SLES102B December 2003 – March 2015 PCM1798
Information in the following applications sections is not part of the TI component specification, and TI does not warrant its accuracy or completeness. TI’s customers are responsible for determining suitability of components for their purposes. Customers should validate and test their design implementation to confirm system functionality.
The PCM1798 device is a hardware-controlled, differential current output DAC that can accept multiple formats of 16- or 24-bit PCM audio data. Because the PCM1798 is a current output part, in most cases a current to voltage stage is required before the signal is passed to the amplifier stage. A microcontroller or DSP can use GPIO to manipulate the control pins CHSL, DEM, FMT0, FMT1, MONO, and MUTE. The PCM1798 requires a 5-V analog supply, as well as a 3.3-V digital supply.
For some applications, it may be desirable to use a programmable digital signal processor as an external digital filter to perform the interpolation function. The following pin settings enable the external digital filter application mode.
The pins used to provide the serial interface for the external digital filter are shown in the connection diagram of Figure 26. The word clock (WDCK) must be operated at 8× or 4× the desired sampling frequency, fS.
Pin assignment when using the external digital filter interface:
The PCM1798 in the external digital filter interface mode supports the 24-bit right-justified audio format as shown in Figure 27.
|800000 (–FS)||000000 (BPZ)||7FFFFF (+FS)|
|Pass-Band Ripple, Sharp Rolloff|
|Transition Characteristics, Slow Rolloff|
The design of the application circuit is very important in order to actually realize the high S/N ratio of which the PCM1798 is capable. This is because noise and distortion that are generated in an application circuit are not negligible.
In the third-order LPF circuit of Figure 32, the output level is 2.1 V RMS, and 123 dB S/N is achieved.
The current of the PCM1798 on each of the output pins (IOUTL+, IOUTL–, IOUTR+, IOUTR–) is 4 mA p-p at 0 dB (full scale). The voltage output level of the I/V converter (Vi) is given by following equation:
TI recommends an NE5534 operational amplifier 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.
The PCM1798 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 low-noise type.