DAC38RF82 datasheet(53/140 Pages) TI1

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0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0.45

0.5

–4

–3

–2

–1

f/f

DAC

G056

sin(x)/x

Corrected

FIR4

DAC38RF82, DAC38RF89

www.ti.com

SLASEA6B – FEBRUARY 2017 – REVISED AUGUST 2017

Product Folder Links: DAC38RF82 DAC38RF89

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Table 42. Register Field Addresses for JESD204B Mode, Interpolation and Clock Phase

Programming (continued)

Bit(s)

Hyperlink

JESD_N_HD_SCR

0x4E

8.5.49

N_M1

4-0

N_M1’ (NPRIME_M1)

12-8

JESD_PHASE_MODE

JESD_LN_EN

0x4A

1-0

8.5.45

All registers are paged!

8.3.18.4 Inverse Sinc Filter

The DAC38RF82 (or DAC38RF89) has a 9-tap inverse Sinc filter (INVSINC) 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 (Figure 42, red line). The inverse sinc filter response

(Figure 42, blue line) has the opposite frequency response from 0 to 0.4 x fDAC, resulting in the combined

response (Figure 42, green line). Between 0 to 0.4 x fDAC, the inverse sinc filter compensates the sample-and-

hold roll-off with less than 0.03 dB error.

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

from full scale to prevent saturation in the filter. The amount of back-off required depends on the signal

frequency, andis 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 INVSINC is at 0.25 x fDAC, the response of INVSINC is 0.9

dB, and the signal must be backed off from full scale by 0.9 dB to avoid saturation. The advantage of INVSINC

having a positive gain at all frequencies is that the user is then able to optimize the back-off of the signal based

on its frequency.

The inverse Sinc filters are enabled by field ISFIR_ENA in register MULTIDUC_CFG1 (8.5.13).

Figure 42. Composite Magnitude Spectrum for INVSINC

8.3.19 PA Protection Block

The DAC38RFxx incorporates an optional power amplifier protection (PAP) block to monitor when the input

signal is two large, for example when an interface error occurs, and reduces the output signal power of the DAC.

The PAP block achieves the functionality of reducing the input signal that crosses the threshold through three

main sub-blocks. These are PAP trigger generation block, PAP gain state machine and GAIN block.

DAC38RF82 Datasheet(PDF) 53 Page - Texas Instruments

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