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ADS8661: input bandwidth

Allen Du
Prodigy 220 points
Part Number: ADS8661

Hi Team, 

May I ask a question about ADS8661, in the datasheet, sampling frequency is 1.25 MSPS, but input bandwidth is only 15k.

Previously I used another ADC IC which sampling frequency is 500ksps and –3dB Input Linear Bandwidth is 1.6Mhz. 

May I ask what's the connection/relationship between sampling frequency and input bandwidth?

And is ADS8661 suitable for DC current signal sensing circuit which bandwidth is 100khz to 1Mhz (PWM controlled circuit)?

Thanks in advance!

  • 0
    TI__Guru**** 168445 points

    Hi Allen,

    The input bandwidth of the ADS8661 is limited by the front end amplifier stage.  Not sure what 'previous ADC' you used, but in general, if you are looking into a SAR ADC that does not have a front end amplifier stage the bandwidth is based on the RDSon of the sample/hold switch and the sampling capacitor.  For your DC current sensing needs, if you are trying to close a control loop, the 15kHz bandwidth may be an issue.  If you are just monitoring the current, I think it would be fine.

  • 0 in reply to Tom Hendrick
    Prodigy 220 points

    Hi Tom,

    Thanks for your reply, it's for current monitoring. previous ADC used is LTC2308, I am looking for another one only need 1~2 channels.

    May I ask another question, what's the ADS8661 response time, for example, host actives SPI CS, how long will ADS8661 sending data to host. Becasue of the current data I am measuring is 10us pulse and want to measure the current of ending of pulse. I have the PWM ctrl signal as reference, so know when the pulse started. just need to know when should send command to read current from ADC.

  • +1 in reply to Allen Du
    TI__Guru**** 168445 points

    Hi Allen,

    That other ADC has 100 ohms of input impedance and a 55pF sample/hold cap, which is where the typical 27MHz -3dB bandwidth comes from. 

    Response time would be the time it takes to do the conversion followed by the time it takes to read out the conversion result.  The conversion time of the ADS8661 is 550ns, which starts after a logic high on the CONVST pin (see Sampling Dynamics on page 7 and Figure 6-3 on page 11).  When you have the conversion back will depend on your SCLK speed.  Running at the max throughput, you would have your data back in ~800ns from the application of CONVST.