PCM1725U Datasheet, PDF (8/14 Pages) Texas Instruments – Stereo Audio DIGITAL-TO-ANALOG CONVERTER

Not Recommended For New Designs
INTERNAL ANALOG FILTER FREQUENCY RESPONSE
(20Hz~24kHz, Expanded Scale)
1.0
0.5
0
–0.5
–1.0
20
100
1k
Frequency (Hz)
10k 24k
FIGURE 7. Low Pass Filter Frequency Response.
INTERNAL ANALOG FILTER FREQUENCY RESPONSE
(10Hz~10MHz)
10
5
0
–5
–10
–15
–20
–25
–30
–35
–40
–45
–50
–55
–60
10
100
1k
10k 100k
1M
10M
Frequency (Hz)
FIGURE 8. Low Pass Filter Wideband Frequency Response.
APPLICATION
CONSIDERATIONS
DELAY TIME
There is a finite delay time in delta-sigma converters. In A/D
converters, this is commonly referred to as latency. For a
delta-sigma D/A converter, delay time is determined by the
order number of the FIR filter stage, and the chosen sampling
rate. The following equation expresses the delay time of
PCM1725:
T_D = 11.125 x 1/f_S
For f_S = 44.1kHz, T_D = 11.125/44.1kHz = 251.4µs
Applications using data from a disc or tape source, such as
CD audio, CD-Interactive, Video CD, DAT, Minidisc,
etc., generally are not affected by delay time. For some
professional applications such as broadcast audio for stu-
dios, it is important for total delay time to be less than 2ms.
OUTPUT FILTERING
For testing purposes all dynamic tests are done on the
PCM1725 using a 20kHz low pass filter. This filter limits
the measured bandwidth for THD+N, etc. to 20kHz. Failure
to use such a filter will result in higher THD+N and lower
SNR and Dynamic Range readings than are found in the
specifications. The low pass filter removes out of band
noise. Although it is not audible, it may affect dynamic
specification numbers.
The performance of the internal low pass filter from DC to
24kHz is shown in Figure 7. The higher frequency rolloff of
the filter is shown in Figure 8. If the user’s application has
the PCM1725 driving a wideband amplifier, it is recom-
mended to use an external low pass filter. A simple 3rd-
order filter is shown in Figure 9. For some applications, a
passive RC filter or 2nd-order filter may be adequate.
BYPASSING POWER SUPPLIES
The power supplies should be bypassed as close as possible
to the unit. It is also recommended to include a 0.1µF ceramic
capacitor in parallel with the 10µF tantalum bypass capacitor.
V_SIN
+ _10kΩ
–
10kΩ
680pF
1500pF
OPA134
10kΩ
100pF
FIGURE 9. 3rd-Order LPF.
®
PCM1725
6
–14
–34
–54
–74
–94
100
GAIN vs FREQUENCY
Gain
Phase
1k
10k
100k
Frequency (Hz)
90
0
–90
–180
–270
–360
1M
8