Forum: µC & Digital Electronics General problem about microcontrollers

von Dragu M. (mircea2012)

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I am a beginer in microcontrollers understanding, and also I am new on 
this forum, so HI all. I have some questions about a few problems :

1. A Sample and Hold circuit is connected to a signal of source 
resistance 2Kohms. Its outputs is connected in turn to an ADC. The 
Sample and Hold capacitor is 90pF.
What aquisition time should be allowed if 10-bit resolution is required 
by the ADC ?

I know that time constant is RC. For this case I obtained 0,18 microsec.

2. A 16F873A microcontroller is operating from a supply of 4V and a 
clock frequency of 1MHz. The ADC is used (a 10-bit ADC module), but only 
8-bit accuracy is required. The signal source resistance is 5Kohms.
Estimate the acquisition time required.
What is the fastest conversion time that is available, assuming each 
conversion is allowed to run to completion ?
Hence estimate the maximum sampling rate possible.

3. Given a sinusoid with 500Hz frequency, what should be the minimum 
sampling frequency for analog-to-digital converter if we want to 
faithfully reconstruct tha analog signal after the conversion ?

Please help me.
Thank you.

von nanotech (Guest)

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Please take your homework elsewhere.

von friedrich (Guest)

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Hello Dragu,

1) If you know that the RC constant is 180ns or 0,18us then you also 
know that the voltage level at that time is roughly 0,63*Vin. If you do 
not know this than please lookup the RC network more in detail.

2) This question looks like from a test or homework. So I will not give 
you the result but help you understanding:
You want to achieve an 8 bit resolution from a 10bit ADC. That gives you 
the relative accuracy you require in %. Then you have given a RC network 
built by the source resistor and the sampling cap. Where would you get 
the sampling cap value from? Now comes 1) into view. You need to 
calculate the RC function of the input in oder to achieve ...

3) If you have done 2) then 3) should be possibel also for you.



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