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LTC1562ACG Datasheet(PDF) 10 Page - Linear Integrated Systems |
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LTC1562ACG Datasheet(HTML) 10 Page - Linear Integrated Systems |
10 / 28 page 10 LTC1562 1562fa Because 2nd order sections with Q ≥ 1 have response peaks near fO, the gain ratio above implies some rules of thumb: fO < 100kHz ⇒ V2 tends to have the larger swing fO > 100kHz ⇒ V1 tends to have the larger swing. The following situations are convenient because the relative swing issue does not arise. The unused output’s swing is naturally the smaller of the two in these cases: Lowpass response (resistor input, V2 output, Figure 5) with fO < 100kHz Bandpass response (capacitor input, V2 output, Figure 6b) with fO < 100kHz Bandpass response (resistor input, V1 output, Figure 6a) with fO > 100kHz Highpass response (capacitor input, V1 output, Figure 7) with fO > 100kHz The LTC1562-2, a higher frequency derivative of the LTC1562, has a design center fO of 200kHz compared to 100kHz in the LTC1562. The rules summarized above apply to the LTC1562-2 but with 200kHz replacing the 100kHz limits. Thus, an LTC1562-2 lowpass filter section with fO below 200kHz automatically satisfies the desirable condition of the unused output carrying the smaller signal swing. APPLICATIONS INFORMATION level inputs require further dynamic range, reducing the value of ZIN boosts the signal gain while reducing the input referred noise. This feature can increase the SNR for low level signals. Varying or switching ZIN is also an efficient way to effect automatic gain control (AGC). From a system viewpoint, this technique boosts the ratio of maximum signal to minimum noise, for a typical 2nd order lowpass response (Q = 1, fO = 100kHz), to 118dB. Input Voltages Beyond the Power Supplies Properly used, the LTC1562 can accommodate input voltage excursions well beyond its supply voltage. This requires care in design but can be useful, for example, when large out-of-band interference is to be removed from a smaller desired signal. The flexibility for different input voltages arises because the INV inputs are at virtual ground potential, like the inverting input of an op amp with negative feedback. The LTC1562 fundamentally responds to input current and the external voltage VIN appears only across the external impedance ZIN in Figure 3. To accept beyond-the-supply input voltages, it is impor- tant to keep the LTC1562 powered on, not in shutdown mode, and to avoid saturating the V1 or V2 output of the 2nd order section that receives the input. If any of these conditions is violated, the INV input will depart from a virtual ground, leading to an overload condition whose recovery timing depends on circuit details. In the event that this overload drives the INV input beyond the supply voltages, the LTC1562 could be damaged. The most subtle part of preventing overload is to consider the possible input signals or spectra and take care that none of them can drive either V1 or V2 to the supply limits. Note that neither output can be allowed to saturate, even if it is not used as the signal output. If necessary the passband gain can be reduced (by increasing the imped- ance of ZIN in Figure 3) to reduce output swings. The final issue to be addressed with beyond-the-supply inputs is current and voltage limits. Current entering the virtual ground INV input flows eventually through the output circuitry that drives V1 and V2. The input current magnitude ( VIN/ZIN in Figure 3) should be limited by design to less than 1mA for good distortion performance. On the other hand, the input voltage VIN appears across the Low Level or Wide Range Input Signals The LTC1562 contains a built-in capability for low noise amplification of low level signals. The ZIN impedance in each 2nd order section controls the block’s gain. When set for unity passband gain, a 2nd order section can deliver an output signal more than 100dB above the noise level. If low Figure 8. 100kHz, Q = 0.7 Lowpass Circuit for Distortion vs Loading Test INV V1 2nd ORDER 1/4 LTC1562 V2 1562 F08 R2 10k CL 30pF RL (EXTERNAL LOAD RESISTANCE) RQ 6.98k RIN 10k VIN VOUT |
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