LMH5401: 3G SDI Amplifier and Alternatives

Hello,

  I was going to point you to our specialized 3G cable driver interfaces, but I found your post on the exact topic. In that case, you would need a high gain bandwidth part capable of accepting a 3G-SDI signal at double the gain. 

  The LMH5401 is able to drive a 75R coax cable. The device has an internal 10ohm resistance at each output, but adding the additional resistance wouldn't be an issue for impedance matching to the coaxial cable. Also the device is able to function in any SE/DE conversion signal format and has enough bandwidth for the gain needed. 

Thank you,
Sima 

Hi Sima,

Thanks for the feedback on the LMH5401.  While it overall looks promising, the datasheet doesn't include recommendations on using it in a SE-SE condition while maintaining correct impedance matching at the input.  For the output it looks like a series 65R resistor from the positive output into the 75R coax, and a 140R from the negative output resistor to ground should basically work.

For the input, the matching network shown doesn't seem to correspond to a 50R matching condition.  What am I missing?  Additionally, the SSS1 shown in Figure 37 climbs substantially with frequency.  Can that be improved with the input matching network, or is that best case for the device?

Figure 29 seems to show a gain of at least 12dB out to about 5GHz, which implies a GBP much higher than 8GHz, what's the limiting factor?

Hello,

The LMH5401 isn't designed for SE-SE line driving applications. If you use the amplifier in this way, HD2 and common mode noise performance will suffer, as will bandwidth under capacitive load. We don't really have any standard line drivers that can deliver these specs. You might want to take a peek at our fixed RF gain blocks, something like the TRF37A73: TRF37A73 data sheet, product information and support | TI.com

Best,

Sam

Hello,

  I agree with Sam, LMH5401 can be used as a single-ended outputs, but there will be an impact to the distortion performance. Another possibility would be to add an external balun to the output of the LMH5401 to preserve device performance depending on your DC requirements.

Thank you,
Sima 

Hi Sima,

I've looked for baluns for this application however I can't seem to find any that have sufficient bandwidth to preserve the 3G SDI edge rates.  Are there any you can suggest?  Additionally the S11 values for most baluns don't meet SDI requirements, but this shouldn't matter if it's only internal between the LMH5401 and the balun.  How low does S11 need to be for the LMH4501 to operate within spec?

Hi,

Are you looking for something you can put onto a PCB? There are quite a few options for RF baluns out there in various packages, sizes, enclosures, etc. The insertion loss will effect the gain you need to introduce to get the desired output.

Best,

Sam

Yes, PCB mount is required.  Would need to a be a balanced to 75R single-ended device.  The closest I've been able to find is the TA-0751003G from iNRCORE.  While specified for 2.973 Gbps, in testing them I've found that they slow edges down below the 135ps max spec for 3G-SDI.

Those appear to be tailored to this application specifically... how are you driving/measuring them in test? Maybe there's an additional time constant showing up that you wouldn't have on a closed system PCB? Is it off by much?

Best,
Sam

If I remember correctly I'm seeing a Tr of about 165ns, so appreciably out of spec.

I'm going to spin up a test PCB with an LMH5401 at 2x gain driving one of these baluns directly and see what things look like.  Since there'll be minimal length (~5 mm) from the LMH5401 to the Balun, do you see any issue forgoing external termination resistors and the associated matching loss?  Section 9.2.2.2 of the datasheet indicates that this is a supported configuration.

Since the source is 50R it looks like the resistor values from Table 2 for 2 V/V gain can just be used directly.  This would be a DC coupled input signal at about 0.4V, +/-2.5V supply voltages, Vcm terminal floated and bypassed to ground, output driving the balun as above.  Any reason you can give for deviating from the table values?

Could you share scope traces and a schematic from your measurement setup? 165ns seems huge. That configuration for the PCB should be fine.

Best,
Sam

Sorry, 165ps*

I wondered if it was a typo :)