Datasheet LTC6400-26 (Analog Devices) - 10

LTC6400-26
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 LTC6400-26 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. The transformer must be placed close to the • Operation from DC to 1.9GHz –3dB bandwidth input pins in order to minimize the refl ection due to input • Fixed gain of 20V/V (26dB) mismatch. Alternatively, one could apply a narrowband impedance match at the inputs of the LTC6400-26 for • Differential input impedance 50Ω frequency selection and/or noise reduction. • Differential output impedance 25Ω Referring to Figure 3, LTC6400-26 can be easily confi gured • Differential impedance of output fi lter 100Ω for single-ended input and differential output without a balun. The signal is fed to one of the inputs through a The LTC6400-26 is composed of a fully differential amplifi er matching network while the other input is connected to the with on chip feedback and output common mode voltage same matching network and a source resistor. Because the control circuitry. Differential gain and input impedance return ratios of the two feedback paths are equal, the two are set by 25Ω/500Ω resistors in the feedback network. outputs have the same gain and thus symmetrical swing. In Small output resistors of 12.5Ω improve the circuit stability general, the single-ended input impedance and termination over various load conditions. They also provide a possible resistor R external fi ltering option, which is often desirable when the T are determined by the combination of RS, RG and R load is an ADC. F. For example, when RS is 50Ω, it is found that the single-ended input impedance is 75Ω and RT is 150Ω in Filter resistors of 50Ω are available for additional fi ltering. order to match to a 50Ω source impedance. Lowpass/bandpass fi lters are easily implemented with just a couple of external components. Moreover, they offer LTC6400-26 25Ω 25Ω 500Ω 12.5Ω single-ended 50Ω matching in wideband applications and 13 +IN +OUT 8 no external resistor is needed. 50Ω IN+ OUT– +OUTF V 7 The LTC6400-26 is very fl exible in terms of I/O coupling. IN 14 +IN + – 50Ω 2.7pF It can be AC- or DC-coupled at the inputs, the outputs or 15 –IN IN– OUT+ –OUTF 6 both. Due to the internal connection between input and 25Ω 25Ω 500Ω 12.5Ω output, users are advised to keep input common mode 16 –IN –OUT 5 voltage between 1V and 1.6V for proper operation. If the 640026 F01 inputs are AC-coupled, the input common mode voltage
Figure 1. Input Termination for Differential 50
Ω
Input Impedance
is automatically biased close to VOCM and thus no external circuitry is needed for bias. The LTC6400-26 provides an LTC6400-26 50Ω 25Ω 500Ω 12.5Ω output common mode voltage set by VOCM, which allows 13 +IN +OUT 8 driving ADC directly without external components such as 50Ω IN+ OUT– transformer or AC coupling capacitors. The input signal 1:1 • +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 MACOM 640026 F02 MABA-007159-000000 The differential input impedance of the LTC6400-26 is 50Ω.
Figure 2. Input Termination for Differential 50
Ω

The interface between the input of LTC6400-26 and 50Ω
Input Impedance Using a Balun
source is straightforward. One way is to directly connect 640026fa 10