• TI Thinks Resolved

TS5A23159 Ringing?

Hello,

     Looking forward to your help in sorting out this issue. 

The objective is to switch between an analog voltage and ground at high switching speeds with reasonable current drive capability (about 20-30mA). 

The signal sequence is as follows: a microcontroller generates the analog voltage (needs to be software configurable and hence the uC). The uC output is connected to one input of a TS5A23159 analog SPDT, the other input being shorted to ground. The output of the switch is connected to a high speed opamp OP4830 configured in unity gain mode. 

The problem is as follows: The output rings as the switch transistions from ground to analog voltage. Following is the oscilloscope screenshot

The output has to transition from 0 to ~3.5V when this happens. Further, the ringing does not end in the output settling to 3.5V there is some sort of capacitor charging behavior following up.

Following is the screenshot (zoomed out and a larger time frame)

  

The above measurements are with no-load at the output of the opamp. 

Please advise on the following:

1) How to reduce the ringing

2) The output has to rise from 0 to 1V in less than 200ns (the switching speeds of the chain components add up to this number). 

What could be the reason for such a response?

Thank you for your time in looking into my case,

  • P.S. By no load I mean other than the scope.
  • In reply to SM:

    Following is the schematic

  • In reply to SM:

    Hello,

            The ringing problem has been solved (in a certain sense). The ringing was being caused due to the oscilloscope probe. Adding a series resistor around 5K (to the tip of the oscilloscope probe) stopped the ringing completely (but also made the rising edge less sharp). 

    However the ~10us capacitive charge like waveform in the transition from zero to high voltage still persists. Please advise. 

    The slew rate of the opamp used is 560V/us so that should not be the cause. 

  • In reply to SM:

    SM,

    To help debug the TS5A23159 can we take a look at the scope shots of the COM1, NC1, NO1, and IN1 on one screen so we can see if there is is an issue with input and output of the switch during switching?

    Thank you,
    Adam
  • In reply to Adam Torma:

    Hello Adam,

             Thank you for your reply and time. 

    The yellow waveform is the input at the NO port and the blue waveform is at COM (note the difference in voltage scales). 

    We also tried the alternate approach of using the trimpot to feed the voltage at the NO port (please refer to the schematic). The behavior is same but the delay is much less now. Following is a picture of the waveforms,

    Looking forward to your input,

    SM

  • In reply to SM:

    SM,

    The wave forms look good from a switch perspective.  The switch is passing the input to the output with no distortion.  We will need to look elsewhere in the circuit for your extra loading. It appears the when you switch to the NO path that your power supply is getting loaded and cant immediately drive the input to the op amp.  I'm going to move this thread to the op amp forum for more assistance on that part of your circuit.  

    Thank you,

    Adam

  • In reply to Adam Torma:

    SM,

      Can you please post a screenshot of the opamp output after adding the series 5kOhm so I can see what we are dealing with. The reason you are seeing a slow rise time is because the 5kohm series resistor along with the probe capacitance forms a low pass filter which will slow down the edge rise/fall time.

    Also, can you configure the amplifier in a gain of 2 (6dB). If you look at figure 2 you will notice that in unity gain config, there is 5-6dB of peaking in the frequency response. This indicates insufficient phase margin and can lead to ringing and other stability issues.

    Please do a before and after screenshot so I can see what issues were resolved with these 2 fixes.

    Finally what is the output impedance of the uC analog output?

  • In reply to Adam Torma:

    Hello Adam,

                   We disconnected the op-amp from the path and the ouput is still the same so I am quite sure that the problem is with the switching circuit. Again, we have tried both the potentiometer and the microcontroller's DAC as sources of voltage and in both cases there is a drop at the output; the only difference being the time required to recover. 

    SM

  • In reply to SM:

    SM,

    Thank you for trying additional debug steps. If the only parts in your circuit are power supplies and the switch you may have uncovered something we haven't seen before. Have you tried other TS5A23159 ICs and they see the same behavior?

    I have ordered some samples of the this device so I can try and bread board your circuit to see if I can see the same slow rise time when switching from NC to NO. Devices take 1-2 days to show up on my desk and will need another day to solder the parts down on some boards. I should have some scope shots for you by the end of next week.

    Thank you,
    Adam
  • In reply to Adam Torma:

    Hello Adam,

                Yes, the circuit now is only the switch and power supply.

    Looking forward to your inputs.

    SM