Hi All, I have a design with 5.3 VCC to SN74123 dual retriggerable monostable multivibrators. Since I am using only one multivibrator, can you recommend a single retriggerable monostable multivibrator with a smaller footprint for me to replace SN74123? I am not sure if the products like SN74LVC1G123 can be used directly.
Thank you for your help!
Hi Joe, thank you so much for your help! I have some additional questions about the Schmitt triggers in my application, Would you advise me?
Currently, in my application, my input is a pure AC signal with zero DC offset. The peak to peak amplitude of this sin signal can be any fixed level between 0.8V to 8V in the steady state operation, while its frequency falls into the range of 1MHz to 5MHz. The sin signal can have some slight distortion in shape.
Right now I am using 74HCT04 to convert this sinusoidal signal to a square wave as the input of 74LS123, to minimize the variation of the sinusoidal amplitude and the distortion.
My question is, do you think using SN74LVC1G123 , with the help of Schmitt triggers, I can remove 74HCT04 inverter? If the Schmitt triggers can converter my input to a square waveform with the same frequency with 0 to 5V amplitude, I guess there is no need for 74HCT04 inverter, the 1nF cap and 100kOhm resistor, is that correct?
Thank you!
Yi Huang,
Yi Huang said:I was trying to use RC as a LPF of the output of the inverter to generate a DC offset at the input of the inverter, to add the offset to the pure AC signal. According to your explanation, I think I should still keep the inverter and the RC components, instead of injecting the AC signal to the SN74LVC1G123, is this correct?
I agree that the AC signal should not be directly connected to the SN74LVC1G123. Can you please tell me more about this AC signal? Why is the AC signal going as low as 0.8 Vp-p? I only ask because even with a DC offset added, the lower voltage signals won't trigger the device.
The SN74LVC1G123 triggers on specified voltage levels VIH and VIL below. This device operating at 5.3V means that in order to register an input as HIGH the voltage needs to hit at least 3.71V, and for a LOW the signal needs to go below 1.59V.
Now, since the AC signal fluctuates in amplitude and frequency, I recommend conditioning the AC signal using DC bias and a comparator instead of the '74HCT04 device with the resistor and capacitor.. This would give you a square wave output that ranges from 0V to 5V, matched to the AC signal frequency. This signal could then be sent to the SN74LVC1G123. The voltage divider resistors can be chosen to achieve a DC bias. I would recommend a bias of 4V to make sure the AC signal is no longer negative. The cap for the HPF should be chosen depending on the frequency of your signal.
Yi Huang said:I have another general question of the operation of this monostable multivibrator. As you know I am using a periodic pulse signal to go to "B" of the vibrator, while A is GND and CLR is high. Can I say the pulse width of my input signal is not important as far as it can generate the pulse with the same frequency at "Q"? Because the pulse width of the signal at "Q" will be determined by REXT and CEXT, the pulse width of the input signal at "B" can tolerate the variation within a reasonable range. Is this a true statement?
The output is triggered based on the rising or falling edge of the input signal. The pulse width of the output is dependent on the Rext and Cext values not the pulse width of your input. You need to also keep in mind the pulse retrigger time. This will increase the output pulse duration if the input is triggered within a specified time after the last trigger. This is also dependent on the Rext and Cext values. Please see page 6 of the datasheet and Figure 7 on page 8 of the datasheet.
Hi Joe,
Thank you very much for your detailed instruction. My AC signal comes from a wireless coil, which may suffer from the misalignment. Therefore, its amplitude may vary within a wide range. You pointed out the item that I did not pay attention before. I will try your proposed circuit to replace the inverter plus RC solution.
I have another question of the description of this circuit block. If my target is to use the rising signal of the input to trigger the output signal only, and to use the REXT and CEXT to determine the pulse width, what is a proper description of the block shown below? Can I simply call it a timer? Thank you!
