Datasheet LTC6400-8 (Analog Devices) - 10
| Manufacturer | Analog Devices |
| Description | 2.2GHz Low Noise, Low Distortion Differential ADC Driver for DC-300MHz |
| Pages / Page | 16 / 10 — APPLICATIONS INFORMATION. Figure 1. Input Termination for Differential … |
| File Format / Size | PDF / 224 Kb |
| Document Language | English |
APPLICATIONS INFORMATION. Figure 1. Input Termination for Differential 50Ω Input Impedance Using Shunt Resistor
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LTC6400-8
APPLICATIONS INFORMATION
LTC6400-8 RS 0.1μF LTC6400-8 25Ω 200Ω 500Ω 12.5Ω 50Ω 200Ω 500Ω 12.5Ω 13 +IN +OUT 8 13 +IN +OUT 8 VIN + 50Ω – R 50Ω T IN+ OUT– IN+ OUT– 59.0Ω +OUTF 7 +OUTF 7 VIN 14 +IN 14 +IN + 57.6Ω – 0.1μF 50Ω 2.7pF 50Ω 2.7pF 15 –IN IN– OUT+ –OUTF 6 15 –IN IN– OUT+ –OUTF 6 0.1μF 25Ω 200Ω 500Ω 12.5Ω 200Ω 500Ω 12.5Ω 16 –IN –OUT 5 16 –IN –OUT 5 64008 F01 27.4Ω 64008 F03
Figure 1. Input Termination for Differential 50Ω Input Impedance Using Shunt Resistor Figure 3. Input Termination for Single-Ended 50Ω Input Impedance
LTC6400-8 25Ω 500Ω The LTC6400-8 is unconditionally stable, i.e. differential 200Ω 12.5Ω 13 +IN +OUT 8 stability factor Kf>1 and stability measure B1>0. However, 50Ω the overall differential gain is affected by both source 1:4 IN+ OUT– +OUTF V 7 IN • impedance and load impedance as shown in Figure 4: 14 +IN + – 402Ω • 50Ω 2.7pF V 1000 R 15 –IN IN– OUT+ –OUTF 6 A OUT L V = = • V R 25Ω 200Ω 500Ω 12.5Ω + 400 25+R IN S L 16 –IN –OUT 5 MINI CIRCUITS 64008 F02 The noise performance of the LTC6400-8 also depends upon TCM4-19 the source impedance and termination. For example, an
Figure 2. Input Termination for Differential 50Ω Input Impedance
input 1:4 transformer in Figure 2 improves SNR by adding
Using a Balun
6dB gain at the inputs. A trade-off between gain and noise is obvious when constant noise fi gure circle and constant order to minimize the refl ection due to input mismatch. gain circle are plotted within the input Smith Chart, based Alternatively, one could apply a narrowband impedance on which users can choose the optimal source impedance match at the inputs of the LTC6400-8 for frequency selec- for a given gain and noise requirement. tion and/or noise reduction. Referring to Figure 3, LTC6400-8 can be easily confi gured LTC6400-8 1/2 R 200Ω 500Ω 12.5Ω S 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 200Ω S 500Ω 12.5Ω 1/2 RL swing. In general, the single-ended input impedance and 16 –IN –OUT 5 termination resistor R 64008 F04 T are determined by the combination of RS, RG and RF. For example, when RS is 50Ω, it is found
Figure 4. Calculate Differential Gain
that the single-ended input impedance is 322Ω and RT is 59Ω in order to match to a 50Ω source impedance. 64008f 10
APPLICATIONS INFORMATION
LTC6400-8 RS 0.1μF LTC6400-8 25Ω 200Ω 500Ω 12.5Ω 50Ω 200Ω 500Ω 12.5Ω 13 +IN +OUT 8 13 +IN +OUT 8 VIN + 50Ω – R 50Ω T IN+ OUT– IN+ OUT– 59.0Ω +OUTF 7 +OUTF 7 VIN 14 +IN 14 +IN + 57.6Ω – 0.1μF 50Ω 2.7pF 50Ω 2.7pF 15 –IN IN– OUT+ –OUTF 6 15 –IN IN– OUT+ –OUTF 6 0.1μF 25Ω 200Ω 500Ω 12.5Ω 200Ω 500Ω 12.5Ω 16 –IN –OUT 5 16 –IN –OUT 5 64008 F01 27.4Ω 64008 F03
Figure 1. Input Termination for Differential 50Ω Input Impedance Using Shunt Resistor Figure 3. Input Termination for Single-Ended 50Ω Input Impedance
LTC6400-8 25Ω 500Ω The LTC6400-8 is unconditionally stable, i.e. differential 200Ω 12.5Ω 13 +IN +OUT 8 stability factor Kf>1 and stability measure B1>0. However, 50Ω the overall differential gain is affected by both source 1:4 IN+ OUT– +OUTF V 7 IN • impedance and load impedance as shown in Figure 4: 14 +IN + – 402Ω • 50Ω 2.7pF V 1000 R 15 –IN IN– OUT+ –OUTF 6 A OUT L V = = • V R 25Ω 200Ω 500Ω 12.5Ω + 400 25+R IN S L 16 –IN –OUT 5 MINI CIRCUITS 64008 F02 The noise performance of the LTC6400-8 also depends upon TCM4-19 the source impedance and termination. For example, an
Figure 2. Input Termination for Differential 50Ω Input Impedance
input 1:4 transformer in Figure 2 improves SNR by adding
Using a Balun
6dB gain at the inputs. A trade-off between gain and noise is obvious when constant noise fi gure circle and constant order to minimize the refl ection due to input mismatch. gain circle are plotted within the input Smith Chart, based Alternatively, one could apply a narrowband impedance on which users can choose the optimal source impedance match at the inputs of the LTC6400-8 for frequency selec- for a given gain and noise requirement. tion and/or noise reduction. Referring to Figure 3, LTC6400-8 can be easily confi gured LTC6400-8 1/2 R 200Ω 500Ω 12.5Ω S 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 200Ω S 500Ω 12.5Ω 1/2 RL swing. In general, the single-ended input impedance and 16 –IN –OUT 5 termination resistor R 64008 F04 T are determined by the combination of RS, RG and RF. For example, when RS is 50Ω, it is found
Figure 4. Calculate Differential Gain
that the single-ended input impedance is 322Ω and RT is 59Ω in order to match to a 50Ω source impedance. 64008f 10
