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  • TI Thinks Resolved

RE: ADS1224: What is the recommendation for enabling the buffer?

Intellectual 910 points

Replies: 1

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Hi Bob

I really appreciate your help. 

There is another question:

Q: Under what condition or application would you recommend enabling the buffer inside ADS1224?

I do understand the enabled buffer will increase the input impedance of ADS1224 greatly, which would guarantee a more accurate sampling that is more close to the actual input signal connected to the input pins. However, compared to the condition without buffer, I also find that an enabled buffer will result in a slightly poor effect on the performance like input voltage range, INL, gain error, and gain error drift, which shown on page 3.

In brief, the buffer is like a double-edged sword and I don't know the suitable usage. This is why I ask this question.

Thanks for your time.

  • Hi Stone,

    I split the thread as the topic changed considerably from the original question.  If the sensor or voltage source has a high impedance output, current through the source could add a voltage drop at the source producing a measurement error.  In other words, you will have a voltage drop at the source and a voltage drop at the analog inputs.  If the ADC input impedance is low, then the voltage drop at the source will introduce a larger error as compared to a high input impedance of the ADC which would limit the overall sampling current and lower the voltage drop of the source.

    As you have mentioned there are some decisions to be made as to whether one error is more significant than another. But there are some other considerations that may dominate.  For example, AVDD is 5V and reference is 2.5V and you need to measure an input signal from 0V to 5V.  In this case you cannot use the buffer enabled as you cannot measure all the way to 0V and the maximum input voltage is 3.5V (5V - 1.5V) with a direct voltage input.

    A different use case may be from a signal source with a 0 to 500mV differential output voltage with a source impedance of 500k Ohms and a common-mode voltage of 2.5V.  In this case you could either use buffer enabled or disabled.  The datasheet shows the input impedance using a 2MHz fCLK is 2.7M Ohm with buffer disabled and 1.2G Ohm with buffer enabled.  As you can see in this case the buffer disabled will have much bigger impact as compared to the buffer enabled.  At the ADC inputs voltage will be Vo (2.7M/3.2M) or about a 15% drop in the sensor output voltage for buffer disabled.  For the buffer enabled Vo(1.2G/1.2005G) or about a 0.04% difference.

    One further consideration is how the input impedance relates to the fCLK frequency of the ADS1224.  The input impedance scales inversely with the fCLK frequency for the buffer disabled.  So the changing the frequency from 2MHz to 870kHz, you would see an approximate increase in the impedance of about 2.3 times 2.7M Ohms (or 6.2M Ohms increase for this case).

    Best regards,

    Bob B

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