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150kHz Current excitation inteference

rahman yunus
Prodigy 130 points
Other Parts Discussed in Thread: LM6172, LMH6624, OPA656, LMH6611, LMH6612

Hi all,

I'm now developing a system which combines two different types of sensors: ultrasonic and impedance sensors. In the system to achieve high-speed acquisition both of the sensors activated at the same time. For ultrasonic sensors I'm using piezoelectric transducer. The transmitted voltage signal is at 328kHz. For the impedance sensors the current excitation is 10mA  at 150kHz. The problem is at the ultrasonic receivers the wave acquired are totally distorted as I activate the impedance current excitation. So far  I have designed 2nd order band pass filter (centre freq=328kHz) but the problem still occurs. Attached is the images of the received ultrasonic signals when the current excitation is turn OFF and ON. Thank you in advance.

Pray for MH370

Fazlul

1665.Current excitation.pdf

  • 0
    TI__Guru* 93105 points

    Hi Fazlul,

    Can you reveal which TI precision amplifier products you are using in your design, and the circuit schematic?

    Regards, Thomas

    PA - Linear Applications Engineering

  • 0 in reply to Thomas Kuehl
    Prodigy 130 points

    Hi Thomas,

    Thank you for the reply. As requested , I attached the schematic diagram of the ultrasonic receiver circuit. Hope you can assist me.

    Regards

    Fazlul

    4201.Schematic -Ultrasonic 328kHz.pdf

  • 0 in reply to rahman yunus
    TI__Guru* 93105 points

    Hello Fazlul,

    There are a couple of things I noticed about your circuit that raise concerns:

    • The 328 kHz input has an input amplitude of 0.5 Vpk and the gain of the first stage is set to 100 V/V. If the input is at the 0.5 Vpk level, the output would attempt to swing 50 Vpk. Certainly that is well beyond the power supply rails of +/-15 V, and beyond the amplifier's capabilities. The output will swing to the rails and then clip causing massive distortion. Operational amplifiers are not designated to operated in that manner.
    • The 1st stage amplifier is set to a gain of 100 V/V as mentioned. That is a closed-loop gain of 40 dB. The LM6172 only has about 43 to 44 dB of open-loop gain at 328 kHz according to the Open-loop Frequency Response graph, in data sheet figure 14. That is entirely too low to support a closed-loop gain. It will result in very large gain error and is insufficient to help reduce distortion.
    • I do not know what parameters you selected for the active band-pass filter other than the 328 kHz center frequency. That said, even if the AV = 1 V/V and the Q is 10, the operational amplifier gain-bandwidth required to support the filter response would be 328 MHz; 3x that of the LM6172. If you selected higher gain and/or Q, the gain-bandwidth requirements will be even higher.
    • Full-wave dc rectifier circuits may have low bandwidth due to the various capacitances that must be charged and discharged; especially when high value resistors are in the path. I suspect that will be the case for the circuit you selected. Take a look at a TI Precision Design for information about precision full-wave rectifiers: http://www.ti.com/lit/ug/tidu030/tidu030.pdf
    • I do believe you are going to need operational amplifiers having much higher gain-bandwidth in most of the stages.

    Regards, Thomas

    PA - Linear Applications Engineering

  • 0 in reply to Thomas Kuehl
    Prodigy 130 points

    Hi Thomas,

    Thank you for your explanation on the circuit. Actually I am not familiar with electronics devices and I really hope you can assist me. I reconsidered your comments and make changes particularly on the op-amp used. In new circuits I select LMH6624 which has better headroom between open loop and closed loop gain (100V/V) at 328kHz.

    On the band-pass filter (MFB), I select OPA656 which has wide 500Mhz unity gain bandwidth which is sufficient to for 328MHz bandwidth. Sorry before this I didn't give any information about the band-pass filter. Band-pass filter: mid-freq=328kHz, Q=10, Bw=32.8kHz, Gain=1V/V

    I attached the update ultrasonic receiver circuits and the transient response at each stages. I also attached the original received signal (captured by digital osc.) before the 1st stage amplification. Hope you can gives some comments on changes that I have made.

    After checking the prices I found that OPA656 is the most expensive device around. Can you suggest other device which has similar performance at lower price? I need to construct 16 channels of ultrasonic receiver and the device costs me much.

    Regards

    Fazlul

    2625.UT response graphs.pdf

    8507.Schematic -Ultrasonic 328kHz rev.b.pdf

    1732.T9R9(b).txt

  • 0 in reply to rahman yunus
    TI__Guru* 93105 points

    Hello Fazlul,

    It looks like you are making good progress. I am not directly involved with the high-speed operational amplifier products such as the LMH6624 and OPA656 so I may not be the best person to comment on them; however, they do appear to have sufficient bandwidth and slew to support your application. The persons providing support for High-Speed Amplifiers E2E forum have greater knowledge about these amplifiers and should be able to assist you with them.

    When it comes to cost you can sort TI's High-Speed operational amplifier tree by cost. The last column on page is the amplifier cost and it can be set to sort ascending, or descending. Here is a link to the page:

    http://www.ti.com/lsds/ti/amplifiers-linear/high-speed-amplifier-greater-than-equal-50MHz-products.page#p512=325;3000

    Doing so provides a number of High-Speed amplifiers that should work in the application at a lower cost than the OPA656. The LMH6611/LMH6612 might be one to consider.

    One little item that I did note about your circuit is the 3rd stage is labeled as having a gain of 10 V/V. The gain is simply -5 V/V.

    Regards, Thomas

    PA - Linear Applications Engineering