ADS1225: Input RC filter.

Kengo.Y

Part Number: ADS1225

Hi team,

Is there an upper limit to the resistance value of the RC filter placed at the input?
If the resistance value is too high, will the ADC result malfunction?

If you have a document that determines the constants, please let me know.

Sincerely.
Kengo.

over 2 years ago

0 Bryan Lizon86 over 2 years ago

TI__Guru 51666 points

Hi Kengo Y

What values do you want to use for your system?

When selecting the RC filter resistor size, you should consider that the ADC leakage current can introduce error, and this error will increase as you increase the size of the filter resistors.

You can review the FAQ below for more information about some considerations for selecting the RC filter components

https://e2e.ti.com/support/data-converters-group/data-converters/f/data-converters-forum/955466/faq-delta-sigma-adc-anti-aliasing-filter-component-selection

-Bryan

0 Kengo.Y over 2 years ago in reply to Bryan Lizon86

Guru 11155 points

Hi Bryan,

Thank you for your comment.

E2E says that ADC leakage current and filter resistance are related.

Actually, the error was larger with 10kΩ and smaller with 1kΩ.
The leakage current of the ADC is not listed in the datasheet, but how much is it?

Sincerely.
Kengo.

+1 Bryan Lizon86 over 2 years ago in reply to Kengo.Y

TI__Guru 51666 points

Hi Kengo Y,

It makes sense that the error is larger with the 10kohm resistor compared to the 1kohm resistor, because the error magnitude should be ~10x worse when using the 10kohm resistor.

The leakage current can be estimated indirectly from the Differential input impedance specification in the ADS1225 datasheet.

With the buffer off, this is 2Mohm (typical), or ~0.5uA. With the buffer on, this is 1Gohm (typical), or ~1nA.

-Bryan

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ADS1225: Input RC filter.