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CD4050B: Input thresholds of the CD4050B - hysteresis?

Alexander A
Intellectual 655 points
Part Number: CD4050B
Other Parts Discussed in Thread: CD40106B

Dear all,

I am trying to properly understand the datasheet of the CD4050B Hex Buffer IC. It is supplied with 15V and decoupled with a 100nF (not shown in the schematic). Input D (pin 9) is connected to a potentiometer such that a voltage between 0V and 15V is applied to pin 9. On pin 10, I measure the output depending on its input (pin 9). the purpose is to see the threshold voltages at which the output changes its state.

My understanding according to the datasheet (Table 6.5):

Output change from high to low:

When input voltage is below 4V

Output change from low to high:

When input voltage is 11V

My measurements, however, show that at 7V on the input, the output changes its state (for both low to high scenario and high to low scenario). This would mean that there is no hysteresis behavior on the CD4050B IC.

Am I correct in my understanding that the CD4050B does not have a hysteresis like behavior? If so, how can I interpret the values from Table 6.5 then?

Thank you.

Below the relevant graphs:

  • 0
    TI__Guru* 96681 points

    Hi Alexander,

    The short answer is that this device does not have hysteresis on the inputs.  You would need a Schmitt-trigger input device for that (such as CD40106B, a Schmitt-trigger input inverter).

    I believe your confusion stems from a definition problem.

    VIH and VIL are defined as guaranteed "high input" and "low input" values -- ie, if your input voltage is equal to or greater than VIH, then the device will recognize that input as a logic high. They do not indicate a switching threshold.

    The threshold voltage (which is what you are measuring) is typically referred to as VT. This value is not typically defined in the datasheet since you should not be operating a CMOS input near the threshold (this can cause damage to the device, excessive power consumption, and unpredictable results). Usually this value is about half of your supply.

    There are three additional values you will find in devices with hysteresis (called 'Schmitt-trigger input' devices) which are VT+, VT- and ΔVT. These values indicate the low to high switching threshold, the high to low switching threshold, and the hysteresis value (respectively). If you see those values in the datasheet, then the device has true Schmitt-trigger inputs, and those inputs have the indicated hysteresis.