THS4541: THS4541 with low vocm

Hello Elhanan,

No greater headroom is needed for the Vocm pin. For a 3V supply at 25C, the typical headroom on the Vocm pin to negative supply needed is 0.88V.

Best,

Hasan Babiker

thank you for the quick respond

can you note me the place in the device data sheet where I can see that?

can you recommend other device (same footprint if there is..) that can handle 0.7V Vcm?

thank you

Hello Elhanan,

This can be found on page 11, under "Output Common-Mode Voltage Control" section and "Common-mode loop supply headroom to negative supply."

Can you clarify which package you are using? And just to confirm you're using the device in single-supply not dual correct?

Note that if you are using single-supply even if the Vocm pin spec is met, your dynamic range will be very limited at such a low output common-mode.

Best,

Hasan Babiker

Hey

my configuration is 3V, single supply, and I'm using the RGT package - THS4541IRGTT

my device (RFSoC) have 0.7V vcm and in some configuration I have to use DC biasing so.. I don't have allot of choice here.

Thank you 

Hello Elhanan,

The THS4551 is also available in an RGT package and can accept a 0.7V Vocm under these conditions, you will be able to achieve a max output differential of 2Vpp.

Best,

Hasan Babiker

Well Elhanan, 

Keep in mind the Vocm control is mainly an FDA output setting issue. Your source can have a different CM voltage and that will get level shifted by the FDA CM loop. A more complete schematic would be helpful., 

Thank you

the Vcm is grate.

unfortunately I need it to be minimum 850MHz Gain Bandwidth (my application BW is 1M to 25MHz FDA MFB HPF with G=10)

do we have solution like that?

Well The THS4541 is probably the right part - if you are doing a high pass filter, you will be level shifting your source CM up to where the Vocm range of the THS4541 will work. 

Hello Elhanan, 

I don't believe there is a particular part that will meet your requirements without modification. If your output range is not a major concern, you can set the Vocm to midsupply and attenuate the output. This will give you the flexibility of using the most suitable FDA without exceeding the Vocm limits. Of course this will mean the output range of the device will be attenuated as well.

Best,

Hasan Babiker

I might have misunderstood here, are you driving an RFSoC with the THS4541 or are you receiving a signal into the FDA from your RFSoC? If you are driving a signal into the RFSoC and it needs a 0.7V DC CM bias, could you not AC couple the THS4541 output (leaving its Vocm to default mid supply) and provide the DC bias with just resistors to 0.7V? Again, schematics help a lot

Hey

I have attached my schematics

Yes I'm driving the RFSoC with the THS4541.

I'm getting the 0.7V from the RFSoC

and yes what I did is add DC Block in the output of my HPF MFB FDA (not showed in the attached schematics) and leave the Vocm of both the RFSoC and the THS4541 unconnected (also not in the attached schematics)

Thank you

Hello Elhanan,

Thank you for clarifying, I assumed you were driving the 0.7V input to the Vocm pin of the THS4541. Using the device in this way with DC blocking caps at the output shouldn't be an issue. 

Best,

Hasan Babiker

Thank you

if my pass band is 1MHz to 25MHz, will 10nF do the work?

Hello Elhanan,

What are your load characteristics following the DC block capacitors.

Best,

Hasan Babiker