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參數資料
| 型號： | AD7880CR |
| 廠商： | ANALOG DEVICES INC |
| 元件分類： | ADC |
| 英文描述： | LC2MOS Single +5 V Supply, Low Power, 12-Bit Sampling ADC |
| 中文描述： | 1-CH 12-BIT SUCCESSIVE APPROXIMATION ADC, PARALLEL ACCESS, PDSO24 |
| 封裝： | SOIC-24 |
| 文件頁數： | 6/16頁 |
| 文件大小： | 337K |
| 代理商： | AD7880CR |
AD7880
–6–
REV. 0
T he AD7880 has two unipolar input ranges, 0 V to 5 V and 0 V
to 10 V. Figure 5 shows the analog input for the 0 V to 5 V
range. T he designed code transitions occur midway between
successive integer L SB values (i.e., 1/2 L SB, 3/2 L SBs,
5/2 LSBs . . . FS –3/2 LSBs). T he output code is straight binary
with 1
LSB = FS/
4096
=
5
V/
4096
=
1.22
mV. T he same applies
for the 0 V to 10 V range, as shown in Figure 6, except that the
LSB size is bigger. In this case
1
LSB = FS/
4096
=
10
V/
4096
=
2.44
mV. T he ideal input/output transfer characteristic for both
these unipolar ranges is shown in Figure 8.
1LSB =
FS
4096
OUTPUT
CODE
0V
111...111
111...110
111...101
111...100
000...011
000...001
000...000
A
FS – 1LSB
A
A
A
A
A
A
A
A
A
A
A
A
A
000...010
V INPUT VOLTAGE
1LSB
+
Figure 8. AD7880 Unipolar Transfer Characteristic
Figure 7 shows the AD7880’s
±
5 V bipolar analog input con-
figuration. Once again the designed code transitions occur mid-
way between successive integer LSB values. T he output code is
straight binary with 1
LSB = FS/
4096
=
10
V/
4096
=
2.44
mV
.
T he ideal bipolar input/output transfer characteristic is shown in
Figure 9.
A
A
A
A
A
A
A
A
1LSB
A
A
A
A
A
A
FS
–
FS = 10V
1LSB =4FS
OUTPUT
CODE
111...111
111...110
100...101
100...000
011...111
011...110
000...001
000...000
FS
2
+
–
0V
V INPUT VOLTAGE
1LSB
–
1LSB
+
Figure 9. AD7880 Bipolar Transfer Characteristic
CLOCK INPUT
T he AD7880 is specified to operate with a 2.5 MHz clock con-
nected to the CLK IN input pin. T his pin may be driven directly
by CMOS or T T L buffers. T he mark/space ratio on the clock
can vary from 40/60 to 60/40. As the clock frequency is slowed
down, it can result in slightly degraded accuracy performance.
T his is due to leakage effects on the hold capacitor in the inter-
nal track-and-hold amplifier. Figure 10 is a typical plot of accu-
racy versus clock frequency for the ADC.
2.5
2.0
1.5
1.0
0.5
0.0
0.5
1.5
2.5
3.5
CLOCK FREQUENCY – MHz
N
Figure 10. Normalized Linearity Error vs. Clock Frequency
T RACK /HOLD AMPLIFIE R
T he charge balanced comparator used in the AD7880 for the
A/D conversion provides the user with an inherent track/hold
function. T he track/hold amplifier acquires an input signal to
12-bit accuracy in less than 3
μ
s. T he overall throughput time is
equal to the conversion time plus the track/hold amplifier acqui-
sition time. For a 2.5 MHz input clock, the throughput time is
15
μ
s.
T he operation of the track/hold amplifier is essentially transpar-
ent to the user. T he track/hold amplifier goes from its tracking
mode to its hold mode at the start of conversion, i.e., on the ris-
ing edge of
CONVST
as shown in Figure 1.
OFFSE T AND FULL-SCALE ADJUST ME NT
In most Digital Signal Processing (DSP) applications, offset and
full-scale errors have little or no effect on system performance.
Offset error can always be eliminated in the analog domain by
ac coupling. Full-scale error effect is linear and does not cause
problems as long as the input signal is within the full dynamic
range of the ADC. Some applications will require that the input
signal range match the maximum possible dynamic range of the
ADC. In such applications, offset and full-scale error will have
to be adjusted to zero.
T he following sections describe suggested offset and full-scale
adjustment techniques which rely on adjusting the inherent off-
set of the op amp driving the input to the ADC as well as tweak-
ing an additional external potentiometer as shown in Figure 11.
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