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  • TI Thinks Resolved

LMH6624: Can it be used in the place of instrument amplifiers

Prodigy 150 points

Replies: 9

Views: 522

Part Number: LMH6624

Hi,

Can this chip be used in the place of instrument amplifiers? Saw on the datasheet it can be used for instrumentation sense amplifiers. Instrument amplifiers I am seeing have very low slew rate. Looking for an instrument amplifier of at least 350V/us slew rate since I am operating at a slightly high frequency and phase of the output signal with respect to the input is of most importance. 

Thanks,

Oro

  • Hi Oro,

    can you show a schematic?

    What common mode rejection ratio do you need? And what do you mean by "slightly high frequency"?

    Kai
  • In reply to kai klaas69:

    Hi Kai,

    There is no schematic as yet. However, I am looking at CMRR of 80 - 100dB. Frequency is between 10 - 15MHz.

    Oro
  • In reply to Oro:

    Hi Oro,

    80...100dB CMRR is nearly impossible if you take discretes. You would need resistors with less than 0.01% production tolerance in the differential amplifier. The temperature drift and the long term drift must also be in this range. And then you have to match unavoidable stray capacitances to achieve a good CMRR at higher frequencies...

    Kai
  • In reply to kai klaas69:

    Thanks Kai. Do you know of any instrumentation amplifier with at least 350V/us?

    Oro
  • In reply to Oro:

    Hi Oro,

    Can you describe the signal in more detail? Do you require both a high impedance input and a single ended output? If not, maybe you could use a fully differential amplifier, which have very good slew rate, bandwidth, and CMRR.

    Best regards,

    Sean
  • In reply to Sean Cashin:

    Hi Sean,

    Thanks for your response. The signal is an 11MHz sinusoidal signal of amplitude in hundreds of mV picked up from a sensor. The output required is not single ended and the impedance does not really matter as a buffer can be used incase of impedance mismatch.

    Oro
  • In reply to Oro:

    Hi Oro,

    An FDA is like two parallel inverting amplifiers. Your dual buffers should have higher bandwidth than the FDA, since their output impedance increases at higher frequencies and it will affect the gain in each inverting half of the FDA. There are many buffer to differential amplifier to differential input ADCs that you could use. I would start with OPA354s, an LMH6551, and an ADS9110 to satisfy your requirements, and modify as necessary. The LMH6551 applications section has a description of FDAs if you are unfamiliar, and there is an FDA series in TI's precision labs video series also.

    Best regards,

    Sean
  • In reply to Sean Cashin:

      Hi Sean,

    Thanks for you response. I looked at your FDA series to gain more insight. For a differential input configuration (see attached image scenario 1), the input was 600mVpp and the gain was 4. As would be expected at the output I had 2.4Vpp. Now for the same input and gain but with single ended input (see scenario 2) the output was 1.2Vpp when it is meant to be 2.4Vpp. If I recall correctly one of the videos did say regardless the input configuration (diff or single ended) the output should be the same provided the input signal has the same amplitude. Wondering why it is not the case in this simulation.

    Thanks,

    Oro

  • In reply to Oro:

    Hi Oro,

    The top simulation shows two signals in series, so the output is naturally double that of the bottom. Connecting the positive end of the generator to one input and the negative end to the other input without any ground will be the differential equivalent of the bottom circuit. In this bottom circuit, the differential output of the bottom actually is is 2.4Vpp, not 1.2Vpp. The absolute difference between these two outputs will range from +1.2V at the peak of a red wave to -1.2V at the peak of a green wave, or 2.4Vpp. This double output swing for a given voltage range is a great benefit of FDAs vs. single ended amplifiers, which must reference to a stationary ground.

    Best regards,

    Sean

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