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參數(shù)資料
| 型號： | AD7721ARZ-REEL |
| 廠商： | Analog Devices Inc |
| 文件頁數(shù)： | 15/16頁 |
| 文件大?。?/td> | 0K |
| 描述： | IC ADC 16BIT SIGMA-DELTA 28SOIC |
| 標(biāo)準(zhǔn)包裝： | 1,000 |
| 位數(shù)： | 16 |
| 采樣率（每秒）： | 468.75k |
| 數(shù)據(jù)接口： | 串行，并聯(lián) |
| 轉(zhuǎn)換器數(shù)目： | 1 |
| 功率耗散（最大）： | 150mW |
| 電壓電源： | 模擬和數(shù)字 |
| 工作溫度： | -40°C ~ 85°C |
| 安裝類型： | 表面貼裝 |
| 封裝/外殼： | 28-SOIC（0.295"，7.50mm 寬） |
| 供應(yīng)商設(shè)備封裝： | 28-SOIC W |
| 包裝： | 帶卷 (TR) |
| 輸入數(shù)目和類型： | 1 個差分，單極；1 個差分，雙極 |
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
(VIN2 + VREFIN – 3/2 LSBs).
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 ideal VIN1 voltage which is (VIN2 + VREFIN/2 – 3/2 LSB).
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
is (VIN2 – VREFIN/2 + 0.5 LSB).
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
to half the sampling frequency (fCLK/2) but excluding the dc com-
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
2 +V
3
2 +V
4
2 +V
5
2 +V
6
2
)
V
1
where V1 is the rms amplitude of the fundamental and V2, V3,
V4, V5 and V6 are the rms amplitudes of the second through the
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
V 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
V 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
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