DAC5687MPZPEP datasheet(35/75 Pages) TI1

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R/W

SDENB

SCLK

SDIO

N1 N0

A3 A2 A1 A0

D6 D5 D4 D3 D2

D0 0

Instruction Cycle

Data Transfer Cycle(s)

SDO

D7 D6 D5 D4 D3 D2 D1 D0 0

3-Pin Configuration

Output

4-Pin Configuration

Output

SDENB

SCLK

SDIO

Data n

Data n−1

SDO

td(DATA)

FIR Filters

DAC5687-EP

SGLS333 – JUNE 2006

Figure 35. Serial Interface Read Timing Diagram

Figure 36 shows the magnitude spectrum response for the identical 51-tap FIR1 and FIR3 filters. The transition

band is from 0.4 to 0.6

× F

IN (the input data rate for the FIR filter) with < 0.002-dB pass-band ripple and > 80-dB

stop-band attenuation. Figure 37 shows the region from 0.35 to 0.45

× F

IN– Up to 0.44 × FIN there is less than

0.5-dB attenuation.

Figure 38 shows the magnitude spectrum response for the 19-tap FIR2 filter. The transition band is from 0.25 to

0.75x FIN (the input data rate for the FIR filter) with < 0.002-dB pass-band ripple and > 80-dB stop-band

attenuation.

The DAC5687 also has an inverse Sinc filter (FIR4) that runs at the DAC update rate (fDAC) that can be used to

flatten the frequency response of the sample and hold output. The DAC sample and hold output set the output

current and holds it constant for one DAC clock cycle until the next sample, resulting in the well known Sin(x)/x

or Sinc(x) frequency response shown in Figure 39 (black solid line). The inverse sinc filter response (Figure 39,

blue dotted line) has the opposite frequency response between 0 to 0.4

× f

DAC, resulting in the combined

response (Figure 39, red dashed line). Between 0 to 0.4

× f

DAC, the inverse sin filter compensates the sample

and hold rolloff with less than < 0.03-dB error.

The inverse sine filter has a gain > 1 at all frequencies. Therefore, the signal input to FIR4 must be reduced from

full scale to prevent saturation in the filter. The amount of backoff required depends on the signal frequency and

is set such that at the signal frequencies the combination of the input signal and filter response is less than 1 (0

dB). For example, if the signal input to FIR4 is at 0.25 × fDAC, the response of FIR4 is 0.9 dB, and the signal will

need to be backed off from full scale by 0.9 dB. The gain function in the QMC block can be used to set reduce

amplitude of the input signal. The advantage of FIR4 having a positive gain at all frequencies is that the user is

then able to optimized backoff of the signal based on the signal frequency.

The filter taps for all digital filters are listed in Table 8.

Note that the loss of signal amplitude may result in lower SNR due to decrease in signal amplitude.

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