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LT6402-12 Datasheet(PDF) 13 Page - Linear Technology |
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LT6402-12 Datasheet(HTML) 13 Page - Linear Technology |
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13 / 16 page  13 640212fa LT6402-12 can also be used, but since it is being driven differentially it will appear at each filtered output as a single-ended capacitance of twice the value. To halve the filter band- width, for example, two 42pF capacitors could be added (one from each filtered output to ground). Alternatively one 21pF capacitor could be added between the filtered outputs, again halving the filter bandwidth. Combinations of capacitors could be used as well; a three capacitor solution of 14pF from each filtered output to ground plus a 14pF capacitor between the filtered outputs would also halve the filter bandwidth (Figure 7). Bandpass filtering is also easily implemented with just a few external components. An additional 560pF and 62nH, each added differentially between +OUTFILTERED and –OUTFILTERED creates a bandpass filter with a 26MHz center frequency, –3dB points of 23MHz and 30MHz, and 1.6dB of insertion loss (Figure 8). Output Common Mode Adjustment The LT6402-12’s output common mode voltage is set by the VOCM pin. It is a high-impedance input, capable of setting the output common mode voltage anywhere in a range from 1.1V to 3.6V. Bandwidth of the VOCM pin is typically 200MHz, so for applications where the VOCM pin is tied to a DC bias voltage, a 0.1μF capacitor at this pin is recom- mended. For best distortion performance, the voltage at the VOCM pin should be between 1.2V and 2.6V. When interfacing with most ADCs, there is generally a VOCM output pin that is at about half of the supply voltage of the ADC. For 5V ADCs such as the LTC17XX family, this VOCM output pin should be connected directly (with the addition of a 0.1μF capacitor) to the input VOCM pin of the LT6402-12. For 3V ADCs such as the LTC22XX families, the LT6402-12 will function properly using the 1.65V from the ADC’s VCM reference pin, but improved Spurious Free Dynamic Range (SFDR) and distortion performance can be achieved by level-shifting the LTC22XX’s VCM reference voltage up to at least 1.8V. This can be accomplished as shown in Figure 9 by using a resistor divider between the LTC22XX’s VCM output pin and VCC and then bypassing the LT6402-12’s VOCM pin with a 0.1μF capacitor. For a common mode voltage above 1.9V, AC coupling capacitors are recommended between the LT6402-12 and LTC22XX ADCs because of the input voltage range constraints of the ADC. Figure 7. LT6402-12 Internal Filter Topology Modified for 1/2x Filter Bandwidth (3 External Capacitors) Figure 8. LT6402-12 Output Filter Modified for Bandpass Filtering (1 External Inductor, 1 External Capacitor) Figure 9. Level Shifting 3V ADC VCM Voltage for Improved SFDR APPLICATIONS INFORMATION VEE VEE 640212 F07 50Ω 14pF LT6402-12 –OUT +OUT 50Ω 14pF 14pF 14pF 14pF 14pF –OUTFILTERED +OUTFILTERED 8 7 6 5 FILTERED OUTPUT (37.5MHz) VEE VEE 640212 F08 50Ω 14pF LT6402-12 –OUT +OUT 50Ω 14pF 14pF –OUTFILTERED +OUTFILTERED 8 7 6 5 FILTERED OUTPUT 640212 F09 IF IN LT6402-12 –INA –INB VOCM 2 31 6 7 1 2 +INB +INA 14 13 15 150Ω 16 10Ω 10Ω LTC22xx 0.1μF 0.1μF +OUTFILTERED –OUTFILTERED AIN+ AIN– 4.02k 11k 1.9V 1.5V 3V VCM |
Similar Part No. - LT6402-12_15 |
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Similar Description - LT6402-12_15 |
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