Datasheet LTC6401-14 (Analog Devices) - 9
| Manufacturer | Analog Devices |
| Description | 2GHz Low Noise, Low Distortion Differential ADC Driver for DC-140MHz |
| Pages / Page | 16 / 9 — APPLICATIONS INFORMATION. Circuit Operation. Input Impedance and … |
| File Format / Size | PDF / 225 Kb |
| Document Language | English |
APPLICATIONS INFORMATION. Circuit Operation. Input Impedance and Matching. Figure 1. Input Termination for Differential 50
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LTC6401-14
APPLICATIONS INFORMATION Circuit Operation Input Impedance and Matching
The LTC6401-14 is a low noise and low distortion fully The differential input impedance of the LTC6401-14 is differential op amp/ADC driver with: 200Ω. Usually the differential inputs need to be terminated to a lower value impedance, e.g. 50 • Operation from DC to 2GHz –3dB bandwidth Ω, in order to provide an impedance match for the source. Several choices are • Fixed gain of 5V/V (14dB) available. One approach is to use a differential shunt resis- • Differential input impedance 200Ω tor (Figure 1). Another approach is to employ a wideband transformer (Figure 2). Both methods provide a wideband • Differential output impedance 25Ω match. The termination resistor or the transformer must • Differential impedance of output fi lter 100Ω be placed close to the input pins in order to minimize the refl ection due to input mismatch. Alternatively, one The LTC6401-14 is composed of a fully differential amplifi er could apply a narrowband impedance match at the inputs with on chip feedback and output common mode voltage of the LTC6401-14 for frequency selection and/or noise control circuitry. Differential gain and input impedance reduction. are set by 200Ω/500Ω resistors in the feedback network. Small output resistors of 12.5Ω improve the circuit stability LTC6401-14 25Ω 100Ω 500Ω 12.5Ω over various load conditions. They also provide a possible 13 +IN +OUT 8 external fi ltering option, which is often desirable when the 50Ω load is an ADC. IN+ OUT– +OUTF V 7 IN 14 +IN + Filter resistors of 50Ω are available for additional fi ltering. 66.5Ω – 50Ω 2.7pF Lowpass/bandpass fi lters are easily implemented with just 15 –IN IN– OUT+ –OUTF 6 a couple of external components. Moreover, they offer 25Ω 100Ω 500Ω 12.5Ω single-ended 50Ω matching in wideband applications and 16 –IN –OUT 5 no external resistor is needed. 640114 F01 The LTC6401-14 is very fl exible in terms of I/O coupling.
Figure 1. Input Termination for Differential 50
Ω
Input Impedance Using Shunt Resistor
It can be AC- or DC-coupled at the inputs, the outputs or both. Due to the internal connection between input and output, users are advised to keep input common mode LTC6401-14 voltage between 1V and 1.6V for proper operation. If the 25Ω 100Ω 500Ω 12.5Ω inputs are AC-coupled, the input common mode voltage 13 +IN +OUT 8 50Ω is automatically biased approximately 250mV above V 1:4 OCM IN+ OUT– +OUTF 7 and thus no external circuitry is needed for bias. The VIN • 14 +IN + – • LTC6401-14 provides an output common mode voltage 50Ω 2.7pF 15 –IN IN– OUT+ –OUTF 6 set by VOCM, which allows driving ADC directly without external components such as transformer or AC coupling 25Ω 100Ω 500Ω 12.5Ω capacitors. The input signal can be either single-ended 16 –IN –OUT 5 MINI CIRCUITS 640114 F02 or differential with only minor difference in distortion TCM4-19 performance.
Figure 2. Input Termination for Differential 50
Ω
Input Impedance Using a Balun
640114f 9
APPLICATIONS INFORMATION Circuit Operation Input Impedance and Matching
The LTC6401-14 is a low noise and low distortion fully The differential input impedance of the LTC6401-14 is differential op amp/ADC driver with: 200Ω. Usually the differential inputs need to be terminated to a lower value impedance, e.g. 50 • Operation from DC to 2GHz –3dB bandwidth Ω, in order to provide an impedance match for the source. Several choices are • Fixed gain of 5V/V (14dB) available. One approach is to use a differential shunt resis- • Differential input impedance 200Ω tor (Figure 1). Another approach is to employ a wideband transformer (Figure 2). Both methods provide a wideband • Differential output impedance 25Ω match. The termination resistor or the transformer must • Differential impedance of output fi lter 100Ω be placed close to the input pins in order to minimize the refl ection due to input mismatch. Alternatively, one The LTC6401-14 is composed of a fully differential amplifi er could apply a narrowband impedance match at the inputs with on chip feedback and output common mode voltage of the LTC6401-14 for frequency selection and/or noise control circuitry. Differential gain and input impedance reduction. are set by 200Ω/500Ω resistors in the feedback network. Small output resistors of 12.5Ω improve the circuit stability LTC6401-14 25Ω 100Ω 500Ω 12.5Ω over various load conditions. They also provide a possible 13 +IN +OUT 8 external fi ltering option, which is often desirable when the 50Ω load is an ADC. IN+ OUT– +OUTF V 7 IN 14 +IN + Filter resistors of 50Ω are available for additional fi ltering. 66.5Ω – 50Ω 2.7pF Lowpass/bandpass fi lters are easily implemented with just 15 –IN IN– OUT+ –OUTF 6 a couple of external components. Moreover, they offer 25Ω 100Ω 500Ω 12.5Ω single-ended 50Ω matching in wideband applications and 16 –IN –OUT 5 no external resistor is needed. 640114 F01 The LTC6401-14 is very fl exible in terms of I/O coupling.
Figure 1. Input Termination for Differential 50
Ω
Input Impedance Using Shunt Resistor
It can be AC- or DC-coupled at the inputs, the outputs or both. Due to the internal connection between input and output, users are advised to keep input common mode LTC6401-14 voltage between 1V and 1.6V for proper operation. If the 25Ω 100Ω 500Ω 12.5Ω inputs are AC-coupled, the input common mode voltage 13 +IN +OUT 8 50Ω is automatically biased approximately 250mV above V 1:4 OCM IN+ OUT– +OUTF 7 and thus no external circuitry is needed for bias. The VIN • 14 +IN + – • LTC6401-14 provides an output common mode voltage 50Ω 2.7pF 15 –IN IN– OUT+ –OUTF 6 set by VOCM, which allows driving ADC directly without external components such as transformer or AC coupling 25Ω 100Ω 500Ω 12.5Ω capacitors. The input signal can be either single-ended 16 –IN –OUT 5 MINI CIRCUITS 640114 F02 or differential with only minor difference in distortion TCM4-19 performance.
Figure 2. Input Termination for Differential 50
Ω
Input Impedance Using a Balun
640114f 9
