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AD8129 noise improvement

I used AD8129 as a differential amplifier in my project. 

My specifications are meet the criteria except for NOISE.

Is there any way to improve the noise for AD8129?

Please teach me how to improve the noise of AD8129.

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  • Hi Jayson,

    Here is what I did:

    1. Took your circuit and simulated it with LTspice.

    2. Since the AD8129 Pspice model does not include noise, I added the input referred input noise voltage, and input noise current (2 places) sources.

    3. I verified your JFET amplifiers functionality using Transient Simulation.

    4. I used LTspice to predict the output noise under two different conditions:

    a) With your JFET buffers in place including the 100:1 attenuator.

    b) With your JFET buffers bypassed and the 100:1 attenuator tied to AD8129 inputs.

    I found that with JFET buffers in place, I measure a much lower output noise density (371nV/RtHz) vs. the case where the 100:1 attenuator is tied to the AD8129 inputs (675nV/RtHz).

    I've shown the noise density plots for these two cases below for comparison:

    Here are the simulation files included:

    ad8129 EZ JFET Noise LTspice

    So, simulations show the opposite of what you found; the JFET buffers do reduce noise. In fact, the simulation difference is exactly the 1pA/RtHz device noise * 40k * Gain, on each input.

    There is an improvement in bandwidth using simulation with the JFET buffers, which may be responsible for you measuring higher noise? Please double check in case you're measuring the noise using time-domain?



  • Hello Hooman,

     May I asked if the simulation includes the noise generated by the FET (LS5911) & PMBT2222A?

     Is it possible that the noise is caused by the transistor?

     My set-up in measuring noise is I short the (+/-) inputs and measured the output by the Oscilloscope (with BW limit = 20MHz). 

    Best regards,


  • Hi Jayson,

    To answer your questions:

    1. Discrete Noise: Yes, the simulation includes all noise sources, including the JFET and the NPN. However, you're dominated by the 40kohm resistor thermal noise (26nV/RtHz) at the Source of the JFETs.

    2. Xsistor Noise: No, the JFET noise (4nV/RtHz) does not factor into this, again because the 40kohm input attenuator noise dominates at 26nV/RtHz. Nor does the NPN noise because there is no gain from the NPN base to its collector (because the collector is looking to the low impedance of the JFET source).

    3. Time-Domain Measurement: With a 20MHz bandwidth, LTspice shows JFET buffered output noise of 1.47mV_RMS (or 9.7mVpp assuming 6.6x factor). That's vs. 780uV_RMS for the 100:1 directly interfacing the AD8129!

    The main reason for what you're measuring can be seen in the plot below where with the JFET's buffers in place, you're extending your circuit's BW from 670kHz to 18MHz! The most likely reason for the low bandwidth with the 100:1 attenuator directly interfacing with AD8129 is the input capacitance of the AD8129 lowering your frequency response. With the JFET buffers in place, you've eliminated this RC pole because of the lower Gate capacitance of the JFET's.

    So, the JFET circuit will have higher bandwidth and thus will measure noisier on the scope. For a true comparison, you must roll-off the bandwidth of the JFET setup to match the un-buffered setup.