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參數資料
| 型號： | AD7703AQ |
| 廠商： | ANALOG DEVICES INC |
| 元件分類： | ADC |
| 英文描述： | LC2MOS 20-Bit A/D Converter |
| 中文描述： | 1-CH 20-BIT DELTA-SIGMA ADC, SERIAL ACCESS, CDIP20 |
| 封裝： | CERDIP-20 |
| 文件頁數： | 8/16頁 |
| 文件大小： | 251K |
| 代理商： | AD7703AQ |
AD7703
–8–
REV. D
DIGIT AL FILT E RING
T he AD7703’s digital filter behaves like an analog filter, with a
few minor differences.
First, since digital filtering occurs after the A to D conversion
process, it can remove noise injected during the conversion
process. Analog filtering cannot do this.
On the other hand, analog filtering can remove noise superim-
posed on the analog signal before it reaches the ADC. Digital
filtering cannot do this and noise peaks riding on signals near
full scale have the potential to saturate the analog modulator
and digital filter, even though the average value of the signal is
within limits. T o alleviate this problem, the AD7703 has over-
range headroom built into the sigma-delta modulator and digital
filter which allows overrange excursions of 100 mV. If noise sig-
nals are larger than this, consideration should be given to analog
input filtering, or to reducing the gain in the input channel so
that a full-scale input (2.5 V) gives only a half-scale input to the
AD7703 (1.25 V). T his will provide an overrange capability
greater than 100% at the expense of reducing the dynamic range
by 1 bit (50%).
FILT E R CHARACT E RIST ICS
T he cutoff frequency of the digital filter is f
CLK
/409600. At the
maximum clock frequency of 4.096 MHz, the cutoff frequency
of the filter is 10 Hz and the data update rate is 4 kHz.
Figure 9 shows the filter frequency response. T his is a 6-pole
Gaussian response that provides 55 dB of 60 Hz rejection for a
10 Hz cutoff frequency. If the clock frequency is halved to give a
5 Hz cutoff, 60 Hz rejection is better than 90 dB.
1
10
100
FREQUENCY – Hz
20
0
–20
–40
–60
–80
–100
–120
–140
–160
G
f
CLK
= 1MHz
f
CLK
= 2MHz
f
CLK
= 4MHz
Figure 9. Frequency Response of AD7703 Filter
Since the AD7703 contains this low-pass filtering, there is a set-
tling time associated with step function inputs, and data will be
invalid after a step change until the settling time has elapsed.
T he AD7703 is, therefore, unsuitable for high speed multiplex-
ing, where channels are switched and converted sequentially at
high rates, as switching between channels can cause a step
change in the input. However, slow multiplexing of the AD7703
is possible, provided that the settling time is allowed to elapse
before data for the new channel is accessed.
T he output settling of the AD7703 in response to a step input
change is shown in Figure 10. T he Gaussian response has fast
settling with no overshoot, and the worst-case settling time to
±
0.0007% is 125 ms with a 4.096 MHz master clock frequency.
P
100
80
60
40
20
0
0
40
80
120
160
TIME – ms
Figure 10. AD7703 Step Response
USING T HE AD7703
SY ST E M DE SIGN CONSIDE RAT IONS
T he AD7703 operates differently from successive approxima-
tion ADCs or integrating ADCs. Since it samples the signal con-
tinuously, like a tracking ADC, there is no need for a start convert
command. T he 20-bit output register is updated at a 4 kHz rate,
and the output can be read at any time, either synchronously or
asynchronously.
C LOC K ING
T he AD7703 requires a master clock input, which may be an
external T T L/CMOS compatible clock signal applied to the
CLK IN pin (CLK OUT not used). Alternatively, a crystal of the
correct frequency can be connected between CLK IN and
CLK OUT , when the clock circuit will function as a crystal
controlled oscillator.
Figure 11 shows a simple model of the on-chip gate oscillator
and T able II gives some typical capacitor values to be used with
various resonators.
AD7703
C2*
C1*
R1
5M
X1
2
3
10pF
10pF
g
m
= 1500
μ
MHO
*SEE TABLE II
Figure 11. On-Chip Gate Oscillator
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