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LT6600-2.5 Datasheet(PDF) 8 Page - Linear Technology |
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LT6600-2.5 Datasheet(HTML) 8 Page - Linear Technology |
8 / 12 page LT6600-2.5 8 660025i APPLICATIO S I FOR ATIO source, provided it remains at least 1.5V above V– and at least 1.5V below V+. An internal resistor divider sets the voltage of Pin 7. While the internal 11k resistors are well matched, their absolute value can vary by ±20%. This should be taken into consideration when connecting an external resistor network to alter the voltage of Pin 7. Pin 2 can be shorted to Pin 7 for simplicity. If a different common mode output voltage is required, connect Pin 2 to a voltage source or resistor network. For 3V and 3.3V supplies the voltage at Pin 2 must be less than or equal to the mid supply level. For example, voltage (Pin 2) ≤ 1.65V on a single 3.3V supply. For power supply voltages higher than 3.3V the voltage at Pin 2 can be set above mid supply. The voltage on Pin 2 should not exceed 1V below the voltage on Pin 7. The voltage on Pin 2 should not be more than 2V above the voltage on Pin 7. Pin 2 is a high impedance input. The LT6600-2.5 was designed to process a variety of input signals including signals centered around the mid-supply voltage and signals that swing between ground and a positive voltage in a single supply system (Figure 1). The range of allowable input common mode voltage (the average of VIN+ and VIN– in Figure 1) is determined by the power supply level and gain setting (see “Electrical Characteristics”). Common Mode DC Currents In applications like Figure 1 and Figure 3 where the LT6600-2.5 not only provides lowpass filtering but also level shifts the common mode voltage of the input signal, DC currents will be generated through the DC path be- tween input and output terminals. Minimize these currents to decrease power dissipation and distortion. Consider the application in Figure 3. Pin 7 sets the output common mode voltage of the 1st differential amplifier inside the LT6600-2.5 (see the “Block Diagram” section) at 2.5V. Since the input common mode voltage is near 0V, there will be approximately a total of 2.5V drop across the series combination of the internal 1580 Ω feedback resis- tor and the external 402 Ω input resistor. The resulting 1.25mA common mode DC current in each input path, must be absorbed by the sources VIN+ and VIN–. Pin 2 sets the common mode output voltage of the 2nd differential amplifier inside the LT6600-2.5, and therefore sets the common mode output voltage of the filter. Since, in the example of Figure 3, Pin 2 differs from Pin 7 by 0.5V, an additional 625 µA (312µA per side) of DC current will flow in the resistors coupling the 1st differential amplifier output stage to filter output. Thus, a total of 3.125mA is used to translate the common mode voltages. A simple modification to Figure 3 will reduce the DC common mode currents by 36%. If Pin 7 is shorted to Pin 2 the common mode output voltage of both op amp stages will be 2V and the resulting DC current will be 2mA. Of course, by AC coupling the inputs of Figure 3, the common mode DC current can be reduced to 625 µA. Noise The noise performance of the LT6600-2.5 can be evalu- ated with the circuit of Figure 6. Given the low noise output of the LT6600-2.5 and the 6dB attenuation of the transformer coupling network, it will be necessary to measure the noise floor of the spectrum analyzer and subtract the instrument noise from the filter noise measurement. Example: With the IC removed and the 25 Ω resistors grounded, Figure 6, measure the total integrated noise (eS) of the spectrum analyzer from 10kHz to 2.5MHz. With the IC inserted, the signal source (VIN) disconnected, and the input resistors grounded, measure the total integrated noise out of the filter (eO). With the signal source con- nected, set the frequency to 100kHz and adjust the ampli- tude until VIN measures 100mVP-P. Measure the output amplitude, VOUT, and compute the passband gain – + 0.1 µF 0.1 µF 2.5V – 2.5V – + LT6600-2.5 3 4 1 7 2 8 5 6 RIN RIN 25 Ω 25 Ω 66002 F06 SPECTRUM ANALYZER INPUT 50 Ω VIN COILCRAFT TTWB-1010 1:1 Figure 6 |
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