LMH6639: Differential Impedance

Hello Murakami,

Do you have any specific concerns or needs for knowing more about the input structure of the LMH6639?  Differential-mode input impedance is typically very high, at least on the order of hundreds of kOhms.

We do not usually specify this value on devices which do not need the additional information.  Typically common-mode input impedance is the spec used for input design considerations.

If there is a direct question or issue which requires support on this, I can continue to look into this.  However, as we do not specify it on the datasheet the information is not readily available.  I also do not know how this information would assist in your design.

Best,

Alec

Hello Murakami,

You do not need to consider the amplifier's common-mode or differential-mode input impedances as strictly.  I would recommend choosing a reasonable Rin so that Rfb is not too large.  A 10k or 100k Rfb would potentially cause stability issues.

An example would be Rfb = 2kOhm and Rin = 20 Ohms.

Best,

Alec

Hello Murakami,

The stability concerns would be seen as oscillations which continue increasing or result in the amplifier overdriving the output.

If you are seeing expected oscillator behavior, it is likely that your circuit is stable.  You may share your simulation file with me if you wish, and I can take a look.

In general, stability issues look like overshoot & ringing on signal edges (like rising or falling edge), as well as gain peaking in frequency domain.

Best,

Alec