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LTC1967 Datasheet(PDF) 11 Page - Linear Technology |
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LTC1967 Datasheet(HTML) 11 Page - Linear Technology |
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11 / 28 page  11 LTC1967 1967f APPLICATIO S I FOR ATIO because of the computation of the square of the input. The typical values shown, 5% peak ripple with 0.05% DC error, occur with CAVE = 1.5µF and fINPUT = 10Hz. If the application calls for the output of the LTC1967 to feed a sampling or Nyquist A/D converter (or other circuitry that will not average out this double frequency ripple) a larger averaging capacitor can be used. This trade-off is depicted in Figure 8. The peak ripple error can also be reduced by additional lowpass filtering after the LTC1967, but the simplest solution is to use a larger averaging capacitor. A 2.2 µF capacitor is a good choice for many applications. The peak error at 50Hz/60Hz will be <1% and the DC error will be <0.1% with frequencies of 10Hz or more. Note that both Figure 6 and Figure 8 assume AC-coupled waveforms with a crest factor less than 2, such as sine waves or triangle waves. For higher crest factors and/or AC + DC waveforms, a larger CAVE will generally be required. See “Crest Factor and AC + DC Waveforms.” Capacitor Type Selection The LTC1967 can operate with many types of capacitors. The various types offer a wide array of sizes, tolerances, parasitics, package styles and costs. Ceramic chip capacitors offer low cost and small size, but are not recommended for critical applications. The value stability over voltage and temperature is poor with many types of ceramic dielectrics. This will not cause an RMS- to-DC accuracy problem except at low frequencies, where it can aggravate the effects discussed in the previous section. If a ceramic capacitor is used, it may be neces- sary to use a much higher nominal value in order to assure the low frequency accuracy desired. Another parasitic of ceramic capacitors is leakage, which is again dependent on voltage and particularly tempera- ture. If the leakage is a constant current leak, the I • R drop of the leak multiplied by the output impedance of the LTC1967 will create a constant offset of the output voltage. If the leak is Ohmic, the resistor divider formed with the LTC1967 output impedance will cause a gain error. For < 0.1% gain accuracy degradation, the parallel impedance of the capacitor leakage will need to be >1000 times the LTC1967 output impedance. Accuracy at this level can be hard to achieve with a ceramic capacitor, particularly with a large value of capacitance and at high temperature. For critical applications, a film capacitor, such as metal- ized polyester, will be a much better choice. Although more expensive, and larger for a given value, the value stability and low leakage make metal-film capacitors a trouble-free choice. With any type of capacitor, the self-resonance of the capacitor can be an issue with the switched capacitor LTC1967. If the self-resonant frequency of the averaging capacitor is 1MHz or less, a second smaller capacitor should be added in parallel to reduce the impedance seen by the LTC1967 output stage at high frequencies. A capacitor 100 times smaller than the averaging capacitor will typically be small enough to be a low cost ceramic with a high quality dielectric such as X7R or NPO/COG. Figure 8. Peak Error vs Input Frequency with One Cap Averaging INPUT FREQUENCY (Hz) 1 –2.0 –1.6 –1.2 –0.8 –0.4 10 100 1967 F08 0 –1.8 –1.4 –1.0 –0.6 –0.2 C = 100 µF C = 47 µF C = 22 µF C = 10 µF C = 4.7 µF C = 2.2 µF C = 1 µF |
Similar Part No. - LTC1967_15 |
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Similar Description - LTC1967_15 |
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