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RF Immunity problems TLV2422

R. Keith Montgomery
Intellectual 270 points
Other Parts Discussed in Thread: TLV2422, TLV2442, OPA333

Hello,

I am trying to diagnose a DC component problem.  I am performing RF immunity tests with TLV2422, and the test fails because there is a DC shift in the output.  The amplifier is configured in a voltage follower/buffer circuit.  The DC shift @ RF frequencies is greater than 20mV in most cases.  This is undesirable because the signal that I am buffering is smaller than the DC offset.

Further experimentation shows that the TLV2442 does not produce the DC shift during RF immunity tests.  The amplifiers are pinout compatible so the same layout and circuit board were used to perform the experiment. 

Can someone comment on the difference between these two amps? The datasheets and equivalent schematics look nearly identical, except the 2442 is a faster amp and uses more power.

 

Thanks,

Keith

  • 0
    TI__Guru* 93105 points

    Hi Keith,

    This is a very good question and observation. Low-bandwidth operational-amplifiers such as the TLV2422 and TLV2442 can respond to strong, conducted RFI and EMI even though the RFI is at a frequency of Megahertz, or Gigahertz well beyond the bandwidth of the amplifier.

    The voltage-offset change is the result of rectification internal to the operational-amplifier. It occurs with the semiconductor junctions of the amplifier itself, or the ESD protection circuitry. Since the operational-amplifier is DC coupled it appears at the output as a component of the voltage-offset, and it may be amplified in proportion to the closed-loop gain as well.

    Different operational-amplifiers have different levels of RFI immunity depending on the design, process, and sensitivity to RF. It is the amplifier design where the immunity can best be dealt with in the form of a low-pass filter. The differential input pair has naturally occurring junction capacitances and there is a small amount of series resistance in the input paths. These series resistances in conjunction with the differential input pair common-mode and differential-mode capacitance form a low-pass filter that is effective on both common-mode and differential-mode RF entering the amplifier inputs. The resistance value can be manipulated to optimize the filter performance and place the cutoff frequency beyond the amplifier's unity-gain bandwidth.

    Modern operational-amplifiers that have been designed in this age of RF intensive applications are optimized to filter and attenuate the RF before it reaches the semiconductor junctions. When the two TLV amplifiers were designed RF close-in RF sources just weren't that common as they are today. If your application involves strong RF fields I will suggest our OPA333 operational-amplifier which by design and measurement provides very high EMI rejection.

    Regards, Thomas

    PA-Linear Applications Engineering