AD7225CQ AD [Analog Devices], AD7225CQ Datasheet - Page 10

• Manufacturer Part Number: AD7225CQ
• Description: LC2MOS Quad 8-Bit DAC with Separate Reference Inputs
• Manufacturer: AD [Analog Devices]

AD7225
A 4-tap programmable transversal filter may be implemented
using the AD7225 (Figure 18). The input signal is first sampled
and converted to allow the tapped delay line function to be pro-
vided by the Am29520. The 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. The resultant prod-
ucts are accumulated to yield the convolution sum output
sample which is held by the AD585.
Low pass, bandpass and high pass filters may be synthesized us-
ing this arrangement. The 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
Figure 19. Predicted (Theoretical) Response
FILTER
I/P
–100
–10
–20
–30
–40
–50
–60
–70
–80
–90
0
0
Figure 20. Actual Response
AD7820
AD584
0.05
ADC
REF
0.1
NORMALIZED FREQUENCY – f/fs
h
h
h
h
SAMPLES
+10V
1
2
3
4
0.15
= 0.117
= 0.417
= 0.417
= 0.417
I/P
V
REF
0.2
GAIN SET
Am29520
0.25
Am7224
DAC
TLD
0.3
0.35
DELAYED
SAMPLES
V
OUT
Figure 18. Programmable Transversal Filter
I/P
V
0.4
REF
h
0.45
1
V
V
REF
OUT
0.5
TAP WEIGHTS
A
h
A
2
QUAD DAC
QUAD DAC
V
V
AD7225
AD7226
OUT
REF
A
h
A
3
–10–
V
V
OUT
REF
A
h
A
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).
DIGITAL WORD MULTIPLICATION
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. This 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
from DACB, V
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:
4
V
where D
digital words in DAC latches A and B respectively.
If D
Y = –(x
where x is the digital code which is applied to all four DAC
latches.
V
V
V
IN
OUT
REF
OUT
V
V
V
V
A
OUT
OUT
OUT
OUT
A
A
B = D
= D
Figure 21. Complex Waveform Generation
A
B
C
D
4
A
+ 2x
FILTER
B
and D
= D then the result is D
A
AGND
V
V
V
V
OUT
I/P
• D
3
REF
REF
REF
REF
h
+
1
+ 3x
ACCUMULATOR
AD7225*
B
A
B
C
D
B
B, can be expressed as:
+15V
are the fractional representations of the
DGND
1
• V
V
X
DD
2
n
O/P
+ 2x + 4) • V
REF
V
V
V
V
h
OUT
OUT
OUT
OUT
2
T
V
SS
A
AD585
A
B
C
D
SHA
X
2
n–1
h
+
T
3
FILTER
O/P
IN
X
2
100k
3
25k
50k
33k
50k
n–2
• V
FILTER
REF
O/P
*DIGITAL INPUTS OMITTED
FOR CLARITY
h
REF
T
4
B then the output
A
X
4
n–3
100k
Y
n
REV. B
Y