When the power is turned off, the Transmitter Output pin is generated oscillation situation.
Is this caused by the internal charge pump?how to improve it?
Under normal circumstances :
The oscillation should be caused by the internal charge pump circuitry as you could find the V- tried to follow the Vcc, did you have large capacitors to bypass the power rail? What's the capacitors value you are using for ADM213E? I can see the long power off time (>100ms from fig.1), I would suggest you decrease the capacitor value to enable a fast power off or just use the shutdown feature which could disable the charge pump switching when shutdown enabled.
1.We use a 0.1uF capacitor
2.Where do you mean the big capacitor? ADM213E VCC-PIN ??
Yes, if you have large bypass capacitors between VCC and GND, it will take more time to discharge. Have you solved this issue?
If you are talking about the large capacitance of the ADM213 VCC-PIN,
we only put a 0.1UF capacitor to the ground.
We will use EN-PIN and SHUTDOWN-PIN to avoid this oscillation,
but we need to explain to the customer why it will oscillate.
What should we explain? Is it because the discharge time of the charge pump is too long?
The long discharge time of charge pump when suddenly unpowered relates to the storage capacitor value, also might be due to the light load consuming not much current on the transmitter output.