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LTC6990HS6-PBF Datasheet(PDF) 18 Page - Linear Technology |
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LTC6990HS6-PBF Datasheet(HTML) 18 Page - Linear Technology |
18 / 28 page LTC6990 18 6990p APPLICATIONS INFORMATION Depending on the application’s requirements, the non- inverting VCO circuit in Figure 13 may be preferred for this wide of a frequency variation as its maximum inac- curacy due to VSET error is only ±9% and can be reduced to only ±3% with a small change to the voltage tuning range specification. Reducing VSET Error Effects in VCO Applications Figure 13 shows a VCO that reduces the effect of ΔVSET by adding an op-amp to make VCTRL dependent on VSET. This circuit also has a positive transfer function (the out- put frequency increases as VIN increases). Furthermore, for positive VIN voltages, this circuit places the greatest absolute frequency error at the highest output frequency. Compared to the simple VCO circuit of Figure 11, the absolute frequency error is unchanged. However, with the maximum absolute frequency error (in Hertz) now occurring at the highest output frequency, the relative frequency error (in percent) is greatly improved. Additionally, by choosing the VCO’s specifications shrewdly, the frequency error (in percent) due to VSET variation is reduced to ΔVSET/VSET = ±3%. To realize this improve- ment, the design must abide by three conditions. First, the VIN voltage must be positive throughout the range. Second, choose VMAX/VMIN ≥ fMAX/fMIN. Last, choose RVCO/RSET ≥ R4/R3. Figure 13 shows a design similar to the previous design example where the VMIN voltage is now specified to be 0.4V. This satisfies the VMAX/VMIN ≥ fMAX/fMIN condition and the design assures that the output frequency error due to VSET variation is only ±3%. Figure 13. VCO with Reduced ΔVSET Sensitivity 6990 F13 LTC6990 OE GND SET OUT V+ DIV C1 0.1μF R1 1M R2 280k DIVCODE = 3 (NDIV = 8, Hi-Z = 0) RSET 249k 3V RVCO 75k – + 3V 3V 10kHz TO 100kHz fOUT VCTRL VSET 1/2 LTC6078 R4 30.1k C4 33pF R3 100k 0.4V TO 4V VIN fOUT 1MHz • 50k NDIV •RVCO • RVCO RSET VIN VSET 1 • R4 R3 IF R4 R3 RVCO RSET , THE EQUATION REDUCES TO: fOUT = 1MHz • 50k NDIV •RSET • VIN VSET VIN •25kHz/V |
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