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ADC121S021: Analog input Front Buffer amp

Part Number: ADC121S021
Other Parts Discussed in Thread: ADS7042

HI staff

In the current app, the NJL7502R phototransistor is directly connected to the analog input terminal of the LTC2361.
ADC121S021 is selected instead of ADI's AD converter.

The ADC121S021 does not have a buffer on the analog input. Is it okay to connect the tactile sensor?
Also, I would be grateful if you could introduce a device that is suitable as an alternative.
(It is especially important to have a small pkg)

NOTE:
At first, I chose ADS7042 as a candidate, but I excluded it from the candidates because a buffer amplifier is required before AD.

best regards
cafain

  • Hello, 

    Apologies for the late response. A phototransistor can be used in many ways, it can be used to control a switch, or to trigger an action using a comparator. Would you share how you are trying to use the phototransistor?  It is most likely that you will need a signal conditioning circuit along with the phototransistor, but the basis of this circuit will be based on the application needs. 

    Regards

    Cynthia

  • Hello Cynthia san

    Please comment and help

    The detailed specifications of the app have not been decided yet.
    It is at the conceptual stage whether the specifications can be realized.

    We plan to adjust the drive current to the phototransistor with a potentiometer and measure the voltage with an ADC.
    The current circuit is planned and the drive current is assumed to be a DC signal.

    best regards

    cafain

  • Hello Cafain san

    This is a normal use case. While designing the circuit, note the sampling rate of the ADC. 

    When the phototransistor is placed in series with the potentiometer, the value of the potentiometer will determine the impedance of the circuit. If the ADC is sampling at a low rate ~10kHz, then this may work fine. But if the ADC is sampling at 1MHZ, then an op amp will be needed to drive the sensor signal to the ADC. 

    Regards

    Cynthia

  • HI, Cynthia san

    A meaningful hint. I would like to make use of it for designing.

    Please introduce the operational amplifier used in the recommended front end. (Vcc = 3.3V, RRIO, Low Offset & Drifte,> 10MHz) .

    thank you very much

    cafain

  • I did a quick parametric search based on supply voltage and GBW, and four devices were listed ~click here~

    but I have reassigned this thread to the op amp team for deeper support on this. 

    Regards

    Cynthia

  • Hey Cafain,

    Here is an updated list of devices that have rail to rail input output and 10MHz BW. What is the targeted spec for offset and drift so I can further refine the search?

    Best,
    Jerry

  • Hi,Jerry san

    Thank you.

    We will review the application specifications again and select the device, so please contact us again at that time.

    regards

    cafain