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PGA305: PGA305 Response Time in Current Mode

Lori Ferger52
Intellectual 495 points
Part Number: PGA305

We have a PGA305 configured for current mode which has a response to a step input as shown below.  This was measured as a voltage across a 250 ohm resistor in the current loop.  

Why is the output in such coarse steps?  As a digital chip,  I expect quantization but a 500 microseconds step duration seems very long.   

Also, the 10 to 90% response time is just under 6 milliseconds.  This is strictly the response of the electronics and not the sensor.  Would you expect the PGA305 to be this slow with no filtering on?

P_GAIN = 100, PADC_GAIN = 1 and the filtering is turned off. 

  

  • 0
    TI__Mastermind 31655 points

    Hi Lori,

    The operating characteristics table (section 6.21) in the datasheet describes some of the parameters you're describing. The output time is 512us, which is about what you're seeing here. The response time for a step response to 2.5V is roughly 1.7ms. This is characterized in 0-5V absolute output voltage mode, so only going to half of the full scale output. If you're going up to the full scale, I would expect it to take longer, and 5.6ms doesn't seem entirely out of the question.

    There is also a non-zero amount of startup time, so if the device has been powered up and immediately given a step response, there will be an additional delay. The time from startup to a settled response is about 9.6ms.

    The timing that you are seeing doesn't seem to be out of the ordinary.


    Regards,

  • 0 in reply to Scott Cummins
    Intellectual 495 points

    Is the "output rate" defined as the rate that the DAC output is updated?

    The output response of the DAC to the step input was measured as 2.475 volts. It seems from your statements that it should take about 1.7 mS.  I will look at analog filtering to see if there is an issue.

  • 0 in reply to Lori Ferger52
    TI__Mastermind 31655 points

    Hi Lori,


    That's correct. The DAC output is updated each time there is a new compensated output from the internal compensation engine.

    The measurements from the datasheet were done unloaded and in voltage mode, so load could have affected the total step response time.


    Regards,