0
\$\begingroup\$

I am trying to convert an input sine wave to a square wave. The sine wave is centered at 2.5V and has an amplitude of 200mV, with a frequency of 100kHz.

I am looking for a hysteresis of 100mV centered around 2.5V. (ie. 50mV below and 50mV above 2.5V)

I am trying to achieve something like the following , where both waves are centered about 2.5V:

enter image description here

To do so I looked through Design with Operational Amplifiers and Analog ICs, and I thought that I could work with a VTC offsetting single-supply inverting Schmitt trigger.

enter image description here

I then calculated the resistance values according to my parameters.

VTH = 2.55V
VTL = 2.45V
VCC = 5V

enter image description here

Where I get:

  • R1 = R2 = 1kΩ
  • R3 = 1kΩ
  • R4 = 50kΩ
  • R5 = 100Ω

Testing this on proteus:

enter image description here

yields undesired results:

enter image description here

I was expecting the hysteresis of 100mV and not 200mV.

On closer inspection, VTL is 2.4Vand VTH is 2.6V. This can be remedied by increasing R4 while keeping R3 constant. I would like to know why my initial calculations were incorrect.

Any suggestions on what I may be doing wrong would be appreciated.

\$\endgroup\$
  • \$\begingroup\$ How did you arrive at the value for R5? It seems much too high to me...like by 100X. \$\endgroup\$ – Elliot Alderson Jan 8 at 21:06
  • \$\begingroup\$ Try making R5 << R4. \$\endgroup\$ – glen_geek Jan 8 at 21:06
  • \$\begingroup\$ Thanks for pointing out. Indeed that is a big mistake. Updating question immediately \$\endgroup\$ – Rrz0 Jan 8 at 21:10
  • \$\begingroup\$ Well 100 ohms is too low. Try something around 5k\$\Omega\$. Be aware that the output low voltage for an LM339 can be as high as 400mV and is typically 130mV (at room temperature, sinking 4mA). \$\endgroup\$ – Elliot Alderson Jan 8 at 21:26

Your Answer

By clicking "Post Your Answer", you acknowledge that you have read our updated terms of service, privacy policy and cookie policy, and that your continued use of the website is subject to these policies.

Browse other questions tagged or ask your own question.