"Chapter 1 - The Op Amp's Place in the World" - HTL Wien 10
"Chapter 1 - The Op Amp's Place in the World" - HTL Wien 10
"Chapter 1 - The Op Amp's Place in the World" - HTL Wien 10
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13.6 Communication D/A Converter Reconstruction Filter<br />
Communication D/A Converter Reconstruction Filter<br />
Modern communication DACs are, effectively, an array of matched current sources optimized<br />
for frequency doma<strong>in</strong> performance. <strong>The</strong> most important dynamic specifications are<br />
SFDR, SNR, THD, IMD, ACPR and settl<strong>in</strong>g time. <strong>The</strong> dc parameters INL and DNL are<br />
considered important because of <strong>the</strong>ir <strong>in</strong>fluence on <strong>the</strong> SFDR parameter. Typical SFDR<br />
figures for 12-bit to 14-bit DACs, with a 5-MHz s<strong>in</strong>gle-tone <strong>in</strong>put at 50 MSPS, ranges from<br />
75 dB to 80 dB. In order to prevent adjacent communication channels from <strong>in</strong>terfer<strong>in</strong>g with<br />
each o<strong>the</strong>r, <strong>the</strong> DAC must exhibit a good SFDR specification.<br />
Communication DACs normally have differential outputs and current-mode architecture<br />
is used to give <strong>the</strong> DAC a higher update rate.<br />
Figure 13–4 shows an <strong>in</strong>terpolat<strong>in</strong>g filter block before <strong>the</strong> DAC and <strong>the</strong> reconstruction<br />
analog filter at <strong>the</strong> output of <strong>the</strong> DAC. <strong>The</strong> <strong>in</strong>terpolat<strong>in</strong>g filter is a digital filter whose system<br />
clock frequency is an <strong>in</strong>teger multiple of <strong>the</strong> filter <strong>in</strong>put data stream and is employed to<br />
reduce <strong>the</strong> DAC’s <strong>in</strong>-band aliased images. This eases <strong>the</strong> job of <strong>the</strong> reconstruction filter<br />
at <strong>the</strong> output of <strong>the</strong> DAC. <strong>The</strong> filter is used to smooth <strong>the</strong> <strong>in</strong>put data — <strong>the</strong> output waveform<br />
frequency is <strong>the</strong> same as <strong>the</strong> <strong>in</strong>put to <strong>the</strong> <strong>in</strong>terpolat<strong>in</strong>g filter. Figure 13–5 shows <strong>the</strong> <strong>in</strong>terpolation<br />
filter output. <strong>The</strong> system clock frequency of <strong>the</strong> DAC and <strong>the</strong> <strong>in</strong>terpolat<strong>in</strong>g filter<br />
are runn<strong>in</strong>g at <strong>the</strong> same rate; <strong>the</strong>refore, <strong>the</strong> frequency spectrum of <strong>the</strong> DAC output signal,<br />
repeated at <strong>in</strong>teger multiples of <strong>the</strong> sampl<strong>in</strong>g rate, becomes <strong>in</strong>creas<strong>in</strong>gly separated as <strong>the</strong><br />
sampl<strong>in</strong>g rate is <strong>in</strong>creased. <strong>The</strong> fur<strong>the</strong>r apart <strong>the</strong> repeated DAC output spectrum, <strong>the</strong>n <strong>the</strong><br />
less steep <strong>the</strong> attenuation characteristic of <strong>the</strong> anti-alias<strong>in</strong>g needs to be. Consequently,<br />
a simpler anti-alias<strong>in</strong>g filter with a less steep rolloff from pass band to stop band can be<br />
used without any <strong>in</strong>crease <strong>in</strong> distortion due to alias<strong>in</strong>g. In Figure 13–4, <strong>the</strong> system clock<br />
is 30.772 MHz (3.84 MSPS x 8). Figure 13–6 shows <strong>the</strong> attenuation characteristics needed<br />
for <strong>the</strong> reconstruction anti-alias<strong>in</strong>g filter.<br />
<strong>The</strong> alias<strong>in</strong>g frequency is ƒ alias = (28.7 – 2) = 26.7 MHz, and <strong>the</strong> required attenuation is<br />
84 dB (14-bit DAC). A third-order anti-alias<strong>in</strong>g low-pass elliptic or Bessel filter could be<br />
used to meet <strong>the</strong> attenuation requirements. Ei<strong>the</strong>r type of high-order filter gives a relatively<br />
flat response up to just below <strong>the</strong> sampl<strong>in</strong>g frequency, followed by a sharp cutoff. But<br />
this arrangement provides no correction for <strong>the</strong> s<strong>in</strong>c function (S<strong>in</strong>x)/x falloff <strong>in</strong> amplitude<br />
naturally produced by <strong>the</strong> sample and hold function <strong>in</strong> <strong>the</strong> DAC.<br />
<strong>The</strong> tradeoff <strong>in</strong> build<strong>in</strong>g a simpler reconstruction filter is that a faster DAC is required to<br />
convert <strong>the</strong> <strong>in</strong>put digital data stream to analog signal.<br />
Wireless Communication: Signal Condition<strong>in</strong>g for IF Sampl<strong>in</strong>g<br />
13-13