LMH2100: VREF value

Hi Amy,

    In the out swing test, Vref refers to Voltage on Ref pin(not the internal Vref on noninverting pin of out stage). When Vref(Voltage on REF pin) = 0, makes output saturated to positive rail, Vref = 2.7 makes output saturated to negative rail.

When you use it for ADC single ended input, you should not force 0V/2.7V on REF pin. You need to interface in either of below configurations:

The low pass output filter config (fig 81)  has the advantage that it preserves the output voltage when the LMH2100 is brought into shutdown. A disadvantage of the low pass output filter is that the series resistor RS limits the output drive capability. This may cause inaccuracies in the voltage read by an ADC when the ADC input impedance is not significantly larger than RS.

You can float the REF pin/ Give feedback to REF from OUT depending on use case

Thanks & Regards,

Hruday

Hi Hruday,

Thank you for your reply.

For ADC single-ended input, the ADC acquired voltage value is Vout = IDET* RTRANS + VREF. 

But how should the VREF value be determined when calculating Vout (Vout = IDET* RTRANS + VREF) ? Does the ADC also need to detect the voltage of the REF pin?

Regards,

Amy Luo

Hi Amy,

    Give a day or two, I will get back to you. I will check internally with the Vref value as it is not mentioned in datasheet.

Regards,

Hruday

Hi Amy,

    ADC not required to detect the VREF pin.

I think internal Vref = ~0.3V from below table.

(Vout = IDET* RTRANS + VREF) is given in datasheet to explain the output stage and that equation may not help to estimate Vout as Idet is still unknown.

You need to estimate Vout as below:

     Vout =  Kslope(Pin - Pintercept) .

so estimate Vout based on Input power, Kslope and Pintercept.

I can support you better if you can elaborate your application.

Regards,

Hruday

Hi Hruday,

Thanks for your information, but I don't understand a bit. If the output needs to be calculated according to the formula

Vout = Kslope(Pin - Pintercept) ,

then the Vout of the ADC single-ended input measurement and the Vout of the ADC differential input measurement differ by one Vref. Which Vout should be used to calculate the Pin by substituting the formula Vout = Kslope(Pin - Pintercept) ? Also, how the Kslope and Pintercept values are determined

Thanks,

Amy