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ads8699: the 2nd order LPF settling time

Liu Chih Wang
Prodigy 20 points
Part Number: ADS8699
Other Parts Discussed in Thread: ADS8691, ADS8695,
AS datasheet  page1
• 18-Bit ADC with Integrated Analog Front-End
• High Speed:
– ADS8691: 1 MSPS
– ADS8695: 500 kSPS
– ADS8699: 100 kSPS
Analog Front-End  had "2nd-Order LPF"
Page 5 
INPUT BANDWIDTH
f–3 dB Small-signal Input bandwidth –3 dB All input ranges 15kHz
f–0.1 dB Small-signal Input bandwidth –0.1 dB All input ranges 2.5kHz
I think that is about the 2nd-Order LPF。
as 18Bit ADC for 0.01% resolution is easy and with 100KMPS,
10us can do it. But how is the 2nd-Order LPF Settling Time, 0.01% ? 
I cannot find it at data sheet.
I could not make sure "2nd-order LPF -3DB 15KHz -0.1DB 2.5KHz "
is OK or no?
I want to do a 10us or 100us settling time to 0.01%  ADC .
To choose this IC good or bad?
How is the settling time to 0.01%, can this ADC's 2nd-order LPF  do?
  • 0
    TI__Mastermind 35550 points

    Hello Liu Chih,

    Thanks to post this query on E2E forum, actually we don’t understand your question very clearly, but  following are important information about ADS8699:

    1. Analog front-end has a 2nd order low pass filter followed by a driver for internal 18-bit ADC, which is shown as below.
    2. Driving and settling requirement is handled completely by the driver but not PGA and filter. Only if the input range setting is modified, 2nd order low pass filter would take long time to settle.
    3. Since the BW is only 15KHz, input frequency range is automatically limited.

    Because of his internal integrated ADC driver, your external circuit will not be critical to settle the signal on sampling capacitor of  internal 18-bit ADC. 

    Thanks and regards

    Dale Li

  • 0 in reply to Dale Li
    Prodigy 20 points

    Hi Dale,

    Thanks For your answer.

    I still do not understand if the LPF 15K HZ BW  would limit the ADC BW or no?

    For example I put a ideal input signal it step from 0V to 10.24V(unipolar full scale).

    If no LPF I think ads8699 can get code 3FFFFH after 10us after the step.

    But with this low pass filter does it need to take more time or no (to get the full scale code 3FFFFH)?

    If it need how long ? and how long to get 0.01% code (10.238976V)?

    Best Regards!

  • 0 in reply to Liu Chih Wang
    TI__Mastermind 35550 points

    Hello Liu Chih,

    The LPF filter is different from switched-capacitor circuit inside SAR ADC. To guarantee the performance without any degradation to the SAR ADC with switched-capacitor input structure, the ADC driver will have to charge ADC's internal sampling capacitor to input signal voltage during during acquisition phase and have to make sure the settling error on sampling capacitor is less than 1LSB, this charging process has to be finished before conversion phase of ADC starts and the internal switch is open. The signal acquisition is critical but ADS8699 has an integrated ADC driver behind 2nd order low pass filter which will execute this function in 5us acquisition time and meet this 18-bit ADC's settling requirement. The 15KHz BW LPF limits the input signal frequency.

    Thanks&Regards

    Dale