ICL7106 / ICL7107 Integrate Values

The integrate capacitor CINT connects to pin 27, and the integrate resistor RINT connects to pin 28. These components are marked C3 and R2 respectively on the documentation circuit, and their purpose is to determine the full-scale reading. There are two options for the full-scale reading, and my design shall be utilising the 2 V option.

Calculate R₂

We require a full-scale swing of 2 V therefore V_INFS is set to 2. The documentation suggests that the optimum value of I_INT is 4 µA. This value comes from the integrators ability to supply 4 µA of drive current and still remain within the linear region of its characteristics. As you can see from the calculation above, by using the optimum recommended values for a 2 V full scale reading, R₂ is calculated to be 500 kΩ. Therefore, a 470-kΩ resistor is the nearest standard value that should work.

Calculate t_INT

Before the value of CINT is calculated, the integration period t_INT is first required, using the formula given. Since the typical oscillator frequency is 48000 Hz, t_INT is 0.08333.

Calculate C₃

We have all the values needed to substitute into this formula. The value of t_INT was calculated earlier, I_INT is the optimum value given for the reasons stated earlier in this page, and V_INT is the full reading of 2 V for this circuit. Therefore, the calculation gives 0.17 µF for the value of the integrate capacitor C₃.

According to the documentation, when the clock frequency is 48 kHz, there will be three readings per second taken, and the best nominal value for CINT is 0.22 µF. When the analogue common provides a reference, a 2 V full-scale reading is fine for this value.

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