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Why the shunt resistor is used in a digital buffer?

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redapplepie

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Hello all:
A digital buffer is always made by two cascaded cmos inverters.
I can always see that a shunt feedback resistor is usually connect from the output of the first inverter to its input.
I want to know why the shunt resistor is used? What's its purpose?

By the way, the digital buffer is only used to buffer the reference clock of 20MHz into the PFD of a pll.
 

what is meant by digital buffer and inverter

redapplepie said:
Hello all:
A digital buffer is always made by two cascaded cmos inverters.
I can always see that a shunt feedback resistor is usually connect from the output of the first inverter to its input.
I want to know why the shunt resistor is used? What's its purpose?

By the way, the digital buffer is only used to buffer the reference clock of 20MHz into the PFD of a pll.
Click to expand...

It may be a self-bias the inverter. the purpose is to amplify a sine wave.
Because some time the clk for the pll reference will be a sine rather a square wave.
 

Re: Digital buffer

This technique is sometimes referred to as Replica bias. The output from the crystal oscillator is a sine wave and the digital buffer is supposed to change it into square wave. However, the duty cycle of the square wave is highly dependant on the sine wave's DC and the switching threshold of the second inverter. The first inverter (the one used for bias) supplies a DC value equal to the switching threshold of the inverter (if the same sizing ratios are used) and tracks well across PVT.
 

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