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LMH2832: LMH2832

TOM FRENCH
Prodigy 40 points
Part Number: LMH2832
Other Parts Discussed in Thread: ADS54J69, , LMH6881

Hello,

The question I have is if the 2 independent channels of the LMH2832 can be used, cascaded (The Output of Input A driving the Input of Input B) with the outputs driving a LMH6881 before ultimately driving an ADC (ADS54J69). The design would be 2 independent Signal channels in relative close proximity (about 2 inches apart on the same PCB) with each channel using this cascade approach as opposed to the two signal channels going through the 2 (two) LMH2832 ICs as Channel A and Channel B respectively.  Each channel would have identical hardware and independent control and emerge at the 2 Channel ADC.  I am wondering about any potential issues with this approach.  Is it too much gain for a channel in such a small space going from A into B?  The desired signal level range is relatively wide and the overall system gain is to be spread out into various sections.  In a worse case the LMH2832 could either be applying maximum attenuation or maximum gain.

The maximum gain applied for 2 cascaded channels is my concern in this arrangement.  Is this a common configuration for the LMH2832?  I understand that the gain and attenuation can be finitely controlled and limited via the SPI bus however if software or firmware has the option to set the above scenario, are there ramification to the signal integrity (harmonics or compression, etc)?  The data sheet typically shows the channels being used independently and in parallel.  Has the LMH2832 or other similar devices been used successfully in this configuration and if so are there any guidelines?  Thank you  

  • 0
    TI__Genius 15555 points
    Hello Tom,

    The LMH2832 can be used in the mentioned cascaded configuration, where the output of one channel is connected to the input of another. Under AC performance in section 7.5, you can see the specifications for the channel-to-channel phase and gain matching. You can use figure 4 of the datasheet to check what the gain matching error will be under certain gain configurations, and see if this meets the scope of your project.

    Best,
    Hasan Babiker
  • 0 in reply to Hasan Babiker31
    TI__Genius 15975 points
    Hi Tom,

    As Hasan mentioned, you can certainly cascade two LMH2832 channels back-to-back. However, the cascade would result in increased noise floor and degraded harmonics. Is there a reason that is requiring you to use such a high gain in the system?

    Best Regards,
    Rohit
  • 0 in reply to Rohit Bhat
    Prodigy 40 points

    I believe the reason that this configuration was chosen is because the dynamic range of this system is very wide,  The input to the first stage can be anywhere from -40 to -7.  There are other VGAs in the chain.  I am looking at the performance and some harmonics even at lower drive levels.  The overall SNR is such that they don't seem to bother the signal of interest.  I know that if they get large enough they will begin to compete or create undesirable intermodulation.  The noise figure for the system is set way up in the front end but I know that other stages may compromise that figure.  For now am just looking at the bandpass at the various stages.  The overall dynamic range is from about -90 to 0dBm.  The output of the 6881 is targeted for around +5 to +13.  An amplifier with that much gain in such a small space is always a challenge.  I wanted to see if this configuration is common practice for this part.  Let me know if you have any other pointers about using and optimizing it this way.  Thank you!

  • 0 in reply to TOM FRENCH
    TI__Genius 15975 points
    Hi Tom,

    If you are band pass filtering the signal chain, then I believe cascading multiple LMH2832's should work because now you are limiting the out-of-band noise from affecting the overall SNR.

    Best Regards,
    Rohit