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ADS8353: About the BW selection for the SAR Drive Amplifier

jian wang75
Prodigy 60 points
Part Number: ADS8353
Other Parts Discussed in Thread: OPA211, OPA320, ADS8860

Hello TI FAE

TI has a tool Analog engineers calculator for drive amplifier BW selection and R C value for the charge bucket filter.

the BW for this tool is based on the Gain is set to 1 for the amplifier.

my question is if the gain is 2 for the driver amplifier, does BW need to be double also?

for example, per tool calculation, the minimum BW for amplifier is 10MHz when gain is equal to 1, if the Gain is equal to 2, the minimum BW is 20MHz, correct?

thanks

  • 0
    TI__Guru 62085 points

    Hello,

    The method you've proposed will be a good conservative approach.  Depending on the amplifier selected and the circuit configuration (e.g. feedback capacitor or not), then there are some 2nd order effects that not require an exactly doubling of the BW, but if you double the BW you should be fine.  Using the simulation methods suggested in the TI Precision Labs videos will help you optimize your driver circuitry from the good starting point provided in the tool.

  • 0 in reply to Collin Wells
    Prodigy 60 points

    thanks Collin.

    so if minimal BW is 10MHz when gain is 1, then 10MHz is not acceptable when gain is changed to 2, but may don't need 20MHz BW,right? 

    regards

  • 0 in reply to jian wang75
    TI__Guru 62085 points

    Hello,

    As mentioned before, the interactions are a little complex but you're summaries are correct.  You won't need <10MHz but you probably won't need all the way up to 20MHz either.  Simulation can be used to determine the best amplifier for your application as described in the TI Precision Labs Videos.

    https://training.ti.com/ti-precision-labs-adcs

  • 0 in reply to Collin Wells
    Prodigy 60 points

    Thanks Collin

    In ti-precision-labs-adcs, there is an example to use the tool for R-C charge bucket values.  the Rflt/2(4.1 ohm to 32.5 ohm) and Cflt(1.1nF) are based on the driver amplifier's gain is equal to 1.

    so question is : 

    if gain is 10, does the Rflt/2(4.1 ohm to 32.5 ohm) and Cflt(1.1nF) value need to be changed? 

    the amplifier + ADC are OPA320 and  ADS8860 in the example, so if gain is changed to 2, which op is suitable, i use OPA211 for the simulation, i only change the amplifier and its gain, others are same with "ADS8860_iNPUT_opa320 -st.TSC" file in the slide.  seems the settling error is very bigger (mV which is much more than 1/2 LSB) in the resistor range(4.1 to 32.5 ohm), so can you help to look at this.

    thanks very much.

    regards

  • 0 in reply to jian wang75
    TI__Guru 62085 points

    Apologies for the delay here. 

    No, the RFilt/CFilt values will not need to be updated, just the BW of the amplifier.