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TC7129 Datasheet(PDF) 7 Page - Microchip Technology |
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TC7129 Datasheet(HTML) 7 Page - Microchip Technology |
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7 / 28 page  © 2006 Microchip Technology Inc. DS21459D-page 7 TC7129 3.0 DETAILED DESCRIPTION (All pin designations refer to 40-pin PDIP.) The TC7129 is designed to be the heart of a high- resolution analog measurement instrument. The only additional components required are a few passive elements: a voltage reference, a LCD and a power source. Most component values are not critical; substitutes can be chosen based on the information given below. The basic circuit for a digital multimeter application is shown in Figure 3-1. See Section 4.0 “Typical Appli- cations”, for variations. Typical values for each component are shown. The sections below give component selection criteria. 3.1 Oscillator (XOSC, CO1, CO2, RO) The primary criterion for selecting the crystal oscillator is to choose a frequency that achieves maximum rejec- tion of line frequency noise. To do this, the integration phase should last an integral number of line cycles. The integration phase of the TC7129 is 10,000 clock cycles on the 200 mV range and 1000 clock cycles on the 2V range. One clock cycle is equal to two oscillator cycles. For 60 Hz rejection, the oscillator frequency should be chosen so that the period of one line cycle equals the integration time for the 2V range. EQUATION 3-1: This equation gives an oscillator frequency of 120 kHz. A similar calculation gives an optimum frequency of 100 kHz for 50 Hz rejection. The resistor and capacitor values are not critical; those shown work for most applications. In some situations, the capacitor values may have to be adjusted to compensate for parasitic capacitance in the circuit. The capacitors can be low-cost ceramic devices. Some applications can use a simple RC network instead of a crystal oscillator. The RC oscillator has more potential for jitter, especially in the least significant digit. See Section 4.5 “RC Oscillator”. 3.2 Integrating Resistor (RINT) The integrating resistor sets the charging current for the integrating capacitor. Choose a value that provides a current between 5 μA and 20 μA at 2V, the maximum full-scale input. The typical value chosen gives a charging current of 13.3 μA: EQUATION 3-1: Too high a value for RINT increases the sensitivity to noise pickup and increases errors due to leakage current. Too low a value degrades the linearity of the integration, leading to inaccurate readings. 1/60 second = 16.7 msec = 1000 clock cycles *2 OSC cycles/clock cycle OSC Frequency ICHARGE = 2V 150 k Ω 13.3 µA |
Similar Part No. - TC7129_06 |
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Similar Description - TC7129_06 |
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