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CD4541B: Frequency difference - calculated vs measured

Lyle Cain
Prodigy 20 points
Part Number: CD4541B
Other Parts Discussed in Thread: SN74LVC1G14, SN74HCT74, CD74HC4040, CD74HC163

I am contracting with a customer and need your help with their circuit that uses the CD4541B. The frequency we measure on the bench does not equate to the formula and configuration details in the data sheet. Rtc is 56K, Ctc is 0.5pF and RS is 113K. The device is configured to divide by 256. The frequency of the square wave being output from the CD4541B measures to be 6.41 kHz. However, using Rtc & Ctc to calculate f, then dividing by 256 gives a calculated output frequency of 60.7 kHz. Off by a factor of 10.

  • 0
    TI__Genius 14140 points

    Hi Lyle,

    Your Ctc value is very low. Our devices typically have a capacitance of at least a few pF just at the I/O. I would increase your capacitance to at least 50pF and adjust your resistor to 143kΩ. You could also use a Schmitt trigger inverter (Ex: SN74LVC1G14) with an RC circuit to produce the desired output frequency as shown below: 

    If you go this route you must adjust the capacitance as needed to get the exact output frequency but 0.75nF should get you pretty close. 

    Regards,

    Sebastian 

  • 0
    Prodigy 20 points

    Excellent! Your recommendation takes me in the direction I wanted to go. I chose 0.5pF through trial & error only because it worked. I was hoping to change to more typical component values, but found that calculations to move them didn't match the circuit operation. I also wanted to find a simpler design since we don't need all the flexibility and precision is not important, just consistency. Let me see what the supply chain allows me to use for this and get back to you. Thanks!

  • 0
    Prodigy 20 points

    I should have included that this is a 5V circuit and I need the clock to output a 5V waveform. I've found a few details on the internet indicating your circuit has a 1V amplitude. What is my next alternative?

  • 0 in reply to Lyle Cain
    TI__Genius 14140 points

    Hi Lyle,

    For the Schmitt trigger oscillator circuit, if you are using the SN74LVC1G14 it is able to output up to 5.5V. 

    Regards,

    Sebastian 

  • 0 in reply to Sebastian Muriel
    Prodigy 20 points

    Okay, good. Is it also capable of collectively driving 1 CP input each of an SN74HCT74, CD74HC163 and CD74HC4040?

  • 0 in reply to Lyle Cain
    Guru 254430 points

    Yes (CMOS inputs have an extremely high impedance).

  • 0 in reply to Clemens Ladisch
    Prodigy 20 points

    I breadboarded the circuit and viewed the square wave. It looks good. I found R & C that gave me the ~6kHz I need. Do you have any idea of its stability? I don't need a precise frequency but do need something that holds itself somewhat steady. About +/-300 Hz would work (roughly 5%). I can find R & C that remains stable over temperature and time. Will stable R & C be enough to accomplish that?

  • 0 in reply to Lyle Cain
    TI__Genius 14140 points

    Hi Lyle,

    Since the stability heavily depends on the RC components, it's likely the circuit won't remain stable over temp/time. You could try using RC components with a tolerance of 1% for the best stability. If that doesn't meet your requirements I would suggest using a crystal oscillator for the best stability.

    Regards,

    Sebastian 

  • 0 in reply to Sebastian Muriel
    Prodigy 20 points

    I appreciate the guidance. It looks like I won't have the time to implement the crystal, or any other change that is significant. Is there a reason I'm being nudged away from the 4541? It remains stable but the formula problem forces me to find the frequency we need through trial and error.

  • +1 in reply to Lyle Cain
    TI__Genius 14140 points

    Hi Lyle,

    The main reason I offered another solution is because the internal oscillator of the 4541 will likely produce a fairly large variability to the calculated time (perhaps more than 20% depending on component tolerances). If the 4541 is stable enough at your desired output frequency then it should work for your application. The formula can be off quite a bit so it's likely you will have to keep finding the frequency through trial and error. 

    Regards,

    Sebastian