Datasheet LTC6401-8 (Analog Devices) - 10
| Fabricante | Analog Devices |
| Descripción | 2.2GHz Low Noise, Low Distortion Differential ADC Driver for DC-140MHz |
| Páginas / Página | 16 / 10 — APPLICATIONS INFORMATION. Figure 4. Calculate Differential Gain. Output … |
| Formato / tamaño de archivo | PDF / 231 Kb |
| Idioma del documento | Inglés |
APPLICATIONS INFORMATION. Figure 4. Calculate Differential Gain. Output Impedance Match and Filter
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LTC6401-8
APPLICATIONS INFORMATION
Referring to Figure 3, LTC6401-8 can be easily confi gured LTC6401-8 1/2 R 200 S Ω 500Ω 12.5Ω 1/2 RL for single-ended input and differential output without a 13 +IN +OUT 8 balun. The signal is fed to one of the inputs through a 50Ω IN+ OUT– matching network while the other input is connected to +OUTF V 7 IN 14 +IN + VOUT the same matching network and a source resistor. Because – 50Ω 2.7pF the return ratios of the two feedback paths are equal, the 15 –IN IN– OUT+ –OUTF 6 two outputs have the same gain and thus symmetrical 1/2 R 200 S Ω 500Ω 12.5Ω 1/2 RL swing. In general, the single-ended input impedance and 16 –IN –OUT 5 64018 F04 termination resistor RT are determined by the combination of R
Figure 4. Calculate Differential Gain
S, RG and RF. For example, when RS is 50Ω, it is found that the single-ended input impedance is 322Ω and RT is and noise is obvious when constant noise fi gure circle 59Ω in order to match to a 50Ω source impedance. and constant gain circle are plotted within the input Smith Chart, based on which users can choose the optimal source RS 0.1μF LTC6401-8 50Ω 200Ω 500Ω 12.5Ω impedance for a given gain and noise requirement. 13 +IN +OUT 8 VIN + – R 50Ω T IN+ OUT–
Output Impedance Match and Filter
59.0Ω +OUTF 7 14 +IN 0.1μF 50Ω 2.7pF The LTC6401-8 can drive an ADC directly without external 15 –IN IN– OUT+ –OUTF 6 output impedance matching. Alternatively, the differential 0.1μF 200Ω 500Ω 12.5Ω output impedance of 25Ω can be made larger, e.g. 50Ω, 16 –IN –OUT 5 by series resistors or LC network. 27.4Ω 64018 F03 The internal low pass fi lter outputs at +OUTF/–OUTF
Figure 3. Input Termination for Single-Ended 50
have a –3dB bandwidth of 590MHz. External capacitors Ω
Input Impedance
can reduce the lowpass fi lter bandwidth as shown in Figure 5. A bandpass fi lter is easily implemented with The LTC6401-8 is unconditionally stable, i.e. differential LTC6401-8 stability factor Kf>1 and stability measure B1>0. However, 200Ω 500Ω 12.5Ω the overall differential gain is affected by both source 13 +IN +OUT 8 50 impedance and load impedance as shown in Figure 4: Ω 8pF IN+ OUT– +OUTF 7 14 +IN FILTERED OUTPUT 12pF (87.5MHz) V 1000 R 50Ω 2.7pF A OUT L V = = • 15 –IN IN– OUT+ –OUTF 6 V R + 400 25 + R 8pF IN S L 200Ω 500Ω 12.5Ω 16 –IN –OUT 5 The noise performance of the LTC6401-8 also depends 64018 F05 upon the source impedance and termination. For example,
Figure 5. LTC6401-8 Internal Filter Topology Modifi ed for Low
an input 1:4 transformer in Figure 2 improves SNR by
Filter Bandwidth (Three External Capacitors)
adding 6dB gain at the inputs. A trade-off between gain 64018f 10
APPLICATIONS INFORMATION
Referring to Figure 3, LTC6401-8 can be easily confi gured LTC6401-8 1/2 R 200 S Ω 500Ω 12.5Ω 1/2 RL for single-ended input and differential output without a 13 +IN +OUT 8 balun. The signal is fed to one of the inputs through a 50Ω IN+ OUT– matching network while the other input is connected to +OUTF V 7 IN 14 +IN + VOUT the same matching network and a source resistor. Because – 50Ω 2.7pF the return ratios of the two feedback paths are equal, the 15 –IN IN– OUT+ –OUTF 6 two outputs have the same gain and thus symmetrical 1/2 R 200 S Ω 500Ω 12.5Ω 1/2 RL swing. In general, the single-ended input impedance and 16 –IN –OUT 5 64018 F04 termination resistor RT are determined by the combination of R
Figure 4. Calculate Differential Gain
S, RG and RF. For example, when RS is 50Ω, it is found that the single-ended input impedance is 322Ω and RT is and noise is obvious when constant noise fi gure circle 59Ω in order to match to a 50Ω source impedance. and constant gain circle are plotted within the input Smith Chart, based on which users can choose the optimal source RS 0.1μF LTC6401-8 50Ω 200Ω 500Ω 12.5Ω impedance for a given gain and noise requirement. 13 +IN +OUT 8 VIN + – R 50Ω T IN+ OUT–
Output Impedance Match and Filter
59.0Ω +OUTF 7 14 +IN 0.1μF 50Ω 2.7pF The LTC6401-8 can drive an ADC directly without external 15 –IN IN– OUT+ –OUTF 6 output impedance matching. Alternatively, the differential 0.1μF 200Ω 500Ω 12.5Ω output impedance of 25Ω can be made larger, e.g. 50Ω, 16 –IN –OUT 5 by series resistors or LC network. 27.4Ω 64018 F03 The internal low pass fi lter outputs at +OUTF/–OUTF
Figure 3. Input Termination for Single-Ended 50
have a –3dB bandwidth of 590MHz. External capacitors Ω
Input Impedance
can reduce the lowpass fi lter bandwidth as shown in Figure 5. A bandpass fi lter is easily implemented with The LTC6401-8 is unconditionally stable, i.e. differential LTC6401-8 stability factor Kf>1 and stability measure B1>0. However, 200Ω 500Ω 12.5Ω the overall differential gain is affected by both source 13 +IN +OUT 8 50 impedance and load impedance as shown in Figure 4: Ω 8pF IN+ OUT– +OUTF 7 14 +IN FILTERED OUTPUT 12pF (87.5MHz) V 1000 R 50Ω 2.7pF A OUT L V = = • 15 –IN IN– OUT+ –OUTF 6 V R + 400 25 + R 8pF IN S L 200Ω 500Ω 12.5Ω 16 –IN –OUT 5 The noise performance of the LTC6401-8 also depends 64018 F05 upon the source impedance and termination. For example,
Figure 5. LTC6401-8 Internal Filter Topology Modifi ed for Low
an input 1:4 transformer in Figure 2 improves SNR by
Filter Bandwidth (Three External Capacitors)
adding 6dB gain at the inputs. A trade-off between gain 64018f 10