LMH5401: Output circuitry to match ADC16DV160

Hello Hasan Babiker,

kindly calrify whether LMH5401 optimum performance can be achieved with power supply(Vs+,Vs-) of +2.5V,-2.5V for the below requirement,

Input : AC coupled

Output : DC coupled

Output Common Mode: 1.15 (fed into Vcm pin of LMH5401)

Output Voltage Swing max: 1 Vpp

if not, kindly suggest the opearting power supply to be used to achieve the desired performance for the above requirement.

Regards,

Devaguru V

Hello Devaguru,

Using the amplifier in this way, you will be hitting the output common-mode limit. For a supply of +/- 2.5V, your output range will be -1.4V to 1.4V (this is for each individual output and not your differential signal). However, with the requirements you suggested, you will want to be able to drive between 0.65V to 1.65V. In short, your positive supply will need to be greater than 2.75V and negative supply lower than -0.45V (without exceeding a 5V supply) you should also add more headroom due to variation between devices. To provide the most headroom on either side you can use a positive supply of 3.65V & negative supply of -1.35V.

Best,

Hasan Babiker

Hello Hasan Babiker,

Kindly find our existing schematic with balun for SE to DE conversion and impedance matching circuit intended to operate in the below requirement,

Frequency range : 1050 to 1150 Mhz
I/P Range : +10 to -75 dBm
It would be greatly helpful if you could clarify some of my doubts,
1.whether the selected part LMH5401 is sensitive and provides amplification for such low Input signal level of -75dBm.

2.Since the signal is pretty much filtered before the FDA, is the Band pass filter between FDA and ADC absolutely necessary? if not, is it enough to terminate the FDA to ADC with just the termination resistors ro+,ro-?

3.If Impedance matching needs to be done, can our existing ADC Impedance matching circuit be used with LMH5401(Replacing Balun) to achieve optimum performance over the mentioned range?

Thanks,

Devaguru V

Hello Devaguru,

1. I calculate input voltage noise from the amplifier to be around 12.5 uV, with your bandwidth of 100MHz. This shouldn't dominate your signal of -75dBm. This does not take into account however noise that will be generated from your passive components and the interaction of these components with current noise. As long as your resistances aren't too high your signal should dominate.

2. The above statement only holds true with the bandpass filter of 100MHz. Without this, the noise will dominate your signal.

3. Best way to test this would be to simulate on TINA. This will not only confirm your performance with the impedance matching circuit, but will also help you know exactly how much noise will be introduced into your system.

Best,

Hasan Babiker