LMH5401: The problem of significant gain drop with increasing frequency in the use scenario of large feedback and input resistors

yanggang5

Part Number: LMH5401

Tool/software:

The circuit when we use LMH5401 are as follow picture

We built a model in ADS for simulation of this usage. The model of LMH5401 is achieved from the official website. The models of PCB wiring and coaxial lines are achieved from simulation extraction and vector network analyzer measurement, respectively. The relationship between the simulated gain and frequency is shown below

However, when testing on actual PCB, we found that the gain of this network would sharply decay with increasing frequency, as shown in the following figure

The difference between the maximum and minimum gain within 300MHz is about 6dB, which cannot be accepted. What is the reason for this problem? Also, is there any way to optimize it？

Thanks

8 months ago

0 Michael Steffes 7 months ago

Guru 48395 points

put 0.2pF parasitic on those large resistors and see what you get sim wise

0 yanggang5 7 months ago in reply to Michael Steffes

Prodigy 110 points

Hi Michael，

According to your suggestion, I added a 0.2pF parallel capacitor beside these large resistor in the ADI simulation, and there was indeed a gain drop similar to the measured results.

Is there any way to compensate for the impact of these parallel capacitors?

Best Regards,

Yang Gang

0 Michael Steffes 7 months ago in reply to yanggang5

Guru 48395 points

Well you could try reducing those R values as far as other contraints will allow

0 yanggang5 7 months ago in reply to Michael Steffes

Prodigy 110 points

The reason for using these large resistors is that the driving ability of the front stage TIA is very weak, and reducing the resistance value will cause performance degradation of the TIA.

But I tried to connect a 1pF capacitor in parallel with the 1KΩ resistor, and the gain flatness of the LMH5401 has been greatly improved. Is this method feasible?

+1 Michael Steffes 7 months ago in reply to yanggang5

Guru 48395 points

yes, that is providing an equalizing zero in the gain path

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LMH5401: The problem of significant gain drop with increasing frequency in the use scenario of large feedback and input resistors