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參數資料
| 型號： | AD7890BR-2 |
| 廠商： | ANALOG DEVICES INC |
| 元件分類： | ADC |
| 英文描述： | LC2MOS 8-Channel, 12-Bit Serial, Data Acquisition System |
| 中文描述： | 8-CH 12-BIT PROPRIETARY METHOD ADC, SERIAL ACCESS, PDSO24 |
| 封裝： | MS-013AD, SOIC-24 |
| 文件頁數： | 18/20頁 |
| 文件大小： | 302K |
| 代理商： | AD7890BR-2 |
AD7890
–18–
REV. A
In the external clocking mode, it is possible to write data to the
Control Register or read data from the output register while a
conversion is in progress. T he same data is presented in Figure
18 as in Figure 17 except that in this case the output data read
for the device occurs during conversion. T hese results are
achieved with a serial clock rate of 2.5 MHz. If a higher serial
clock rate is used, the code transition noise will degrade from
that shown in the plot of Figure 18. T his has the effect of inject-
ing noise onto the die while bit decisions are being made and
this increases the noise generated by the AD7890. T he histo-
gram plot for 8192 conversions of the same dc input now shows
a larger spread of codes with the rms noise for the AD7890-2
increasing to 170
μ
V. T his effect will vary depending on where
the serial clock edges appear with respect to the bit trials of the
conversion process. It is possible to achieve the same level of
performance when reading during conversion as when reading
after conversion depending on the relationship of the serial clock
edges to the bit trial points (i.e., the relationship of the serial
clock edges to the CL K IN edges). T he bit decision points
on the AD7890 are on the falling edges of the master clock
(CLK IN) during the conversion process. Clocking out new
data bits at these points (i.e. the rising edge of SCLK ) is the
most critical from a noise standpoint. T he most critical bit deci-
sions are the MSBs, so to achieve the level of performance out-
lined in Figure 18, reading within 1
μ
s after the rising edge of
CONVST
should be avoided.
(X–3)
(X–4)
(X+4)
(X+3)
(X+2)
(X+1)
X
(X–1)
(X–2)
O
CODE
8000
0
2000
1000
4000
3000
5000
6000
7000
SAMPLING FREQUENCY =
102.4kHz
T
A
= +25°C
Figure 18. Histogram of 8192 Conversions with Read
During Conversion
Writing data to the Control Register also has the effect of intro-
ducing digital activity onto the part while conversion is in
progress. However, since there are no output drivers active dur-
ing a write operation, the amount of current flowing on the die
is less than for a read operation. T herefore, the amount of noise
injected into the die is less than for a read operation. Figure 19
shows the effect of a write operation during conversion. T he his-
togram plot for 8192 conversions of the same dc input now
shows a larger spread of codes than for ideal conditions but
smaller than for a read operation. T he resulting rms noise for
the AD7890-2 is 110
μ
V. In this case, the serial clock frequency
was 10 MHz.
(X–3)
(X–4)
(X+4)
(X+3)
(X+2)
(X+1)
X
(X–1)
(X–2)
O
CODE
8000
0
2000
1000
4000
3000
5000
6000
7000
SAMPLING FREQUENCY =
102.4kHz
T
A
= +25°C
Figure 19. Histogram of 8192 Conversions with Write
During Conversion
Dynamic Performance
T he AD7890 contains an on-chip track/hold, allowing the part
to sample input signals up to 50 kHz on any of its input chan-
nels. Many of the AD7890’s applications will simply require it
to sequence through low frequency input signals across its eight
channels. T here may be some applications, however, for which
the dynamic performance of the converter out to 40 kHz input
frequency is of interest. It is recommended for these wider band
sampling applications that the hardware conversion start method
is used for reasons outlined previously.
T hese applications require information on the ADC’s effect on
the spectral content of the input signal. Signal to (Noise +
Distortion), total harmonic distortion, peak harmonic or spuri-
ous and intermodulation distortion are all specified. Figure 20
shows a typical FFT plot of a 10 kHz, 0 V to +2.5 V input after
being digitized by the AD7890-2 operating at a 102.4 kHz sam-
pling rate. T he signal to (Noise + Distortion) is 71.5 dB and the
total harmonic distortion is –85 dB. It should be noted that
reading data from the part during conversion at 10 MHz serial
clock does have a significant impact on dynamic performance.
T herefore, for sampling applications, it is recommended not to
read data during conversion.
0
51.2
–60
–120
–90
0
–30
25.6
F = /2
SAMPLE RATE = 102.4 kHz
INPUT FREQUENCY = 10 kHz
SNR = 71.5 dB
T
A
= +25°C
S
FREQUENCY – kHz
SNR IS SIGNAL TO (NOISE + DISTORTION) RATIO.
Figure 20. AD7890 FFT Plot
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