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ADS1235: Some questions about VCOM, noise measurement

Part Number: ADS1235
Other Parts Discussed in Thread: ADS1261

Tool/software:

Hello, professionals.

I have some questions about ADS1235, especialy about VCOM.

Q.1

I saw the Q&A about VCOM and that was answerd VCOM is voltage that is divided by a resistor connected to AVDD and AVSS.

Do you know that internal resister value?

Q.2

Is the datacheet parameters, such as noise was measured by analog input which is internaly connected VCOM?

"input short" in the data sheet means this?

Q.3

(It's not question, wish advice) I wanted to measure ADS1235 input short noise, by short connnected at EVM's input coonector point.

In a different case, I know that the ADS1261 IC has a pin called AINCOM which outputting half the power supply voltage, but the ADS1235 does not have such a pin.

How should I measure it? Should I simply use a resistor divider?

  • Hello Miyashita-san,

    Q1: This information is not readily available, but typically, internal resistor dividers will be in the range of 100kOhm to keep power consumption low.  The VCOM signal is not available outside the device, and the resistor values are chosen to ensure low noise and low power.

    Q2: For both input offset and noise, the term 'input short' is applicable to both an external short at the ADC input pins to mid supply, or using the internal short to VCOM.

    Q3: You have two options.  Option 1 would be to use a resistor divider powered from the AVDD/EXC+ and AVSS/EXC- test points, and then connect to 2 of the 6 available ADS1235 inputs.  The resistor divider can be made up of two equal values in the range of 1kOhm to 100kOhm.  Option 2 is to select +/-2.5V power mode by sliding switch S1 to the left.  Now, both analog inputs can be connected to GND, since GND is now the mid-supply point of the ADS1235 when AVDD=+2.5V and AVSS=-2.5V.

    Regards,
    Keith Nicholas
    Precision ADC Applications

  • Hello, Mr.Nicholas.

    I'm appriciate for your answer and advice.
    I could know new infomations about these devices.

    Thank you,
    Best regards.

    Kazuyuki Miyashita