LMP90080-Q1: input path filtering

Sharan Ghimire

Other Parts Discussed in Thread: LMP90080, ADS1148, LMP90080-Q1

Hello Bryan,

Since I will be doing radiometric measurements and also implement some low pass filtering. I am reading an app note from TI (reference SBAA 201).

I believe this app report refers to ADC family ADS 1248 ,1148 and not the one I am planning to use. So I am wondering what would be the recommended values for differential and common mode capacitors and resistors (reference and/or bias resistors) used in the filter.

Best,

Sharan

over 3 years ago

0 Bryan Lizon86 over 3 years ago

TI__Mastermind 40191 points

Hi Sharan,

Yes, the ADS1148 would be the next generation version of the LMP90080.

You can review the following FAQ for more information about selecting the resistor/cap values: https://e2e.ti.com/support/data-converters/f/73/t/955466?tisearch=e2e-sitesearch&keymatch=faq%3Atrue

-Bryan

0 Sharan Ghimire over 3 years ago in reply to Bryan Lizon86

Prodigy 110 points

Bryan,

That helped.

But going through the LMP90080 I don't see a modulator frequency specified like on the ADS124S08 ( one you have on example. like how clock frequency is divided by 4).

So i am just taking internal clock frequency as that to calculate the cutoff frequency. Let me know if that's a correct way to go for those ADC who don't specify mod frequency.

Thanks.

0 Bryan Lizon86 over 3 years ago in reply to Sharan Ghimire

TI__Mastermind 40191 points

Hi Sharan,

Yes, I looked through the datasheet and some old e2e posts and could not find a definitive fMOD for the LMP90080-Q1.

Another option is to design the cutoff frequency to be ~10x greater than the data rate you need to run at. Fortunately the LMP90080-Q1 is a relatively slow ADC (214 SPS max at CLK = 3.5 MHz), so setting the cutoff frequency to be 2kHz provides 20dB of rejection at 20kHz, which should be far below the ADC's actual modulator frequency. You could likely set the filter cutoff to be 10kHz for extra margin, but I will leave that determination up to you.

-Bryan

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LMP90080-Q1: input path filtering