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LMH2832: Questions of LMH2832 flatness

Prodigy 640 points

Replies: 2

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Part Number: LMH2832

Hello,

Here are two questions on LMH2832 flatness. Please help on them.

1. On the page 9 of LMH2832 datasheet, the curve of Voltage gain vs Frequency only shows the performance above 1MHz. How is the curve between 0~1MHz? Do we have any test data to show?

2. The EC Table of Bandwidth for 0.1-dB flatness states the bandwidth is typically 300MHz. Does it mean that the 0.1dB flatness should cover 0~300MHz bandwidth?

Thanks!

Paul

  • Hi Paul,

    Two great questions, and ones I can answer here succinctly.  With high speed parts, the datasheet often reflects the use case and operating conditions of the part.  Higher speed parts are typically valuable because of large bandwidth, thus the parts of the curve you see are highlights from the data taken.  If you download the TI reference design for the LMH2832 and simulate AC performance from 1 HZ to 10 GHz, you will see how flat and uninteresting the majority of the curve (on a log scale) is.  So, to answer your questions:

    1. The curve is flat and mimics the behavior of the curve between 1MHz and around 60MHz, which is to say the voltage gain at frequencies less than 1MHz is the voltage gain at 1MHz.  The data was most likely omitted as a calculated choice to showcase the poles in the voltage gain at higher frequency resolution in the same size image on the datasheet.

    2. You have a good handle on this one, from my perspective.  BW is inherently a measure of a width, or span of frequency.  So, this data value on the datasheet is describing how to maintain a relative flatness of 0.1dB, the limiting BW is 300 MHz.  Increasing the span of frequencies you operate at will start to engage the pole and decrease your voltage gain.  If you simulate this part, you can place a cursor at 300 MHz and experience the shift in gain at higher frequencies, which exceeds the 0.1-dB flatness parameter.

    Hopefully this makes some sense, in short you understand how 0.1-dB flatness works and just need to extrapolate the flatness for sub-MHz performance of voltage gain.

    Thanks,

    Alec

  • In reply to Alec Saebeler:

    Hi Alec,

    Thanks for explanation!

    B.R

    Paul Ren

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