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LT1993-4 Datasheet(PDF) 11 Page - Linear Technology |
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LT1993-4 Datasheet(HTML) 11 Page - Linear Technology |
11 / 16 page LT1993-4 11 19934fa APPLICATIO S I FOR ATIO Circuit Description The LT1993-4 is a low noise, low distortion differential amplifier/ADC driver with: • DC to 900MHz –3dB bandwidth • Fixed gain of 4V/V (12dB) independent of RLOAD • 100 Ω differential input impedance • Low output impedance • Built-in, user adjustable output filtering • Requires minimal support circuitry Referring to the block diagram, the LT1993-4 uses a closed- loop topology which incorporates 3 internal amplifiers. Two of the amplifiers (A and B) are identical and drive the differential outputs. The third amplifier (C) is used to set the output common mode voltage. Gain and input imped- ance are set by the 100 Ω/200Ω resistors in the internal feedback network. Output impedance is low, determined by the inherent output impedance of amplifiers A and B, and further reduced by internal feedback. The LT1993-4 also includes built-in single-pole output filtering. The user has the choice of using the unfiltered outputs, the filtered outputs (175MHz –3dB lowpass), or modifying the filtered outputs to alter frequency response by adding additional components. Many lowpass and bandpass filters are easily implemented with just one or two additional components. The LT1993-4 has been designed to minimize the need for external support components such as transformers or AC-coupling capacitors. As an ADC driver, the LT1993-4 requires no external components except for power-supply bypass capacitors. This allows DC-coupled operation for applications that have frequency ranges including DC. At the outputs, the common mode voltage is set via the VOCM pin, allowing the LT1993-4 to drive ADCs directly. No output AC-coupling capacitors or transformers are needed. At the inputs, signals can be differential or single-ended with virtually no difference in performance. Furthermore, DC levels at the inputs can be set independently of the output common mode voltage. These input characteristics often eliminate the need for an input transformer and/or AC-coupling capacitors. Input Impedance and Matching Networks Because of the internal feedback network, calculation of the LT1993-4’s input impedance is not straightforward from examination of the block diagram. Furthermore, the input impedance when driven differentially is different than when driven single-ended. When driven differentially, the LT1993-4’s input impedance is 100 Ω (differential); when driven single-ended, the input impedance is 75 Ω. For single-ended 50 Ω applications, a 150Ω shunt match- ing resistor to ground will result in the proper input termination (Figure 1). For differential inputs there are several termination options. If the input source is 50 Ω differential, then input matching can be accomplished by either a 100 Ω shunt resistor across the inputs (Figure 3), or a 49.9 Ω shunt resistor on each of the inputs to ground (Figure 2). 19934 F01 IF IN LT1993-4 –INA –INB –OUT +OUT 8 5 +INB +INA 14 13 15 16 Figure 1. Input Termination for Single-Ended 50 Ω Input Impedance Figure 2. Input Termination for Differential 50 Ω Input Impedance 19934 F02 IF IN– IF IN+ LT1993-4 –INA –INB –OUT +OUT 8 5 +INB +INA 14 13 15 16 Figure 3. Alternate Input Termination for Differential 50 Ω Input Impedance 19934 F03 IF IN– IF IN+ LT1993-4 –INA –INB –OUT +OUT 8 5 +INB +INA 14 13 15 16 |
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