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OPA140: Is OPA140 a EMI hardned op-amp ?

Alberto Aloisio
Intellectual 900 points
Part Number: OPA140
Other Parts Discussed in Thread: LMV851

Dear Support,

in my application, I use an OPA140 configured as Voltage Follower to monitor avoltage level  with a scope (HiZ input) driving a 1m-long coax cable.

The bandwidth is limited to 1MHz. Dual supply is +/- 12V.

The entire circuit is shielded in a metallic box.

Unfortunately, the (1m-long) coax cable seems injecting RF noise in the circuit, disrupting the entire circuit (a low noise voltage amplifier).

I decided to decouple the internal node from the scope by using a voltage follower. However, as matter of fact, in my case it is not enough.

The cable injects noise onto the buffer output and, somehow, it back propagates to the rest of the circuit.

I also inserted a ferrite bead at the output of the OPA, with no luck. I have seen a small improvement by inserting a 1K resistor in series

with the OPA output. I have the scope in HiZ (1M), so 1K does not change the picture.

I am wandering if the OPA140 is an 'EMI hardned' opamp or not. Devices like LMV851/861 clearly spec this feature in the datasheet. What about OPA140 ?

Should I try to redesign the PCB in order to change the OPA with the LMV ? Do you suggest other devices ?

Any advice is very welcome.

Best Regards,

Alberto

  • 0
    TI__Guru 53451 points

    Hi Alberto,

    Question:

    The entire circuit is shielded in a metallic box.

    Is the entire circuit shielded in a metal box, include your power rails? Are the battery sources for OPA140 inside of metal box? If you send DC power rails into the box through the wall of metal box, did you place 0.1u bypass capacitors from the power rails to the metal box's shield? If you pass through OPA140's output via BNC terminal, did you place a bypass capacitor at the output of the cable. 

    Yes, OPA140  EMIRR rejection ratio plot, see Section 7.3.9. Also, please see the attached application link below. 

    What RF frequency ranges tend to interfere the operation of OPA140? What kind of RF environmental does the circuit is operating under? If you still have issues to figure it out, please wrap around your circuit with Aluminum foil, and see if the op amp's RF interference is reduced. Aluminum foil will form a faraday cage and shield the RF interference away from the internal Op Amp circuit (wrap wire harness good, sometimes I use the copper foil with adhesives).  Please let  us know what you find. 

    http://www.ti.com/lit/an/sboa128a/sboa128a.pdf

    best,

    Raymond

  • 0 in reply to Raymond Zhang1
    Intellectual 900 points

    Dear Raymond,

    thank you for the prompt reply.

    The circuit is powered by a +/-15V low-noise DC power supply. Inside the box, two 7x12 regulators provide for +/- 12V.

    The noise from this supply scheme is the same of a battery pack self contained in the box, so we decided to go with the external

    supply.

    We are in a typical lab, with many test & measurements running at the same time. We have WiFi and cellular phones in the lab,

    a couple of meters away from the circuit. Two PCs are running nearby (half a meter away). There is no  RF high-power equipment.

    The aggressors are at 50 MHz  and  the range 80-100 Mhz (FM broadcast ?). The problem appears when we connect the

    BNC cable to the OPA output and it gets worse when we connect it to the scope. Wrapping the box into aluminum foil does not

    change really much.

    Thanks for your help,

    Alberto

  • 0 in reply to Alberto Aloisio
    TI__Guru 53451 points

    Hi Alberto,

    what kind of metal box do you have? Please make sure inner metal box is not coated or conductive to the outer metal box. Please try the following. 

    1. Connect the metal shell to a true earth ground. 

    2. place all input and output connection (power rails, input/output signals) to the metal box with 0.1uF bypass capacitors. Normally I use terminals with build-in bypass capacitors.

    3. Try to shield the 1 meter cable as well with Al foil and ground it on both ends of the instrument.  

    I am treating OPA140 circuit similar to a mini EMI chamber. It should work, unless RF signals have a path to get into the OPA140 and inside of metal box. If you still have issues, please send me a picture of your setup. 

    Best,

    Raymond

  • 0 in reply to Raymond Zhang1
    TI__Guru 53451 points

    Hi Alberto,

    I hope that your issues have been resolved. I am going to close this inquiry. If you still have questions, you may continue to post on this thread,  or create a new one.

    Best,

    Raymond