Datasheet LTC6400-8 (Analog Devices) - 9

LTC6400-8
BLOCK DIAGRAM
V– ENABLE V+ V– 12 11 10 9 BIAS CONTROL RG RF ROUT +IN +OUT 200Ω 500Ω 12.5Ω 13 8 RFILT +OUTF 50Ω +IN IN+ OUT– 7 14 C R FILT FILT –IN 2.7pF –OUTF 50Ω 15 6 IN– OUT+ RG RF ROUT –IN –OUT 200Ω 500Ω 12.5Ω 16 5 2k COMMON MODE CONTROL 5.3pF 64008 BD 1 2 3 4 V+ VOCM V+ V–
APPLICATIONS INFORMATION Circuit Operation
The LTC6400-8 is very fl exible in terms of I/O coupling. It can be AC- or DC-coupled at the inputs, the outputs or The LTC6400-8 is a low noise and low distortion fully both. Due to the internal connection between input and differential op amp/ADC driver with: output, users are advised to keep input common mode • Operation from DC to 2.2GHz –3dB bandwidth voltage between 1V and 1.8V for proper operation. If the • Fixed gain of 2.5V/V (8dB) inputs are AC-coupled, the input common mode voltage is automatically biased approximately 450mV above VOCM • Differential input impedance 400Ω and thus no external circuitry is needed for bias. The • Differential output impedance 25Ω LTC6400-8 provides an output common mode voltage set by V • Differential impedance of output fi lter 100Ω OCM, which allows driving ADC directly without external components such as transformer or AC coupling The LTC6400-8 is composed of a fully differential amplifi er capacitors. The input signal can be either single-ended with on chip feedback and output common mode voltage or differential with only minor difference in distortion control circuitry. Differential gain and input impedance performance. are set by 200Ω/500Ω resistors in the feedback network. Small output resistors of 12.5Ω improve the circuit stability
Input Impedance and Matching
over various load conditions. They also provide a possible The differential input impedance of the LTC6400-8 is 400Ω. external fi ltering option, which is often desirable when the Usually the differential inputs need to be terminated to a load is an ADC. lower value impedance, e.g. 50Ω, in order to provide an Filter resistors of 50Ω are available for additional fi ltering. impedance match for the source. Several choices are avail- Lowpass/bandpass fi lters are easily implemented with just able. One approach is to use a differential shunt resistor a couple of external components. Moreover, they offer (Figure 1). Another approach is to employ a wideband single-ended 50Ω matching in wideband applications and transformer and shunt resistor (Figure 2). Both methods no external resistor is needed. provide a wideband match. The termination resistor or the transformer must be placed close to the input pins in 64008f 9