AD7225(RevB) Просмотр технического описания (PDF) - Analog Devices
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AD7225 Datasheet PDF : 12 Pages
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AD7225
FILTER
I/P
I/P
AD7820
ADC
DELAYED
I/P
Am29520
TLD
SAMPLES
SAMPLES
AD7225
QUAD DAC
VOUT A
VOUT B
VOUT C
VOUT D
VREF A VREF A VREF A VREF A
ACCUMULATOR
O/P
AD585
+
SHA
FILTER
O/P
h1
h2
h3
h4
VOUT A VOUT A VOUT A VOUT A
AD584
REF
+10V
Am7224
DAC
VREF
VOUT
VREF
AD7226
QUAD DAC
FILTER
I/P
h1
Xn
Xn–1
Xn–2
Xn–3
T
T
T
1
2
3
4
h2
h3
h4
+
GAIN SET
TAP WEIGHTS
FILTER
O/P
Yn
Figure 18. Programmable Transversal Filter
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.
0
–10
–20
–30
–40
h1 = 0.117
–50
h2 = 0.417
h3 = 0.417
–60
h4 = 0.417
–70
–80
–90
–100
0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 0.5
NORMALIZED FREQUENCY – f/fs
Figure 19. Predicted (Theoretical) Response
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 VREF B then the output
from DACB, VOUT B, can be expressed as:
VOUT B = DA • DB • VREF A
where DA and DB are the fractional representations of the
digital words in DAC latches A and B respectively.
If DA = DB = D then the result is D2 • VREF 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 = –(x4 + 2x3 + 3x2 + 2x + 4) • VIN
where x is the digital code which is applied to all four DAC
latches.
VIN
Figure 20. Actual Response
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
–10–
+15V
VDD
VREF A
VOUT A
AD7225*
VREF B
VOUT B
VREF C
VOUT C
VREF D
VOUT D
AGND DGND VSS
25kΩ
50kΩ
33kΩ
50kΩ
100kΩ
100kΩ
Y
*DIGITAL INPUTS OMITTED
FOR CLARITY
Figure 21. Complex Waveform Generation
REV. B
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