Datasheet LTC6401-20 (Analog Devices) - 10

LTC6401-20
APPLICATIONS INFORMATION Circuit Operation
the differential inputs may need to be terminated to a lower value impedance, e.g. 50 The LTC6401-20 is a low noise and low distortion fully Ω, in order to provide an imped- ance match to the source. Several choices are available. differential op amp/ADC driver with: One approach is to use a differential shunt resistor (Figure • Operation from DC to 1.3GHz –3dB bandwidth 1). Another approach is to employ a wideband transformer impedance (Figure 2). Both methods provide a wideband match. The • Fixed gain of 10V/V (20dB) termination resistor or the transformer must be placed close to the input pins in order to minimize the refl ection • Differential input impedance 200Ω due to input mismatch. Alternatively, one could apply a • Differential output impedance 25Ω narrowband impedance match at the inputs of the LTC6401- 20 for frequency selection and/or noise reduction. • Differential impedance of output fi lter 100Ω Referring to Figure 3, LTC6401-20 can be easily confi gured The LTC6401-20 is composed of a fully differential amplifi er for single-ended input and differential output without a with on chip feedback and output common mode voltage balun. The signal is fed to one of the inputs through a control circuitry. Differential gain and input impedance are matching network while the other input is connected to set by 100Ω/1000Ω resistors in the feedback network. the same matching network and a source resistor. Because Small output resistors of 12.5Ω improve the circuit stability the return ratios of the two feedback paths are equal, the over various load conditions. They also provide a possible external fi ltering option, which is often desirable when the LTC6401-20 load is an ADC. 25Ω 100Ω 1000Ω 12.5Ω 13 +IN +OUT 8 Filter resistors of 50Ω are available for additional fi ltering. 50Ω Lowpass/bandpass fi lters are easily implemented with just IN+ OUT– V +OUTF 7 IN a couple of external components. Moreover, they offer 14 + +IN 66.5Ω – 50Ω 1.7pF single-ended 50Ω matching in wideband applications and 15 –IN IN– OUT+ –OUTF 6 no external resistor is needed. 25Ω 100Ω 1000Ω 12.5Ω The LTC6401-20 is very fl exible in terms of I/O coupling. 16 –IN –OUT 5 It can be AC- or DC-coupled at the inputs, the outputs or 640120 F01 both. Due to the internal connection between input and
Figure 1. Input Termination for Differential 50
Ω
Input Impedance
output, users are advised to keep input common mode
Using Shunt Resistor
voltage between 1V and 1.6V for proper operation. If the inputs are AC-coupled, the input common mode voltage LTC6401-20 is automatically biased close to VOCM and thus no external 25Ω 100Ω 1000Ω 12.5Ω circuitry is needed for bias. The LTC6401-20 provides an 13 +IN +OUT 8 output common mode voltage set by V 50Ω OCM, which allows 1:4 IN+ OUT– driving an ADC directly without external components such • • V +OUTF 7 IN 14 + +IN as a transformer or AC coupling capacitors. The input – 50Ω 1.7pF signal can be either single-ended or differential with only 15 –IN IN– OUT+ –OUTF 6 minor differences in distortion performance. 25Ω 100Ω 1000Ω 12.5Ω 16 –IN –OUT 5
Input Impedance and Matching
640120 F02 The differential input impedance of the LTC6401-20 is
Figure 2. Input Termination for Differential 50
Ω
Input Impedance
200Ω. If a 200Ω source impedance is unavailable, then
Using a 1:4 Balun
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