Datasheet

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SLES117A – AUGUST 2004 – REVISED NOVEMBER 2006
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19
APPLICATION INFORMATION
APPLICATION CIRCUIT
The design of the application circuit is very important in order to actually realize the high S/N ratio of which the
PCM1794A is capable. This is because noise and distortion that are generated in an application circuit are not
negligible.
In the circuit of Figure 24, the output level is 2 V RMS, and 127 dB S/N is achieved. The circuit of Figure 25 can realize
the highest performance. In this case the output level is set to 4.5 V RMS and 129 dB S/N is achieved (stereo mode).
In monaural mode, if the output of the L-channel and R-channel is used as a balanced output, 132 dB S/N is achieved
(see Figure 26).
I/V Section
The current of the PCM1794A on each of the output pins (I
OUT
L+, I
OUT
L–, I
OUT
R+, I
OUT
R–) is 7.8 mA p-p at 0 dB
(full scale). The voltage output level of the I/V converter (Vi) is given by following equation:
Vi = 7.8 mA p–p × R
f
(R
f
: feedback resistance of I/V converter)
An NE5534 operational amplifier is recommended for the I/V circuit to obtain the specified performance. Dynamic
performance such as the gain bandwidth, settling time, and slew rate of the operational amplifier affects the audio
dynamic performance of the I/V section.
Differential Section
The PCM1794A voltage outputs are followed by differential amplifier stages, which sum the differential signals for
each channel, creating a single-ended I/V op-amp output. In addition, the differential amplifiers provide a low-pass
filter function.
The operational amplifier recommended for the differential circuit is the Linear Technology LT1028, because its input
noise is low.