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| MAX1213 |
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 MAXIM
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13 page 
1.8V, 12-Bit, 170Msps ADC for Broadband Applications ______________________________________________________________________________________ 13 Applications Information FSR Adjustments Using the Internal Bandgap Reference The MAX1213 supports a full-scale adjustment range of 10% (±5%). To decrease the full-scale signal range, an external resistor value ranging from 13k Ω to 1MΩ may be added between REFADJ and AGND. A similar approach can be taken to increase the ADC’s full-scale range (FSR). Adding a variable resistor, potentiometer, or predetermined resistor value between REFADJ and REFIO increases the FSR of the data converter. Figure 6a shows the two possible configurations and their impact on the overall full-scale range adjustment of the MAX1213. Do not use resistor values of less than 13k Ω to avoid instability of the internal gain regulation loop for the bandgap reference. See Figure 6b for the results of the adjustment range for a selection of resis- tors used to trim the full-scale range of the MAX1213. Table 1. MAX1213 Digital Output Coding INP ANALOG INPUT VOLTAGE LEVEL INN ANALOG INPUT VOLTAGE LEVEL OUT-OF-RANGE ORP (ORN) BINARY DIGITAL OUTPUT CODE (D11P/N–D0P/N) TWO’S COMPLEMENT DIGITAL OUTPUT CODE (D11P/N–D0P/N) > VCM + VFS / 4 < VCM - VFS / 4 1 (0) 1111 1111 1111 (exceeds +FS, OR set) 0111 1111 1111 (exceeds +FS, OR set) VCM + VFS / 4 VCM - VFS / 4 0 (1) 1111 1111 1111 (+FS) 0111 1111 1111 (+FS) VCM VCM 0 (1) 1000 0000 0000 or 0111 1111 1111 (FS/2) 0000 0000 0000 or 1111 1111 1111 (FS/2) VCM - VFS / 4 VCM + VFS / 4 0 (1) 0000 0000 0000 (-FS) 1000 0000 0000 (-FS) < VCM + VFS / 4 > VCM - VFS / 4 1 (0) 00 0000 0000 (exceeds -FS, OR set) 10 0000 0000 (exceeds -FS, OR set) VON OGND OVCC VOP 2.2k Ω 2.2k Ω Figure 5. Simplified LVDS Output Architecture MAX1213 REFERENCE BUFFER ADC FULL SCALE = REFT-REFB REFT: TOP OF REFERENCE LADDER. REFB: BOTTOM OF REFERENCE LADDER. 1V AVCC AVCC/2 G CONTROL LINE TO DISABLE REFERENCE BUFFER REFERENCE SCALING AMPLIFIER REFIO REFADJ 13k Ω TO 1M Ω 0.1 μF REFT REFB 13k Ω TO 1M Ω Figure 6a: Circuit Suggestions to Adjust the ADC’s Full-Scale Range FS VOLTAGE vs. FS ADJUST RESISTOR FS ADJUST RESISTOR (k Ω) 875 750 500 625 250 375 125 1.39 1.41 1.43 1.45 1.47 1.49 1.51 1.53 1.55 1.57 1.37 0 1000 RESISTOR VALUE APPLIED BETWEEN REFADJ AND REFIO INCREASES VFS RESISTOR VALUE APPLIED BETWEEN REFADJ AND AGND DECREASES VFS Figure 6b: FS Adjustment Range vs. FS Adjustment Resistor |
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