Nevion DAC-AES - Manual (Page 6)

DAC-AES Rev. 7
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3 Technical description
The DAC-AES card was designed as a reference audio converter with a highly audio
optimized signal path. Careful design of the output circuitry was needed to maintain a high
dynamic range whilst minimizing distortion. Jitter of the sampling clock is kept to a minimum
by the use of a digital receiver with a large degree of hysteresis. The clock recovery scheme
allows the sample clock to vary with a few PPM between an upper and lower limit.
The two D/A converters are totally independent and may be used with completely different
input signals.
The AES receiver circuit signals the most important information about the received signal to
the microcontroller, which sets up the appropriate parameters in the D/A converter. Non-
audio and pre-emphasis are interpreted and used by the converter.
The output of the D/A converter feeds a differential second-order filter. This solution
suppresses high frequency noise and the turn- on and off transients. The choice of op-amp is
quite important in order to achieve a low output noise. The signal is then AC coupled to a
non-inverting variable gain stage with gains ranging from 0 dB to 18 dB. The gain variation is
performed by switching extra resistors in parallel to ground from the inverting input. The
output is inverted with an inverting stage and the output stage is fed differentially.
The output circuit is a electronic differential amplifier with output level compensation. If either
of the output phases is connected to 0V, the level of the other phase will increase to maintain
the output differential signal level. Each converter has dual independent analogue outputs.
The card has a microcontroller that sets up the parameters of the D/A converters and
controls the LEDs on the front of the card. It also monitors the

12V voltage level generated
on the card and the 5V power voltage and reports to the Multicon Gyda controller when
present.