ADS8920B: ADC KICKBACK FILTER DESIGN

HARI T O1

Part Number: ADS8920B
Other Parts Discussed in Thread: ADS8922B, ADS8924B

Hi Team,

I have a question regarding the KICKBACK Filter design in the ADS892xB data sheet. According to the datasheet it has three sampling rates.

ADS8920B: 1-MSPS =1Mhz
ADS8922B: 500-kSPS =500Khz
ADS8924B: 250-kSPS=250Khz

As per my knowledge the cut off frequency of the KICKBACK Filter is sampling frequency/2. Please correct me if I am wrong.

Please see the below figure obtained from the datasheet. If we calculate the cut off frequency of the RC filter given in the below
figure it is 7Mhz. Which is more than half of the sampling frequency of all three models.

How the resistor value became 2.2ohm

May I know where I went wrong.

Regards
HARI

over 2 years ago

0 Collin Wells over 2 years ago

TI__Guru 62475 points

Hi Hari,

Your understanding of anti-aliasing filters is correct. However, an anti-aliasing filter is usually applied much before the ADC drive inputs in the signal conditioning circuitry. The primary purpose of the Rflt/Cflt components and the driver amplifier before it, is for charge kick-back to properly settle the internal sampling cap to <1/2 LSB within the acquisition period (tacq). The full theory is reviewed in this video series:

Successive-approximation-register (SAR) analog-to-digital converter (ADC) input driver design: These videos describe how to design the input driver circuitry for a SAR ADC. https://training.ti.com/node/1139106?context=1139747-1140267-1128375-1139106

0 HARI T O1 over 2 years ago in reply to Collin Wells

Expert 1265 points

Hi Collin,

Thank you.

Regards

HARI

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ADS8920B: ADC KICKBACK FILTER DESIGN