Datasheet LTC6401-26 (Analog Devices) - 10

LTC6401-26
APPLICATIONS INFORMATION Circuit Operation
them if the source is differential (Figure 1). Another ap- proach is to employ a wideband transformer if the source The LTC6401-26 is a low noise and low distortion fully is single ended (Figure 2). Both methods provide a wide- differential op amp/ADC driver with: band match. Alternatively, one could apply a narrowband • Operation from DC to 1.6GHz –3dB bandwidth impedance match at the inputs of the LTC6401-26 for • Fixed gain of 20V/V (26dB) frequency selection and/or noise reduction. • Differential input impedance 50Ω Referring to Figure 3, LTC6401-26 can be easily confi gured for single-ended input and differential output without a • Differential output impedance 25Ω balun. The signal is fed to one of the inputs through a • Differential impedance of output fi lter 100Ω matching network while the other input is connected to the same matching network and a source resistor. Because the The LTC6401-26 is composed of a fully differential amplifi er return ratios of the two feedback paths are equal, the two with on chip feedback and output common mode voltage outputs have the same gain and thus symmetrical swing. In control circuitry. Differential gain and input impedance general, the single-ended input impedance and termination are set by 25Ω/500Ω resistors in the feedback network. resistor R Small output resistors of 12.5Ω improve the circuit stability T are determined by the combination of RS, RG and R over various load conditions. They also provide a possible F. For example, when RS is 50Ω, it is found that the single-ended input impedance is 75Ω and R external fi ltering option, which is often desirable when the T is 150Ω in order to match to a 50Ω source impedance. load is an ADC. LTC6401-26 Filter resistors of 50Ω are available for additional fi ltering. 25Ω 25Ω 500Ω 12.5Ω Lowpass/bandpass fi lters are easily implemented with just 13 +IN +OUT 8 a couple of external components. Moreover, they offer 50Ω IN+ OUT– single-ended 50Ω matching in wideband applications and +OUTF V 7 IN 14 +IN + no external resistor is needed. – 50Ω 2.7pF 15 –IN IN– OUT+ –OUTF 6 The LTC6401-26 is very fl exible in terms of I/O coupling. It can be AC- or DC-coupled at the inputs, the outputs or 25Ω 25Ω 500Ω 12.5Ω both. Due to the internal connection between input and 16 –IN –OUT 5 640126 F01 output, users are advised to keep input common mode voltage between 1V and 1.6V for proper operation. If the
Figure 1. Input Termination for Differential 50
Ω
Input Impedance
inputs are AC-coupled, the input common mode voltage is automatically biased close to VOCM and thus no external LTC6401-26 circuitry is needed for bias. The LTC6401-26 provides an 50Ω 25Ω 500Ω 12.5Ω output common mode voltage set by V 13 +IN +OUT 8 OCM, which allows driving ADC directly without external components such as 50Ω IN+ OUT– 1:1 • transformer or AC coupling capacitors. The input signal +OUTF V 7 IN 14 +IN + – • can be either single-ended or differential with only minor 50Ω 2.7pF difference in distortion performance. 15 –IN IN– OUT+ –OUTF 6 25Ω 500Ω 12.5Ω
Input Impedance and Matching
16 –IN –OUT 5 M/A-COM 640126 F02 MABA-007159-000000 The differential input impedance of the LTC6401-26 is 50Ω. The interface between the input of LTC6401-26 and 50Ω
Figure 2. Input Termination for Differential 50
Ω
Input Impedance
source is straightforward. One way is to directly connect
Using a Balun
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