ADC31JB68: Suggestion for input drivers

Hi Amy,

The issue is that I need to operate since DC and I did not find any balun specified from DC. The MABA starts from 4.5MHz.

I've found some baluns starting from 100kHz. Do you think I should have problem using that kind of baluns witn DC?

Thanks,

Rodrigo

Hi Rob,

Looking for the FDAs, I've found the LMH6553. It seems ok for my design, mainly because of the clamping function.

I'm trying to simulate it at TINA-TI but I'm able to make the common-mode works.

I'm driving the CM input with 1.6Vdc but the output signal is around 0Vdc, as you can see at the images below. I would like to see an offset of 1.6Vdc in the output signal.

This FDA will drive the ADC31JB68. 

Can you help me with the simulations?

Thank you,

Rodrigo

Hello Rodrigo,

I support the High Speed Amplifiers product line, I can assist with your simulation and design.  I have drawn up your schematic in TINA-TI; could you elaborate on the setup for your voltage signal input?  I am not able to replicate your signal; could you confirm your circuit setup & model version number/year?

Have you also referenced the ADC31JB68 datasheet's section on Driving the Analog Input, starting at Section 9.1.4.2?

Best,

Alec

Hi Alec,

The voltage signal input is 1.7Vpp (850mV Amplitude).

The TINA version I'm using is:

You can find my simulation file attached.

lmh6553.TSC

Thanks for the reply.

Rodrigo

Hello Rodrigo,

Thank you for the information.  I can replicate what you see now, please allow me some time to investigate.

Best,

Alec

Hello Alec, do you have any update about the issue?

Thank you,

Rodrigo

Hello Rodrigo,

I have found the root of the problem: you were measuring the differential output voltage using a voltmeter across both output channels.  This will only show the DC voltage offset between each output stage, which you can confirm by your observed output.  In order to view the VOCM properly, please use the file I attached:

lmh6553_customer.TSC

As you can see, when I use a voltage pin for each output, I can view the two outputs at a 1.6V DC level.

Please confirm this behavior for me, and mark the post as resolved if you have no further questions.

This behavior highlights the importance of knowing what we are measuring, and what we expect from a measurement.  The differential measurement you performed captures the variance in phase & offset between the two output stages, while the separate voltage measurements I sent over will show the proper output common mode and signal level.

Best,

Alec

Hi Alec, thank you for find the problem.

I will use this circuit to drive the AD31JB68. I am simulating the input differential termination of the AD with R3 and the VCM with the V4.

Do you see any problem to implement this DC coupled circuit as is in my design?

Another question is about the connection with the AD. As the drive will be very close to it, does I need to add extra resistor to match the impedance?

Regards,

Rodrigo

Hello Rodrigo, 

As long as you have read the guidance for using both the LMH6553 as an ADC driver and the ADC's datasheet on how it needs to be driven, you circuit is likely fine.  

I do not know the ADC part as well as I do the amplifier; I think DC coupling is alright if your common mode is set properly, which we determined is the case.

Best,

Alec