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ADS8881: Advises on hardware design for noise filter

Part Number: ADS8881
Other Parts Discussed in Thread: INA317, INA819, INA818, OPA365, INA331, REF6050

Dear TI Support,

I am designing a custom prototype board to read ADC from an input using ADS8881. I attached the schematic below. My problem is that I got so much noise when reading the ADC; when I shorted the 2 inputs together (2 inputs from the opamp in front of the ADS8881), the data was still not stable, I got the value from 0x3D090 to 0x2328. For the C7 capacitor in the schematic, I replaced it with the 4.7uF. I am not really good at hardware design.

I use the 50 MHz clock on SCK line, I read the data from the ADS8881 through 3-wire SPI mode every 100us and I want the make the chip work as fast as possible. Can you give me some advises with my design, do I have to add any components or any kind of filter?

Thank you and best regards,


  • Duy,

    1. You mentioned that you want to read the ADS8881 as fast as possible. The fastest data rate for the ADS8881 is 1Msps.  But you connect the input of the data converter to the INA317. In your configuration the INA317 Gain is 100. The bandwidth of the INA317 in a gain of 100 is 3.5kHz.   Why do you want to sample the low frequency signal from the INA317 at such a high data rate? Normally, you would reduce the ADS8881 sampling rate to a rate that is closer to the bandwidth of the amplifier.
    2. The “noise” that you are seeing is probably settling errors on the SAR ADC. In order to settle you will need to select an amplifier that has a sufficiently wide bandwidth then you will need to properly select the RC input filter (R1, R3, and C7 in your design). In your case the large C7 value (0.1uF) and the small value for R1 and R3 ( it looks like zero ohms) is likely causing your INA317 to become unstable.
    3. I strongly suggest that you use a cookbook circuit as the basis for your design. These are design examples that we put together to help you get the circuit correct as soon as possible. I think Circuit for driving a switched-capacitor SAR ADC with a buffered instrumentation amplifier is very close to what you want. Notice that the INA in that example is followed by an op amp buffer. I think you will need this in your design.
    4. I also strongly suggest that you use an ADS8881EVM and a DIP Adaptor EVM. You can build your amplifier circuit on a breadboard and wire it to the input of the ADS8881EVM to check performance. The reason I think you should do this is so that you can separate the software / firmware design part from the hardware design part.
    5. I suggest that you go through Op Amp and Data Converter precision labs video training. This is a lot of material, but I think it may be beneficial to you. Specifically, for SAR data converters you need to understand SAR ADC input component selection. This section covers the importance of choosing an amplifier with the correct bandwidth and selecting the RC filter circuit.
    6. I didn’t provide any specific recommendations on your circuit as you need to decide what sampling rate you want to run at and you need to consider the bandwidth of the amplifier. If you choose this, I can provide a circuit recommendation.

  • Hi ,

    Firstly, I would like to thank you for your kind reply, I got a better view for your provided links. 

    6. I didn’t provide any specific recommendations on your circuit as you need to decide what sampling rate you want to run at and you need to consider the bandwidth of the amplifier. If you choose this, I can provide a circuit recommendation.  

    => For this, I sampling at least 128 *  5000 = 640000 samples per second (128 samples every 200 us), for the bandwidth I prefer the bandwidth of 100kHz, is it OK? 

    Thank you and best regards,


  • Duy,

    1. I suggest that you use the INA818 or INA819. These devices will give you a bandwidth of 270kHz in a gain of 100. Your schematic showed a gain of 100, so I assume that is the gain you want. There are many other INA that meet the bandwidth requirement. I selected INA818 / INA819 as it is a new device with good cost / performance tradeoff. If you have other key specifications that you need please suggest an alternative. However make sure that the Bandwidth at Minimum Gain Specification (click for list) is greater than 1MHz and confirm that the bandwidth in gain of 100 is greater than 100kHz.
    2. Regarding the sampling rate of the ADC. The input of a SAR ADC has a switched capacitor circuit that creates transient current spikes. The amplifier driving the SAR needs to respond to these spikes, so normally the bandwidth of the amplifier is greater than the sampling rate (i.e. typically 20x sampling rate). If you want to use a 100kHz INA to drive a SAR running at 640ksps, than you will need to have a wide bandwidth buffer at the output of the INA. For example, see Circuit for driving an ADC with an instrumentation amplifier in high gain or Circuit for driving a switched-capacitor SAR ADC with a buffered instrumentation amplifier. If you do not want this additional buffer you will need to reduce your sampling rate. Please confirm if you prefer a buffer or reduced sampling rate.

    Best regards,

  • HI ,

    Thank you once again for your reply, I prefer the buffer, but are there any higher bandwidth opamps that match 20x the sample rate? I want to make the prototype simple and in the future I will definitely upgrade it to the buffer. My gain is 100 and it will not be changed. For the INA818/INA819, I guess I have to wait for a while to buy them since they are not available at the moment.

    Best regards,

  • Duy,

    1. Attached is a power point file that shows a proposed circuit and simulation results.
    2. The TINA files are imbedded into the presentation. Just click to open the files if you have TINA TI.
    3. I recommended a different INA (INA317). Part of the reason I changed the recommendation is that the INA317 is optimized for 5V operation, and the other device I recommended was optimized for +/-15V operation. Also you mentioned that you cannot purchase the device yet (you can get samples). In any case, the INA317 has sufficient bandwidth and is optimized for 5V operation. One thing to keep in mind is that the INA317 (and many other INA) has a linear output swing vs common mode limitation. For this device the output swing is limited to 20mV to 3.8V on a 5V supply.
    4. Note that most INA do not have a very wide bandwidth. Also, many are optimized to operate on a +/-15V supply. So the selection that have a bandwidths of 100kHz in a gain of 100V/V with a 5V supply are limited.
    5. The power point shows transient settling from the ADC charge kickback. The buffer amplifier (opa365) in needed to absorbed these transients as the INA will not have sufficient bandwidth to do this.
    6. The power point shows the expected noise (2.6mVrms or 16mVpp). This is a fairly large noise compared to 1LSB for the converter. However, this is fairly typical for a INA in high gain. The only way to reduce the noise further is to limit the bandwidth further.
    7. I hope this helps.

    Best regards,


    ina + opa365 + ads8881.pptx

  • Dear ,

    I got all the information and material that are needed for my development and thank you very much for your kind support.
  • Hi ,

    Just to make sure, the OPA365 and the INA317 must have the same V+ = Vref and V- = Vref/2.
    Is it right?

    Thank you.
  • Tran,

    1. The Ref pin on INA331 is used to shift the signal.  This should be connected to Vref/2.

    2. The positive supply on INA331 is connected to a voltage equal to Vref.  In the example circuit I showed 5V.  Note: I would use an LDO to generate the supplies and a REF6050 to generate the reference voltage on the ADC.

    3.  The negative supply on the INA331 is 0V.

    4.  The positive supply on the amplifier is Vref (5V) and the negative supply is 0V.  Again, I recomend that you use an LDO for the power supplies and a voltage reference (REF6050) for the reference input on the ADC.

    5.  The attachment below has a full schematic and TINA files for this circuit.

    ina + opa365 + ads8881 (1).pptx