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LT1997-2 Distortion Specs

Crowbar
on Jan 15, 2020

Hello!

In a LT1997-2 datasheet I see image on a page 1 with a THD spec on 1 kHz and on page 18 there is a Distortion section with a general words: "The LT1997-2 features excellent distortion performance when the internal op amp is operating in the normal operating region. Operating the LT1997-2 with the internal op amp in the over the top region will increase distortion due to the lower loop gain of the op amp". I'm interested in a more detailed specification: THD vs F for a given gain, amplitude and load as it done for conventional INAs: where can i find so?

Thanks.

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    •  Analog Employees 
    on Jan 15, 2020 10:39 PM

    Unlike traditional INA, an over-the-top (OTT) opamp has a transition region where the input stage goes between a common-emitter pair and a common-base pair as the common-mode approaches the upper supply. Since the offset is different between the two (usually much worse in the OTT mode) there is a region of significant non-linearity. Any signals that cross the transitional common-mode will undergo instances of distortion that would be difficult to characterize in a useful way, though I suppose a plot of the pure OTT mode distortion might have been of some value. We don't usually expect OTT mode to be the dominant operating condition for precision applications, but rather a means of avoiding hard clips or phase reversal under overdrive conditions (like servos for example). If your application is distortion sensitive and would tend to cross an OTT transition, then a different solution may be better. One alternative would be HV opamps (like LTC6090) along with LT5400 resistor networks.

  • 0 on Jan 16, 2020 9:23 AM in reply to jon.munson

    In my application LT1997-2 internal OPAMP will operate inside [V– ; V+ – 1.75V] so it won't transition into OTT mode. The LT5400 solution will provide worse CMRR inside 0-1kHz range. I don't have oppurtiny to do the THD benchmarking on LT1997-2 eval, so I'm searching for any useful information here.