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Voltage comparator and shmitt trigger

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wanneng

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shmitt trigger

I am now designing a degital frequency measurer,the digital part is completed ,and the analog channel is now under process,and I have some problem ,the input signal is from 1hz-10Mhz,and I must make the sine wave to rectangular wave,but the frequency below 1khz is mesured
rong,(the last two digital is random,5 digital total),but if the input signal is rectangular wave ,and the measurement is right,but the wave is the same as that the input signal sine wave. :) ,I want to need some information about voltage comparator and the shmitt trigger.
 

voltage comparator

Look at the digital waveform out of your comparator to see if it is oscillating on the transitions.

To add the hystersis to your comparator circuit, put a large value resistor from the digital output to the non-inverting analog input. Then put a small resistor from the non-inverting analog input to ground. Apply the signal to me be measured to the inverting input of the comparator.

The amount of hystersis is the logic level swing (such as 5 V) multiplied by the resistor ratio. Using 1 Meg and 1 k ohms will give you 5 mV. There is a tradeoff of the hystersis value. It sets the smallest analog signal that you can mesure. Making it too small will not prevent oscillations at the transitions.
 

shmitt triger

hi
look at the national semiconductor v/f f/v convertor and comparo tor linear IC products app.
 

trigger comparator

Tkanks,but the output waveform have no oscillating,but its seting time is about 250ns,I realy don't konw what to do next?
 

lm311 digital trigger

It may be an unstable source you are using for test. Try measuring your test oscillator with a commercial frequency counter. Also try making your own low frequency source with a crystal oscillator, digital divider, and low pass filter to make the output a sinewave.
 

comparator

Thanks,I measure the wave with a commerical frequency counter,and the result is right,is the voltage compatator is not precise? I am using the mxl1016,and then attach to a lm311.but the precise is not meet my require.(0.01%)
 

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