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TC850CPL 数据表(PDF) 8 Page - Microchip Technology |
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TC850CPL 数据表(HTML) 8 Page - Microchip Technology |
8 / 26 page TC850 DS21479C-page 8 © 2006 Microchip Technology Inc. 3.0 DETAILED DESCRIPTION The TC850 is a multiple-slope, integrating A/D con- verter (ADC). The multiple-slope conversion process, combined with chopper-stabilized amplifiers, results in a significant increase in ADC speed, while maintaining very high resolution and accuracy. 3.1 Dual-Slope Conversion Principles The conventional dual-slope converter measurement cycle (shown in Figure 3-1) has two distinct phases: 1. Input signal integration 2. Reference voltage integration (de-integration). FIGURE 3-1: Dual-Slope ADC Cycle The input signal being converted is integrated for a fixed time period, measured by counting clock pulses. An opposite polarity constant reference voltage is then de-integrated until the integrator output voltage returns to zero. The reference integration time is directly proportional to the input signal. In a simple dual-slope converter, complete conversion requires the integrator output to “ramp-up” and “ramp- down.” Most dual-slope converters add a third phase, auto-zero. During auto-zero, offset voltages of the input buffer, integrator and comparator are nulled, thereby eliminating the need for zero offset adjustments. Dual-slope converter accuracy is unrelated to the inte- grating resistor and capacitor values, as long as they are stable during a measurement cycle. By converting the unknown analog input voltage into an easily mea- sured function of time, the dual-slope converter reduces the need for expensive, precision passive components. Noise immunity is an inherent benefit of the integrating conversion method. Noise spikes are integrated, or averaged, to zero during the integration period. Inte- grating ADCs are immune to the large conversion errors that plague successive approximation converters in high-noise environments. A simple mathematical equation relates the input signal, reference voltage and integration time: EQUATION 3-1: 3.2 Multiple-Slope Conversion Principles One limitation of the dual-slope measurement tech- nique is conversion speed. In a typical dual-slope method, the auto-zero and integrate times are each one-half of the de-integrate time. For a 15-bit conver- sion, 214 +214 +215 (65,536) clock pulses are required for auto-zero, integrate and de-integrate phases, respectively. The large number of clock cycles effectively limits the conversion rate to about 2.5 conversions per second, when a typical analog CMOS fabrication process is used. The TC850 uses a multiple-slope conversion technique to increase conversion speed (Figure 3-2). This tech- nique makes use of a two-slope de-integration phase and permits 15-bit resolution up to 40 conversions per second. During the TC850’s de-integration phase, the integra- tion capacitor is rapidly discharged to yield a resolution of 9 bits. At this point, some charge will remain on the capacitor. This remaining charge is then slowly de-integrated, producing an additional 6 bits of resolution. The result is 15 bits of resolution achieved with only 29 +26 (512 + 64, or 576) clock pulses for de-integration. A complete conversion cycle occupies only 1280 clock pulses. In order to generate “fast-slow” de-integration phases, two voltage references are required. The primary refer- ence (VREF1) is set to one-half of the full scale voltage (typically VREF1 = 1.6384V, and VFS = 3.2768V). The secondary voltage reference (VREF2) is set to VREF1/64 (typically 25.6 mV). To maintain 15-bit linearity, a tolerance of 0.5% for VREF2 is recommended. FIGURE 3-2: “Fast Slow” Reference De- Integration Cycle Auto Zero 0V Integrator Output End of Conversion Reference De-integrate Signal De-integrate Time 1 RINTCINT∫ TINT 0 VIN(T)DT = VREF TDEINT RINTCINT where: VREF = Reference voltage TINT = Signal integration time (fixed) TDEINT = Reference voltage integration time (variable). Auto Zero 0V Integrator Output End of Conversion "Slow" Reference De-integrate (6-Bit Resolution) "Fast" Reference De-integrate (9-Bit Resolution) Signal Integrate Time |
类似零件编号 - TC850CPL |
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类似说明 - TC850CPL |
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