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TC7129CKW Datasheet(PDF) 11 Page - Microchip Technology |
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TC7129CKW Datasheet(HTML) 11 Page - Microchip Technology |
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11 / 28 page  © 2006 Microchip Technology Inc. DS21459D-page 11 TC7129 Figure 4-6: Temperature Compensating Circuits. 4.5 RC Oscillator For applications in which 3-1/2 digit (100 μV) resolution is sufficient, an RC oscillator is adequate. A recom- mended value for the capacitor is 51 pF. Other values can be used as long as they are sufficiently larger than the circuit parasitic capacitance. The resistor value is calculated as: EQUATION 4-1: For 120 kHz frequency and C = 51 pF, the calculated value of R is 75 k Ω. The RC oscillator and the crystal oscillator circuits are shown in Figure 4-7. Figure 4-7: Oscillator Circuits. 4.6 Measuring Techniques Two important techniques are used in the TC7129: successive integration and digital auto-zeroing. Successive integration is a refinement to the traditional dual-slope conversion technique. 4.7 Dual-Slope Conversion A dual-slope conversion has two basic phases: inte- grate and de-integrate. During the integrate phase, the input signal is integrated for a fixed period of time; the integrated voltage level is thus proportional to the input voltage. During the de-integrate phase, the integrated voltage is ramped down at a fixed slope, and a counter counts the clock cycles until the integrator voltage crosses zero. The count is a measurement of the time to ramp the integrated voltage to zero and is, therefore, proportional to the input voltage being measured. This count can then be scaled and displayed as a measure- ment of the input voltage. Figure 4-8 shows the phases of the dual-slope conversion. Figure 4-8: Dual-Slope Conversion. The dual-slope method has a fundamental limitation. The count can only stop on a clock cycle, so that mea- surement accuracy is limited to the clock frequency. In addition, a delay in the zero-crossing comparator can add to the inaccuracy. Figure 4-9 shows these errors in an actual measurement. TC7129 1N4148 5 k Ω 75 k Ω 200 k Ω 39 k Ω 19 36 24 23 V– V+ VDISP DGND TC7129 2N2222 39 k Ω 19 36 24 23 V– V+ VDISP DGND 20 k Ω 18 k Ω R = 0.45 Freq * C TC7129 TC7129 1 40 2 270 k Ω 10 pF V+ 120 kHz 5 pF V+ 1 40 2 51 pF 75 k Ω De-integrate Zero Crossing Time Integrate |
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