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ADS127L11: Ask about ADC delay

ETHAN ZHOU
Prodigy 40 points
Part Number: ADS127L11
Other Parts Discussed in Thread: ADS8900B, ADS8910B, ADS8920B

Tool/software:

Hi, I have three questions about ADS127L11.
1️⃣In one-shot mode, is there a latency time every time the START pin becomes high level？
2️⃣ What is the relationship between programmable conversion-start delay time and latency time? Are they superimposed or overlapping?
3️⃣ The PWM frequency of controller is 200kHz. When the waveform oscillates, the sampling window is only 1 microsecond per cycle. In this case, what configuration should I use? Can the sampling be realized by controlling the delay?

  • +1
    TI__Mastermind 46760 points

    Hello Ethan,

    1. Yes, each time START is asserted high, the conversion time will be equal to the latency, specified in Tables 8-3, 8-5, 8-6, and 8-7 depending on the filter configuration.

    2.  The conversion-start delay is in addition to the latency time.

    3.  The ADS127L11 uses an internal digital filter, which will filter all harmonics of the PWM signal.  The resulting conversion results will look more like the fundamental sinewave, than a square wave.  If you want to recreate the PWM signal with more frequency content, then a SAR data converter such as ADS8900B will be more appropriate.

    Regards,
    Keith Nicholas
    Precision ADC Applications

  • +1 in reply to Keith Nicholas
    Prodigy 40 points

    Hi, Keith, thank you for your reply. As for the third question, I didnt describe it clearly. A clearer description is that the PWM voltage of 200kHz is applied to the motor winding to generate current, which can be regarded as an LR load. In a very short time, the current can be regarded as direct current. Due to the high-frequency (several MHz) oscillation of the current during the switching process of the power device, the ADC must be sampled within a 1 microsecond window after the oscillation ends. In this case, can it be sampled by configuration delay? How to configure this ADC?

  • 0 in reply to ETHAN ZHOU
    TI__Mastermind 46760 points

    Hello Ethan,

    Since you need to wait for the oscillation to end, and then capture the data within a 1us window, this will not be possible with the ADS127L11.  The ADS127l11 continuously samples data, and when using the highest data rate setting to achieve 1MSPS, each result is the average of the current reading plus the three previous readings, and will require at least 4us.  Since you need to start sampling at a precise time, you would need to use the START signal to control the timing, and in this case, the total conversion time will be 4.38us.

    We have several other ADCs that are designed for this exact type of measurement.  I had mentioned the ADS8900B, which is a 20b SAR ADC and can capture the result and provide a reading in 1us time.  There are also lower cost versions of this ADC, including the ADS8910B (18b) and the ADS8920b (16b) that would work as well.

    Regards,
    Keith