LMH5401: Simulation Confusion

Shoaf Robinson

Part Number: LMH5401

Tool/software:

Hello,

I am working on a project where we need to use an FDA to perform SE-DE conversion on a DC-coupled input signal. Naturally, I am trying to get a better understanding of how to interface with this chip using one of the datasheet's example circuits and recreating it in PSpice for TI and simulating.

I must be doing something wrong, however, as I keep getting a gain different than what it seems it should be based on the documentation (I am getting a gain of 1 here rather than 4).

Could anyone provide some insight on what I may be doing wrong here? Also please let me know if any clarification is needed or if the images included are not showing up. Would love some help here - thank you!

3 months ago

0 Keyur Tejani 3 months ago

TI__Intellectual 1790 points

Thank you for doing your due diligence in referring to the data sheet information.

One immediate issue I see, is outputs, etc is all railed out. This is because the device needs headroom from the supplies, and Vcm is set to 0V, while VS+ = 5V, and VS- = 0V.

Adjust the supplies to VS+ = 2.5V and VS- = -2.5V so that everything that is currently ground referenced, falls at midsupply.

Once that is resolved, be sure to refer to this section of the data sheet to understand where the gain, etc reference points are.

Keyur

0 Shoaf Robinson 3 months ago in reply to Keyur Tejani

Prodigy 20 points

I appreciate your prompt reply, but I could use a bit more clarification on this. I am not clear on this section of the datasheet referring to the gain reference points. How do I know what VOUT_AMP is if I cannot probe it? The only change I have made thus far is adjusting Vs+ and Vs- to 2.5V and -2.5V respectively, but that did not affect the output as far as I can tell.

0 Keyur Tejani 3 months ago in reply to Shoaf Robinson

TI__Intellectual 1790 points

Here are the reference points at various points of the output network... 1 is at VOUT_AMP, 2 is at OUT+/-, and 3 is at load. For all intents and purposes, you may only care about the voltage/gain at the load, or at the OUT+/- points.

I also noticed the input dc voltage was 0V in the previous circuit; was that intentional? Did you try changing that to say 100mV or something?

Keyur

0 Shoaf Robinson 3 months ago in reply to Keyur Tejani

Prodigy 20 points

The input source I am using is a pulse wave with the following parameters to mimic the actual source that will be input:

I am not sure why it says 0V there at the input. I just re-simulated after changing the DC parameter seen above to 1, but there was no change in the sim results.

0 Keyur Tejani 3 months ago in reply to Shoaf Robinson

TI__Intellectual 1790 points

I believe you are doing a dc analysis, as opposed to a transient analysis, where the pulse response can be analyzed.

You can try putting a dc battery, with say 100mV, in series with the pulse source for dc analysis and check the dc behavior of the circuit.

Keyur

0 Keyur Tejani 3 months ago in reply to Keyur Tejani

TI__Intellectual 1790 points

Also, here are some PSPICE for TI's training video series, if it helps.

Precision labs series: PSpice for TI

Explore PSpice® for TI design and simulation tool

Keyur

0 Shoaf Robinson 3 months ago in reply to Keyur Tejani

Prodigy 20 points

Okay so then when I try an AC analysis the gain is still just one:

Let me ask you this, if you were in my shoes would you be confused as well? Or how would you be testing this circuit in simulation? There must be something I am doing wrong or the SPICE model is funky... not sure though. Let me know what you think.

0 Keyur Tejani 3 months ago in reply to Shoaf Robinson

TI__Intellectual 1790 points

At least the circuit is not railed and output is responding to input/stimuli. And yes, differential amplifiers can be a bit confusing.

Believe you are taking gains as voltage at reference point 4 with respect to 1. That will result in a gain of 1 as there is a voltage division by 2 at reference points 2 and then another one at reference point 4.

If you take gains from reference point 3 to reference point 2, as mentioned in figure 60, and throughout the data sheet, that should result in a gain of about 4V/V. Reason I say about 4V/V, as you can only probe OUT+/- points, and there will be a minor voltage division of 20Ω (internal output resistors) to the external 40+40+100Ω.

And though LMH5401 data sheet is very detailed, if you want to delve deeper into the workings of differential amplifiers, below is some collateral we have:

Precision labs series: Fully differential amplifiers

Output impedance matching with fully differential operational amplifiers

Input impedance matching with fully differential amplifiers

Hope this helps.

Keyur

0 Shoaf Robinson 3 months ago in reply to Keyur Tejani

Prodigy 20 points

I think that was it! Wow I cannot believe we were probing the wrong points this whole time - lesson learned. Thank you so much for your help I really appreciate it.

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LMH5401: Simulation Confusion