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PGA281: Gain setting time

Roberto Zugarini
Prodigy 10 points
Part Number: PGA281

Hi,

I would like to implement the analog input front-end solution explained in the SBAA551 application note with the PGA281 amplifier:
www.ti.com/.../sbaa551.pdf

This would allow me to go directly to my MCU AD converter and set the gain to G=0.250 for 0-10V input configuration, G=0.5 for 0-5V input configuration and G=2 for 4-20mA input configuration.

In worst case to read 8 analog inputs every 1ms (1KHz) I've to change gain every 125us (4KHz). Is this timing compatible with PGA281? 

Thanks in advance, best regards.

  • 0
    TI__Genius 9435 points

    Hi Roberto,

    From the PGA281 datasheet, the switching time of the gain logic is ~1µs. However, the time to respond to the gain change is effectively the settling time of the amplifier driving the output to the new voltage.

    The settling time for various gain settings is given in the datasheet's electrical table, and is also shown in the typical characteristics section.

    Depending on your settling requirements, the settling time can be 20-40μs for an 8V output step. This is well within your ~125μs acquisition window.

    Keep in mind that you must also consider the 1V/μs input slew rate limitation of the PGA281. For a mux input, the input voltage can change from 0-10V almost instantly, and the amplifier is not fast enough to maintain the "virtual short" between the input and feedback. This produces a large offset voltage that forward biases the back-to-back input protection diodes and turns on the input clamp.

    You will need to add an RC input filter that reduces the slew rate the input to 1V/μs. This input filter will determine the settling time of your system, and can be sized to accommodate the desired acquisition time of your ADC.

    Regards,

    Zach