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LT1819 Datasheet(PDF) 11 Page - Linear Technology |
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LT1819 Datasheet(HTML) 11 Page - Linear Technology |
11 / 16 page LTC6401-20 11 640120f two outputs have the same gain and thus symmetrical swing. In general, the single-ended input impedance and termination resistor RT are determined by the combination of RS, RG and RF. For example, when RS is 50Ω, it is found that the single-ended input impedance is 200Ω and RT is 66.5Ω in order to match to a 50Ω source impedance. The LTC6401-20 is unconditionally stable. However, the overall differential gain is affected by both source imped- ance and load impedance as shown in Figure 4: A V VR R R V OUT IN S L L == ++ 2000 200 25 • The noise performance of the LTC6401-20 also depends upon the source impedance and termination. For example, an input 1:4 balun transformer in Figure 2 improves SNR by adding 6dB of gain at the inputs. A trade-off between gain and noise is obvious when constant noise figure circle and constant gain circle are plotted within the same Figure 4. Calculate Differential Gain input Smith Chart, based on which users can choose the optimal source impedance for a given gain and noise requirement. Output Match and Filter The LTC6401-20 can drive an ADC directly without external output impedance matching. Alternatively, the differential output impedance of 25Ω can be matched to higher value impedance, e.g. 50Ω, by series resistors or an LC network. The internal low pass filter outputs at +OUTF/–OUTF have a –3dB bandwidth of 590MHz. External capacitor can reduce the low pass filter bandwidth as shown in Figure 5. A bandpass filter is easily implemented with only a few components as shown in Figure 6. Three 39pF capacitors and a 16nH inductor create a bandpass filter with 165MHz center frequency, –3dB frequencies at 138MHz and 200MHz. APPLICATIONS INFORMATION Figure 3. Input Termination for Single-Ended 50Ω Input Impedance Figure 5. LTC6401-20 Internal Filter Topology Modified for Low Filter Bandwidth (Three External Capacitors) 640120 F03 IN+ OUT– IN– OUT+ 100 Ω RT 66.5 Ω 0.1 μF 0.1 μF 12.5 Ω 1000 Ω LTC6401-20 100 Ω RS 50 Ω RS 50 Ω VIN 1000 Ω 12.5 Ω 50 Ω 50 Ω 13 14 15 16 7 5 6 8 + – 0.1 μF RT 66.5 Ω +OUT +OUTF –OUTF –OUT +IN +IN –IN –IN 1.7pF 640120 F04 IN+ OUT– IN– OUT+ 100 Ω 12.5 Ω 1000 Ω LTC6401-20 100 Ω 1/2 RS 1/2 RS VIN VOUT 1000 Ω 12.5 Ω 50 Ω 50 Ω 13 14 15 16 7 5 6 8 + – 1/2 RL 1/2 RL +OUT +OUTF –OUTF –OUT +IN +IN –IN –IN 1.7pF 640120 F05 IN+ OUT– IN– OUT+ 100 Ω 12.5 Ω 1000 Ω LTC6401-20 100 Ω 1000 Ω 12.5 Ω 50 Ω 50 Ω 13 14 15 16 7 5 6 8 8.2pF 8.2pF 12pF FILTERED OUTPUT (87.5MHz) +OUT +OUTF –OUTF –OUT +IN +IN –IN –IN 1.7pF Figure 6. LTC6401-20 Application Circuit for Bandpass Filtering (Three External Capacitors, One External Inductor) LTC2208 10 Ω 10 Ω 4.99 Ω 4.99 Ω 39pF 16nH 39pF 39pF 640120 F06 IN+ OUT– IN– OUT+ 100 Ω 12.5 Ω 1000 Ω LTC6401-20 100 Ω 1000 Ω 12.5 Ω 50 Ω 50 Ω 13 14 15 16 7 5 6 8 +OUT +OUTF –OUTF –OUT +IN +IN –IN –IN 1.7pF |
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