011
where N is the number of bits. Thus, for an ideal 12-bit converter, Signal to (Noise + Distortion) = 74 dB.
000...010 Total Harmonic Distortion 000...001
Total Harmonic Distortion (THD) is the ratio of the rms sum
000...000
of harmonics to the rms value of the fundamental. For the
0V REF IN–1LSB
AD7721, THD is defined as
DIFFERENTIAL INPUT VOLTAGE (VIN1–VIN2)
2 2 2 2 2 (V +V +V +V +V ) THD = 20 log 2 3 4 5 6 V1 Figure 3. AD7721 Unipolar Mode Transfer Function –8– REV. A