MAX3222E: MAX3222E Output Signal Fluctuation

Hi Blanche,

This device (and many other RS-232 transceivers) use a charge pump to take the VCC rail and generate the higher-voltage V+/V- rails that are switched onto the "DOUT" pins to transmit high or low levels to the RS-232 line.  These charge pumps will have some ripple associated with them, and the ripple frequency and amplitude are dependent on the capacitance values used and the load current.  In this case when the DOUT line is high it is switched to V+, and this V+ rail is sourcing some current to the RS-232 line.  This causes the voltage to drop as current is pulled from the V+ charge storage capacitor.  Once the voltage reached a level that is too low, it is recharged by the charge pump and the voltage increases back to its regulated level.

You can read more about the operation of the charge pump circuitry and what factors influence the ripple waveforms in this blog:

https://e2e.ti.com/blogs_/b/analogwire/archive/2018/06/28/how-the-rs-232-transceiver-s-regulated-charge-pump-circuitry-works

Please let us know if you have any further questions.

Regards,
Max