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AD1877 Folha de dados(PDF) 9 Page - Analog Devices |
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AD1877 Folha de dados(HTML) 9 Page - Analog Devices |
9 / 20 page AD1877 REV. A –9– The ground planes should be tied together at one spot under- neath the center of the package with an approximately 3 mm trace. This ground plane technique also minimizes RF transmis- sion and reception. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 28 27 26 25 24 23 22 21 20 19 18 17 16 15 L RCK WCLK BCLK DGND1 DVDD1 RDEDGE S/ M 384/ 256 AVDD VINL CAPL1 CAPL2 AGNDL VREFL CLKIN TAG SOUT DVDD2 RESET MSBDLY R LJUST AGND VINR CAPR1 CAPR2 AGNDR VREFR DGND2 DIGITAL GROUND PLANE ANALOG GROUND PLANE Figure 4. Recommended Ground Plane Each reference pin (14 and 15) should be bypassed with a 0.1 µF ceramic chip capacitor in parallel with a 4.7 µF tantalum capaci- tor. The 0.1 µF chip cap should be placed as close to the pack- age pin as possible, and the trace to it from the reference pin should be as short and as wide as possible. Keep this trace away from any analog traces (Pins 10, 11, 12, 17, 18, 19)! Coupling between input and reference traces will cause even order har- monic distortion. If the reference is needed somewhere else on the printed circuit board, it should be shielded from any signal dependent traces to prevent distortion. Wherever possible, minimize the capacitive load on the digital outputs of the part. This will reduce the digital spike currents drawn from the digital supply pins and help keep the IC sub- strate quiet. How to Extend SNR A cost-effective method of improving the dynamic range and SNR of an analog-to-digital conversion system is to use multiple AD1877 channels in parallel with a common analog input. This technique makes use of the fact that the noise in independent modulator channels is uncorrelated. Thus every doubling of the number of AD1877 channels used will improve system dynamic range by 3 dB. The digital outputs from the corresponding deci- mator channels have to be arithmetically averaged to obtain the improved results in the correct data format. A microprocessor, either general-purpose or DSP, can easily perform the averaging operation. Shown below in Figure 5 is a circuit for obtaining a 3 dB improvement in dynamic range by using both channels of a single AD1877 with a mono input. A stereo implementation would require using two AD1877s and using the recommended input structure shown in Figure 2. Note that a single microproces- sor would likely be able to handle the averaging requirements for both left and right channels. AD1877 RECOMMENDED INPUT BUFFER SINGLE CHANNEL INPUT DIGITAL AVERAGER AD1877 VINR VINL SINGLE CHANNEL OUTPUT Figure 5. Increasing Dynamic Range By Using Two AD1877 Channels DIGITAL INTERFACE Modes of Operation The AD1877’s flexible serial output port produces data in two’s-complement, MSB-first format. The input and output sig- nals are TTL logic level compatible. Time multiplexed serial data is output on SOUT (Pin 26), left channel then right chan- nel, as determined by the left/right clock signal LRCK (Pin 1). Note that there is no method for forcing the right channel to precede the left channel. The port is configured by pin selec- tions. The AD1877 can operate in either master or slave mode, with the data in right-justified, I 2S-compatible, Word Clock controlled or left-justified positions. The various mode options are pin-programmed with the Slave/ Master Pin (7), the Right/Left Justify Pin (21), and the MSB Delay Pin (22). The function of these pins is summarized as follows: |
Nº de peça semelhante - AD1877_15 |
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Descrição semelhante - AD1877_15 |
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