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THS4541: Low noise, low distortion band pass filter for high precision ADC

Yong Chen2
Prodigy 50 points
Part Number: THS4541
Other Parts Discussed in Thread: ADS8881, , THS4551, THS2630

Hi TI expert , 

  I am designing a MFB band pass filter for 18bit ADC( ADS8881),  the center frequency of BPF is 1kHz,  Q = 4, gain = 1,  pass bandwidth = 250Hz,  stop bandwidth = 2500Hz, 

stop band attenuation = -20dB,  Butterworth  to achieve minimum ripple.  My final goal is to get datasheet performance(SNR, THD)  of ADS8881, so I need to design the band pass filter with lowest noise and lowest distortion, do a lot of search in the TI forum, then THS4551 or THS4541 will be the best choice for the filter amplifier. From the datasheet, THS4541 should have a better noise performance than THS4551,  but when I do the simulation with TINA, THS4541 give me more output noise then THS4551,  please help on that, why this happening?  Did I do something wrong here? 

 
If possible, Could you help to recommend a better BPF design for this goal?   My sine wave source is good, but not good enough to achieve ADS8881 datasheet performance, so I need a low noise & low distortion BPF to get the best ADC performance.

Here is the TINA simulation file for both of these amplifiers.

1khz BPF MFB THS4541.TSC

Noise analysis of THS4541

Noise analysis of THS4551

sine wave source

  • 0
    Prodigy 50 points

    1khz BPF MFB THS4551.TSC

    Here is the TINA file with THS4551 

  • 0 in reply to Yong Chen2
    TI__Expert 6050 points

    Hi Yong,

    It is expected that the THS4551 has the better noise performance. You will see from Figure 7-39 of the THS4551 datasheet, the input spot noise is much lower at 1kHz than the input spot noise of the THS4541 as shown in Figure 6-39 in the datasheet at 1kHz. Therefore, the simulation results match expectations.

    After looking at the frequency response of the simulations, I believe that you are seeing good BPF performance and I have no obvious suggestions for improving the design. Increasing it from a 2nd order BPF to 4th order BPF stage would show improved filter performance, but I don't believe that will have a significant impact on noise or distortion. If you are concerned about these, you could also explore the THS2630. It has better distortion performance over the THS4551, but has an increased noise.

    Thanks,

    Nick

  • 0 in reply to Nick Nauman
    Prodigy 50 points

    Hi Nick,

      Thanks a lot for the reply,  I will use THS4551 as the amplifier to get better noise performance.  For this band pass filter at the front end of the ADC(ADS8881), do I still need a unity gain driver plus RC filter ?  Can I use the Band pass filter to replace the unity gain driver?  It seem that its better to keep the RC filter, but I want to remove the unity gain driver, because my BPF gain also equal to 1,  is it a good idea to remove the unity gain driver? 

  • 0 in reply to Yong Chen2
    TI__Expert 6050 points

    Hi Yong,

    Instead of using the unity gain THS4551 stage that you have circled in the schematic above, you can use the BPF stage that you designed. This stage will still act as a buffer since it also in a G=1V/V, but will have the added benefit of additional filtering. You should still keep the RC filter stage that follows the THS4551 stage.

    Thanks,

    Nick