Datasheet

LOW-PASS FILTER AND DIFFERENTIAL-TO-SINGLE-ENDED CONVERTER FOR DAC OUTPUTS
10 Fm
47W
R
1
R
3
R
2
C
2
AnalogOutput
(2V )
RMS
VOUTx+
(4V )
PP
+
10 Fm
R
1
R
3
VOUTx-
(4V )
PP
+
C
2
R
2
C
1
47W
R
1
R
3
R
2
C
2
AnalogOutput
(2V )
RMS
VOUTx+
(4V )
PP
R
1
R
3
VOUTx-
(4V )
PP
C
2
R
2
C
1
PCM1789
www.ti.com .............................................................................................................................................. SBAS451A – OCTOBER 2008 – REVISED JANUARY 2009
Δ Σ DACs use noise-shaping techniques to improve in-band signal-to-noise ratio (SNR) performance at the
expense of generating increased out-of-band noise above the Nyquist frequency, or f
S
/2. The out-of-band noise
must be low-pass filtered in order to provide optimal converter performance. This filtering is accomplished by a
combination of on-chip and external low-pass filters.
Figure 38 and Figure 39 show the recommended external differential-to-single-ended converter with low-pass
active filter circuits for ac-coupled and dc-coupled applications. These circuits are second-order Butterworth
filters using a multiple feedback (MFB) circuit arrangement that reduces sensitivity to passive component
variations over frequency and temperature. For more information regarding MFB active filter designs, please
refer to Applications Bulletin SBAA055 , Dynamic Performance Testing of Digital Audio D/A Converters, available
from the TI web site (www.ti.com ) or your local Texas Instruments' sales office.
Because the overall system performance is defined by the quality of the DACs and the associated analog output
circuitry, high-quality audio op amps are recommended for the active filters. Texas Instruments ’ OPA2134 ,
OPA2353 , and NE5532A dual op amps are shown in Figure 38 and Figure 39 , and are recommended for use
with the PCM1789.
NOTE: Amplifier is an NE5532A x 1/2 or OPA2134 x1/2; R
1
= 7.5 k Ω ; R
2
= 5.6 k Ω ; R
3
= 360 Ω ; C
1
= 3300 pF; C
2
= 680 pF; Gain = 0.747;
f
– 3 dB
= 53 kHz.
Figure 38. AC-Coupled, Post-LPF and Differential to Single-Ended Buffer
NOTE: Amplifier is an NE5532A x 1/2 or OPA2134 x1/2; R
1
= 15 k Ω ; R
2
= 11 k Ω ; R
3
= 820 Ω ; C
1
= 1500 pF; C
2
= 330 pF; Gain = 0.733;
f
– 3 dB
= 54 kHz.
Figure 39. DC-Coupled, Post-LPF and Differential to Single-Ended Buffer
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Product Folder Link(s): PCM1789