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PCM1750P Datasheet(PDF) 10 Page - Texas Instruments |
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PCM1750P Datasheet(HTML) 10 Page - Texas Instruments |
10 / 21 page ® PCM1750 10 A Gain Change Rotates Transfer Function Offset Change Shifts Transfer Function 1FFFF H 1FFFE H 00001 00000 H 3FFFF H 20001 H 20000 H * Gain drift (mostly due to reference drift) rotates the transfer function around the bipolar zero code (00000 ). NOTE: As the power supply voltages change (mostly due to the +V supply), the transfer function rotates around BPZ. See the power supply rejection specification in the spec table. 0.00V +2.749979 –20.98µV –2.75 HEX H bit-1. The switches of the CDAC and the switches of the TDAC operate concurrently with each other, that is, when a decision is made to keep or reject bit-1, the same decision is made for the correction voltage for bit-1. Even though the ratio stability of the nichrome resistors used in the TDAC may not be as good as the poly capacitors, it is inconsequen- tial because the correction voltage of each bit has a limited range of adjustment. The DLE at the major carry (a code change from 111...111 to 000...000; in binary two’s complement coding) is typi- cally ±1/2 LSB at the 16-bit level, which is sufficient to provide 90dB SNR and –30dB low level distortion (–60dB input). For applications requiring less DLE at the major carry, a pin is provided for each channel to make an external MSB adjustment. DISCUSSION OF SPECIFICATIONS RESOLUTION AND DYNAMIC RANGE The theoretical resolution of the PCM1750 is 18-bits. The maximum possible number of output codes or counts at 18- bits is 262,144 or 108dB (calculated by raising 2 to the 18th power). The relative accuracy of any A/D converter, how- ever, is more a function of it’s absolute linearity and signal- to-noise ratio than how many bits of resolution it has. These more pertinent specifications are described later in this section. Dynamic range, as it is usually defined for digital audio converters, is the measure of THD+N at an effective input signal level of –60dB referred to 0dB. For the PCM1750 this value is typically 90dB and a minimum of 88dB (for audio bandwidth = 20Hz to 24kHz, THD+N at –60db = –30 db typ, –28dB max; fIN = 1kHz and fS = 192kHz). Resolution is also commonly used as a theoretical measure of dynamic range, but it does not take into account the effects of distortion and noise at low signal levels. ANALOG INPUT RANGE The analog input range for the PCM1750 is a bipolar ±2.75V (nominal). Table I gives the precise input/output and volt- age/code relationships for the PCM1750. Figure 5 shows these same relationships in a graphical format. It should be noted that the computed voltage input levels represent center values (the midpoint between code transitions). Output cod- ing is in binary two’s complement. DIGITAL OUTPUT ANALOG INPUT VOLTAGE INPUT 262144 LSBs Full Scale Range 5.50000000V 1 LSB Minimum Step Size 20.98083496 µV 1FFFFHEX +Full Scale +2.74997902V 00000HEX Bipolar Zero 0.00000000V 3FFFFHEX Bipolar Zero –1LSB –0.00002098V 20000HEX –Full Scale –2.75000000V TABLE I. Analog Input to Digital Output Relationships. From Figure 5, the effects of offset and gain errors can be visualized. These errors can change value in response to changes in temperature and/or supply voltage. In addition, gain error (or the full scale range, FSR) changes in direct proportion to the VREFIN voltage value. SAMPLE AND HOLD PARAMETERS Aperture Delay and Uncertainty Aperture delay is the time required to switch from the SAMPLE to HOLD mode. This time is typically 10ns for the PCM1750 and it is constant. Aperture uncertainty (jitter) is the amount of uncertainty associated with the aperture delay. Aperture uncertainty affects the overall accuracy of the converter and is greatest at the maximum input frequency of the converter. The formula for determining the maximum input frequency (fMAX) for a given error contribution due to aperture uncertainty is: fMAX=(2 X π X tjitter X 2N)–1 where tjitter is the RMS aperture uncertainty and 2N is the desired SNR (signal-to-noise ratio) expressed in total number of quantization levels. A 15-bit SNR, therefore, would be expressed as 215 or 32768. Using the typical PCM1750 aperture jitter of 50psrms and an SNR at the 15-bit level, fMAX = (2 X π X 50ps X 32768)–1 or 97.1kHz. This matches very closely with the rated dynamic accuracy of the PCM1750 where THD+N = –88dB max. This means the typical aper- ture jitter of PCM1750 only becomes a factor when input signals to it exceed 97kHz and/or an SNR greater than 15 bits is desired. Input Bandwidth The full power bandwidth of the PCM1750 is that input frequency above which significant distortion is observed (THD+N > 10-bits or –60dB for a full scale input signal). In the data sheet, this number is specified as typically being 500kHz. In wideband operation (when no digital filter is used) the additional full power bandwidth of the PCM1750 FIGURE 5. Analog Input to Digital Output Diagram. |
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