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ADC121C021-Q1: Min/Max Input Impedance and Max Conversion Time

Art Mecina
Genius 16585 points
Part Number: ADC121C021-Q1

Hello Guys,

Good day.

One of our customer is looking to find the formula for the acquisition time and saw that it can be extracted from the definition of the Throughput Time of the ADC121C021-Q1 datasheet. Wherein THROUGHPUT TIME = acquisition time + conversion time.

1) He wants to know the maximum conversion time to be use in his calculations of acquisition time. 

2) What is the Min/Max Impedance for this part? If his signal source has an output impedance of 10k Ohm, will this ADC function properly?

Thanks in advance!

Art

  • 0
    TI__Mastermind 24970 points

    Hello,

    1. the conversion time is 1us. this is the value the customer should use. 

    2. This device is a SAR architecture, which means it is uses a switched capacitor input to sample the input. because of this, the input impedance varies, based on the sampling rate. The input must be able to drive the input signal and have it settle within the acquisition time. The input usually has a small capacitor (pico Farads) to help supplement the sample-and-hold capacitor's charge.  

    Regards

    Cynthia

  • 0 in reply to Cynthia
    TI__Genius 16585 points

    Thanks for looking into this Cynthia!

    For item#2, Is there any range of input impedance that can be provided to the customer?

    If the input impedance is large, how will the ADC behave?

    Thanks to confirm!

    Art

  • 0 in reply to Art Mecina
    TI__Mastermind 24970 points

    There is not a range that can be provided.

    If you share what the input source is and the sampling rate the device is using, I can help optimize the circuit to make sure the input is settling. 

    Is the customer seeing a settling issue? meaning the output conversion data is close to the input voltage but not quite it?

    I would initially suggest adding a 680 pf or 860 pf cap at the ADC input to ground. 

    Regards

  • 0 in reply to Cynthia
    TI__Genius 16585 points

    Hi Cynthia,

    Thank you very much for the response. I have relayed to the customer your suggestion and inquiry.

    I might point the customer to this thread directly so that he could explain his use case better, if he still needs further assistance.

    Thanks!

    Art

  • 0 in reply to Cynthia
    Prodigy 20 points

    Hello, regarding the source driving the ADC121C021Q; I will be using the ADC to sample the analog output of the LX3302A Inductive Sensor IC with data sheet attached below. Note that the output impedance of the LX3302A analog output is 10K Ohm typical, with min and max values of 0.9k Ohm and 100K Ohm. I would like to implement a first order RC filter and have done some analysis regarding choosing R and C vales to ensure that the ADC121C021Q can adequately charge while sampling. I would prefer not to use a buffer if not necessary due to space limitations for my design. The sampling rate of the ADC121C021Q will be 22.2ksps for my current design. Note that I have not implemented any circuitry with the ADC121C021Q yet, I am still in the design phase.

    Please let me know if I can provide any more information regarding my design. Thank You very  much for your assistance. 20006306B.pdf

  • 0 in reply to mitchell jefferies
    TI__Mastermind 24970 points

    Hello Mitchell, 

    At this low speeds of signals, a buffer might not be needed. 

    The RC circuit at the output of the sensor should be sized to help replenish the ADC's sample and hold capacitor, thus the cut off would not really help as filter.

    I would suggest using an capacitor, in the hundreds of pico Farads (300 - 910). The capacitor will help to replenish the C_sh capacitor. Note that the bigger the capacitor, the longer the start up time will be for the system as the sensor will need to initially charge this cap. 

    Regards

    Cynthia

  • 0 in reply to Cynthia
    Prodigy 20 points

    Thank You, Cynthia.

    This was very helpful.