Question on LMH6554

Hi Felix,

Question 1: Yes, the equations you've copied do apply to the LMH6554 in the configuration you've described.

Here is another E2E post which has a spreadsheet calculator to simplify the task of computing the inter-related resistor values you have asked about:

In fact, I believe your conditions match the spreadsheet values in that E2E post directly, although you can manipulate the spreadsheet, and shows the values of RG, RF, and RT that fit your application:

http://e2e.ti.com/cfs-file.ashx/__key/communityserver-discussions-components-files/10/0576.FDA-Design-Tool-12_5F00_23_5F00_13.xlsx

Question 2: In the disable mode you describe, the output will be high impedance (as noted in the datasheet page 14). The 200ohm Rin value that you've quoted is the result of the action of the LMH6554 in keeping the input(s) at a virtual short, and thus the 200ohm will not be maintained under disable condition. I don't believe that we have made any measurements of the input impedance in the disable mode. So, unfortunately you may have to make some measurements yourself. I don't even know how to guestimate what happens to input impedance in disable mode? Maybe other people on the forum have a better understanding on what happens to a FDA in disable mode and can chime in?

 

Regards,

Hooman

Hi Felix,
I just noticed that you've mentioned that your configuration is "fully differential" and I had addressed your question as if you had a single ended to differential amplifier. Sorry I had missed that the first time around!

Your configuration would be similar to Figure 30 in the datasheet.
For a gain of 6dB with close to 2GHz bandwidth, you'd want to keep RF low (RF= 200ohm, see Figure 1 frequency response). That means RG= 100ohm to get the right gain of 6dB. This dictates RT (termination from each input to ground) to be 33.3ohm so that the differential input impedance as seen by your 50ohm differential (25 ohm single ended) source is 25ohm (33.3ohm || 100ohm = 25ohm).
The output resistors (Ro) would be 50ohm (similar to Figure 30).

Regards,
Hooman

Hello Hooman,
Is there a problem with using RG=25 Ohms (no RT) & RF=50. This will give a gain of 6dB and if more is desired RF can be increased to 100 or 200 Ohms. The rational to eliminate RT is that we do not want to loose incoming signal in terminating resistors.
Regards,
Arvind

Hi Arvind,

The LMH6554 is of the Current Feedback Amplifier (CFA) kind. So, the range of allowed values of RF is limited. I think you will run into stability issues if you were to use RF=50ohm that you are suggesting. You could see that in Figure 22 (Open Loop Transimpedance), if you were to draw a horizontal line at 50ohm (= 34 dB.ohm), you will intersect the Gain / Phase plots beyond 3GHz which is unchartered territory (meaning there will not be enough phase margin). This application note gets into the details of the loop gain analysis of a CFA, if you are interested:

So, I'd stick with the lowest RF value the datasheet mentions (RF= 200ohm) to make sure the LMH6554 remains stable.

Regards,

Hooman