ICL7106 / ICL7107 Oscillator Frequency

The ICL7106 and ICL7107 require an oscillator clock pulse to operate, and the designers have provided pin 38, pin 39, and pin 40 for this purpose.

External Clock Oscillator

You can inject a clock signal in pin 40 of this IC using an external generator to make it work, and usually a 5 V peak-to-ground square wave is recommended. There is also facility for connecting a crystal between pins 39 and 40 if an ultra-precise timing is required, however, the application of resistor R3, and capacitor C4, is usually adequate for most projects.

RC Oscillator

The simplest, best, and cost-effective way of generating a clock signal is by using a resistor R3, and capacitor C4, connected to the pins in the manner shown above. As you can see, the generated clock signal requires further processing by internal circuits, and the first stage divides the oscillator frequency by four, and the output of that is further divided by the following stages. Eventually it boils down to the number of sample readings per second the internal analogue-to-digital converter (ADC) takes.

The beauty of this IC is that it can reject RF noise in the environment and thereby provide an extremely accurate and stable reading free of any jitter. To reject 60 Hz noise, the circuit performs well when the oscillator frequency is a multiple of 60, hence, 48 kHz, 60 kHz, 80 kHz, 120 kHz, and 240 kHz works best. To reject 50 Hz noise, 40 kHz, 50 kHz, 100 kHz works well. Interestingly, a clock frequency of 40 kHz can reject both 50 Hz and 60 Hz noise, but at that speed, the ADC takes 2.5-samples-per-second. At 48 kHz clock speed, the ADC takes three readings per second, which is good enough for most applications. Hence, a clock frequency between 40 kHz to 48 kHz works best because it rejects 50 Hz and 60 Hz noise. Hence, this is what I shall be using in my circuit. The documentation recommends the optimum value of R3 to be 100 kΩ for all clock frequencies; hence, I shall be using this value in my design.

Calculating Oscillator Frequency

Given that the best clock frequency to be is between 40 kHz to 48 kHz, and the optimum recommended value of R3 is 100 kΩ, a capacitor of 100 pf is all that required. The calculation above shows that using those values for the R3 and C4 gives a clock frequency of 45 kHz. The actual clock frequency might vary slightly due to the tolerance values of the components and stray capacitance. However, I shall be using these values for my digital meter circuit.

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