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AD1878JD 데이터시트(PDF) 9 Page - Analog Devices |
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AD1878JD 데이터시트(HTML) 9 Page - Analog Devices |
9 / 16 page AD1878/AD1879 REV. 0 –9– between each set of input pins (12 to 13, and 17 to 16) to complete the input bypassing. This input bypassing mini- mizes the RF transmission and reception capability of the AD1878/AD1879 inputs. • For best performance, do not use a socket with the AD1878/ AD1879. If you must socket the part, use pin clips to keep the part flush with the board, thus keeping bypassing as close to the chip as possible. • The AD1878/AD1879 should be placed on a split ground plane as illustrated in Figure 5. The digital ground plane should be placed under the top end of the package and the analog ground plane should be placed under the bottom end of the package as shown in Figure 5. The split should be be- tween Pins 7 and 8 and between Pins 21 and 22. The ground planes should be tied together at one spot under- neath the center of the package. This ground plane tech- nique also minimizes RF transmission 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 LRCK BCK S0 DV DD 64/32 DGND NC AV SS 1 AV SS 2 AGND APD VINR– VINR+ REFR WCK DATA CLK S1 DGND DV DD AV SS 1 AV DD 2 AV DD 1 AGND VINL– VINL+ REFL RESET DIGITAL GROUND PLANE ANALOG GROUND PLANE Figure 5. AD1878/AD1879 Recommended Ground Plane • Each reference pin (14 and 15) should be bypassed with a resistor and a capacitor. One end of the resistor should be placed as close to the package 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 input pin traces! Cou- pling between input and reference traces will cause second harmonic distortion. The resistor is used to reduce the high frequency coupling into the references from the board. • Wherever possible, minimize the capacitive load on digital outputs of the part. This will reduce the digital spike cur- rents drawn from the digital supply pins. 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 mul- tiple AD1879 channels in parallel with a common analog input. (The same technique would work with the AD1878. However, this would be of little value since using a single AD1879 would be more effective.) This technique makes use of the fact that the noise in independent modulator channels is uncorrelated. Thus every doubling of the number of AD1879 channels used will im- prove system dynamic range by 3 dB. The digital outputs from the corresponding decimator channels have to be arithmetically averaged to obtain the improved results in the correct data for- mat. A digital processor, either general-purpose or DSP, can easily perform the averaging operation. Shown below in Figure 6 is a circuit for obtaining a 3 dB im- provement in dynamic range by using both channels of a single AD1879 with a mono input. The minus (–) output from the in- put buffer is sent to both right and left minus AD1879 inputs; the plus (+) output from the input buffer is sent to both right and left plus AD1879 inputs. A stereo implementation would require using two AD1879s and using the full recommended in- put structure shown above in Figure 2. Note that a single digital processor would likely be able to handle the averaging require- ments for both left and right channels. PIN 1 0.580 (14.73) 0.485 (12.32) 1 14 15 2 8 0.625 (15.87) 0.600 (15.24) 0.015 (0.381) 0.008 (0.204) 0.195 (4.95) 0.125 (3.18) 0.250 (6.35) MAX 0.022 (0.558) 0.014 (0.356) 0.100 (2.54) BSC 0.200 (5.05) 0.125 (3.18) 0.070 (1.77) MAX 0.060 (1.52) 0.015 (0.38) 0.150 (3.81) MIN SEATING PLANE 1.565 (39.70) 1.380 (35.10) Figure 6. Increasing Dynamic Range by Using Two AD1879 Channels DIGITAL INTERFACE Modes of Operation The AD1878/AD1879’s flexible serial output port produces data in twos-complement, MSB-first format. Output signals are to TTL/CMOS logic levels. The port is configured by pin selec- tions. The AD1879 can operate in either master or slave modes. Each 16-/18-bit output word of a stereo pair can be formatted within a 32-bit field as right-justified, as I 2S-compatible, or at user-selected positions. The two 32-bit fields constitute a 64-bit frame (64-bit mode). The output can also be truncated to 16 bits and formatted in a 16-bit field with two 16-bit fields in a 32-bit frame (32-bit mode). The various mode options are pin-programmed with the S0 Mode Select Pin (3), the S1 Mode Select Pin (25), and the 64/32 Bit Rate Select Pin (4). The function of these pins is summarized: Serial Port Operation Mode 64/32 S0 S1 64-Bit Master Mode—Word Clock Output 1 0 0 64-Bit Master Mode—Word Clock Input 1 1 0 64-Bit Slave Mode 1 1 1 Reserved 1 0 1 32-Bit Master Mode—Word Clock Out HI 0 0 0 32-Bit Master Mode—Word Clock Ignored 0 1 0 32-Bit Slave Mode 0 1 1 Reserved 0 0 1 Serial Port Data Timing Sequences In the “master modes,” the bit clock (BCK) and left/right clock (LRCK) are always outputs, generated internally in the AD1878/ AD1879 from the master clock (CLOCK) input. The word clock (WCK) may either be an internally generated output or a user-supplied input, depending on the pin-programmed mode selected. |
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