AD7225LN Просмотр технического описания (PDF) - Analog Devices

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AD7225LN

LC2MOS Quad 8-Bit DAC with Separate Reference Inputs

Analog Devices 

AD7225

APPLICATIONS INFORMATION

AD7820

ADC

INPUT

DELAYED

INPUT

AM29520

TLC

SAMPLES

SAMPLES

AD7225

QUAD DAC

VOUTA

VOUTB

VOUTC

VOUTD

ACCUMULATOR

OUTPUT

FILTER

+

AD585 OUTPUT

SHA

AD584 10V

REF

VREFA VREFA VREFA VREFA

h1

h2

h3

h4

VOUTA VOUTA VOUTA VOUTA

AM7224 VOUT

VREF

DAC

VREF

AD7225

QUAD DAC

Xn

FILTER

INPUT

1

h1

Xn – 1

T

2

h2

Xn – 2

T

3

h3

Xn – 3

T

4

h4

GAIN SET

TAP WEIGHT

+

FILTER

OUTPUT

y(n)

Figure 20. Programmable Transversal Filter

PROGRAMMABLE TRANSVERSAL FILTER

A discrete time filter can be described by either multiplication

in the frequency domain or by convolution in the time domain:

N

Y (ω) = H(ω)X(ω) or yn = ∑hk Xn−k +1

k=1

The convolution sum can be implemented using the special struc-

ture known as the transversal filter (see Figure 21). It consists

of an N-stage delay line with N taps weighted by N coefficients,

the resulting products being accumulated to form the output.

The tap weights or coefficients hk are the nonzero elements of

the impulse response and therefore determine the filter transfer

function. A particular filter frequency response is realized by

setting the coefficients to the appropriate values. This property

leads to the implementation of transversal filters whose fre-

quency response is programmable.

Xn

FILTER

INPUT

1

h1

Xn – 1

Xn – 2

T

2

T

3

h2

h3

Xn – N

Xn – N + 1

N –1

T

N

hN – 1

hN

+

FILTER

OUTPUT

N

yn =

hk Xn – k + 1

k=1

Figure 21. Transversal Filter

A four-tap programmable transversal filter can be implemented

using the AD7225 (see Figure 20). The input signal is first sampled

and converted to allow the tapped delay line function to be

provided 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 products

are accumulated to yield the convolution sum output sample,

which is held by the AD585.

0

h1 = 0.117

–10

h2 = 0.417

h3 = 0.417

–20

h4 = 0.417

–30

–40

–50

–60

–70

–80

–90

–100

0

0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.45 0.50

NORMALIZED FREQUENCY (f/fS)

Figure 22. Predicted (Theoretical) Response

0

–10

–20

–30

–40

h1 (DAC A) = 00011110

–50

h2 (DAC B) = 01101011

h3 (DAC C) = 01101011

–60

h4 (DAC D) = 00011110

–70

–80

–90

–100

0

0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.45 0.50

FREQUENCY (f/fS)

Figure 23. Actual Response

Low-pass, band-pass, and high-pass filters can be synthesized

using this arrangement. The particular up weights needed for

any desired transfer function can be obtained using the standard

Remez exchange algorithm. Figure 22 shows the theoretical

low-pass frequency response produced by a four-tap transversal

filter with the coefficients indicated. Although the theoretical

prediction does not take into account the quantization of the

input samples and the truncation of the coefficients, neverthe-

Rev. C | Page 17 of 24

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