DAC5687IPZP datasheet(34/79 Pages) TI

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FIR Filters

f/fIN

−160

−140

−120

−100

−80

−60

−40

−20

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

G046

f/fIN

−0.6

−0.5

−0.4

−0.3

−0.2

−0.1

0.0

0.1

0.35

0.37

0.39

0.41

0.43

0.45

G047

DAC5687

SLWS164E – FEBRUARY 2005 – REVISED SEPTEMBER 2006

Figure 35 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 36 shows the region from 0.35 to 0.45

× f

IN. Up to 0.44 × fIN, there is less than

0.5 dB of attenuation.

Figure 37 shows the magnitude spectrum response for the 19-tap FIR2 filter. The transition band is from 0.25 to

0.75

× f

IN (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 sets 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 38 (red dash-dotted line). The inverse sinc filter response (Figure 38,

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

× f

DAC, resulting in the combined response

(Figure 38, green dotted line). Between 0 to 0.4

× f

DAC, the inverse sinc filter compensates the sample-and-hold

rolloff with less than 0.03-dB error.

The inverse sinc 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

× f

DAC, the response of FIR4 is 0.9 dB, and the signal must

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

optimize 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.

Figure 35. Magnitude Spectrum for FIR1 and FIR3

Figure 36. FIR1 and FIR3 Transition Band

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