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參數(shù)資料
| 型號： | AD7225CQ |
| 廠商： | ANALOG DEVICES INC |
| 元件分類： | DAC |
| 英文描述： | LC2MOS Quad 8-Bit DAC with Separate Reference Inputs |
| 中文描述： | QUAD, PARALLEL, 8 BITS INPUT LOADING, 8-BIT DAC, CDIP24 |
| 封裝： | CERDIP-24 |
| 文件頁數(shù)： | 10/12頁 |
| 文件大?。?/td> | 339K |
| 代理商： | AD7225CQ |
AD7225
REV. B
–10–
FILTER
I/P
I/P
SAMPLES
Am29520
TLD
AD7820
ADC
SAMPLES
AD7225
QUAD DAC
DELAYED
I/P
AD7226
QUAD DAC
V
REF
A
h
1
h
2
V
OUT
A
V
OUT
A
V
OUT
B
V
OUT
C
V
OUT
D
ACCUMULATOR
O/P
AD585
SHA
FILTER
O/P
TAP WEIGHTS
Am7224
DAC
AD584
REF
GAIN SET
+10V
V
OUT
V
REF
V
REF
+
T
T
T
1
2
3
4
h
4
h
3
h
2
h
1
+
FILTER
O/P
Y
n
FILTER
I/P
X
n–1
X
n–2
X
n
X
n–3
V
REF
A
h
3
V
OUT
A
V
REF
A
h
4
V
OUT
A
V
REF
A
V
OUT
A
Figure 18. Programmable Transversal Filter
A 4-tap programmable transversal filter may be implemented
using the AD7225 (Figure 18). T he input signal is first sampled
and converted to allow the tapped delay line function to be pro-
vided by the Am29520. T he multiplication of delayed input
samples by fixed, programmable up weights is accomplished by
the AD7225, the four coefficients or reference inputs being set
by the digital codes stored in the AD7226. T he resultant prod-
ucts are accumulated to yield the convolution sum output
sample which is held by the AD585.
0
–100
0.5
–70
–90
0.05
–80
0
–40
–60
–50
–30
–20
–10
0.45
0.4
0.35
0.3
0.25
0.2
0.15
0.1
NORMALIZED FREQUENCY – f/fs
G
h
1
= 0.117
h
2
= 0.417
h
3
= 0.417
h
4
= 0.417
Figure 19. Predicted (Theoretical) Response
Figure 20. Actual Response
Low pass, bandpass and high pass filters may be synthesized us-
ing this arrangement. T he particular up weights needed for any
desired transfer function may be obtained using the standard
Remez Exchange Algorithm. Figure 19 shows the theoretical
low pass frequency response produced by a 4-tap transversal
filter with the coefficients indicated. Although the theoretical
prediction does not take into account the quantization of the in-
put samples and the truncation of the coefficients, nevertheless,
there exists a good correlation with the actual performance of
the transversal filter (Figure 20).
DIGIT AL WORD MULT IPLICAT ION
Since each DAC of the AD7225 has a separate reference input,
the output of one DAC can be used as the reference input for
another. T his means that multiplication of digital words can be
performed (with the result given in analog form). For example,
if the output from DACA is applied to V
REF
B then the output
from DACB, V
OUT
B, can be expressed as:
V
OUT
B
=
D
A
D
B
V
REF
A
where
D
A
and
D
B
are the fractional representations of the
digital words in DAC latches A and B respectively.
If D
A
= D
B
= D then the result is D
2
V
REF
A
In this manner, the four DACs can be used on their own or in
conjunction with an external summing amplifier to generate
complex waveforms. Figure 21 shows one such application. In
this case the output waveform, Y, is represented by:
Y
= –(
x
4
+ 2
x
3
+ 3
x
2
+ 2
x
+ 4)
V
IN
where
x
is the digital code which is applied to all four DAC
latches.
V
OUT
A
V
OUT
B
V
OUT
C
V
OUT
D
V
REF
A
V
REF
B
V
REF
C
V
REF
D
+15V
V
DD
AD7225*
DGND
AGND
V
SS
V
IN
25k
50k
33k
50k
100k
100k
Y
*DIGITAL INPUTS OMITTED
FOR CLARITY
Figure 21. Complex Waveform Generation
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