Datasheet LT6402-20 (Analog Devices) - 10

LT6402-20
U U W U APPLICATIO S I FOR ATIO Circuit Description
the outputs, the common mode voltage is set via the VOCM pin, allowing the LT6402-20 to drive ADCs directly. No The LT6402-20 is a low noise, low distortion differential output AC-coupling capacitors or transformers are needed. amplifi er/ADC driver with: At the inputs, signals can be differential or single-ended • –3dB bandwidth with virtually no difference in performance. Furthermore, DC to 300MHz DC levels at the inputs can be set independently of the • Fixed gain independent of R output common mode voltage. These input characteristics LOAD 10V/V (20dB) often eliminate the need for an input transformer and/or AC-coupling capacitors. • Differential input impedance 100Ω
Input Impedance and Matching Networks
• Low output impedance Calculation of the input impedance of the LT6402-20 is • Built-in, user adjustable output fi ltering not straightforward from examination of the block diagram because of the internal feedback network. In addition, the • Requires minimal support circuitry input impedance when driven differentially is different than Referring to the block diagram, the LT6402-20 uses a when driven single-ended. closed-loop topology which incorporates 3 internal am- Differential Single-Ended plifi ers. Two of the amplifi ers (A and B) are identical and LT6402-20 100Ω 85.9Ω drive the differential outputs. The third amplifi er is used to set the output common mode voltage. Gain and input For single-ended 50Ω applications, a 121Ω shunt matching impedance are set by the 500Ω and 100Ω resistors in resistor to ground will result in the proper input termina- the internal feedback network. Output impedance is low, tion (Figure 1). For differential inputs there are several determined by the inherent output impedance of amplifi ers termination options. If the input source is 50Ω differential, A and B, and further reduced by internal feedback. then the input matching can be accomplished by either a 100Ω shunt resistor across the inputs (Figure 3), or The LT6402-20 also includes built-in single-pole output equivalent 49.9Ω shunt resistors on each of the inputs fi ltering. The user has the choice of using the unfi ltered to ground (Figure 2). If additional AC gain is desired, an outputs, the fi ltered outputs (75MHz –3dB lowpass), or impedance ratio transformer can also be used to better modifying the fi ltered outputs to alter frequency response match impedances. by adding additional components. Many lowpass and bandpass fi lters are easily implemented with just one or two additional components. 13 –INB 8 –OUT 14 The LT6402-20 has been designed to minimize the need –INA 0.1µF for external support components such as transformers or LT6402-20 AC-coupling capacitors. As an ADC driver, the LT6402-20 15 IF IN +INB +OUT requires no external components except for power-supply 16 5 +INA 121Ω bypass capacitors. This allows DC-coupled operation for ZIN = 50Ω SINGLE-ENDED 6402 F01 applications that have frequency ranges including DC. At
Figure 1. Input Termination for Single-Ended 50
Ω
Input Impedance
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