LMH9135: Need confirmation on the DC Bias Pull UP Resistor.

Dear RF Amps team,

This is a joint project between AFE7769D team and VVDN team. Please help with the following:

1. What is the expected common mode input range of the LMH9135 amps?

2. is there a center tap equivalent to the LMH9135 input pins so we can use the center tap to bias the AFE7769D output at 1.8V?

3. does LMH9135 have internal bias voltage for the input differential pins?

The AFE7769D output must have 1.8V bias. If the LMH9135 does not have an active bias, then we will need to R29 and R49 into 0402 size 56nH inductor to 1.8V to actively bias the AFE7769D.

-Kang

Hello Kang,

1. What is the expected common mode input range of the LMH9135 amps?
>> LMH9135 is Ac coupled internally, so common mode should not matter

2. is there a center tap equivalent to the LMH9135 input pins so we can use the center tap to bias the AFE7769D output at 1.8V?
>> No there is no pin equivalent to center tap.

3. does LMH9135 have internal bias voltage for the input differential pins?
>>From below block diagram of LMH9135, inputs are connected to Balun inside, so I don't think they have any bias voltage at inputs. But, let me double check this with design and confirm.


Regards,
Hruday

Hello Hruday,

The AFE7769D TX output requires a 1.8V bias. Therefore, we will need an inductor RF choke for each leg of the AFE7769D TX output to bias the output at 1.8V, and then connect to the LMH9135 input.

Please advise if the VCM of the LMH9135 input can accept external 1.8V bias.

Hi Kang,
   Upon Checking with design, if there is AC coupling cap between AFE and LMH, then we don’t need to worry about the CM mismatch. So please go ahead with 56nH on R29 and R49 and LMH9135 can accept 1.8V bias

Best,
Hruday

Hi Kang and Hruday,

Thanks for your valuable feedback. Summarizing the above threads we are advised to use 56nH Inductor for both R29 and R49, please correct me if i am wrong.

Thanks and regards

- Teja 

Hi Hruday and Kang,

we need one more confirmation regarding LMH9135 . Please share the maximum PD pin input current.

Hi Sreejitha,
    We have not captured PD pin bias current data for the datasheet. Our validation team is working to get that, I will share the results next week.

-Hruday

Hi Team,

Thanks for your quick responses.

Based on our previous threads, we are recommended to use 56nH Inductor and populate both R29 and R49. For this inductor as this is using for the RF signals generally we use the inductor parts which has 2X greater SRF(self resonance frequency) in this case we have to use approx 7GHz. That High SRF inductors are not available. Can you please advise us whether we need to consider this Higher SRF Inductor or any lower value is possible.

Thanks and Regards

- Teja

Hi Teja,

You may use a different physical size inductor (i.e. 0402 size or 0603 size) to allow for higher SRF frequency. You may also see if a slight reduction of inductance value would allow you get to higher SRF.

The goal is to have different combination of physical size/actual inductance to achieve SRF, while at the same time maximize the impedance above 3.5GHz RF band so the pull-up impedance is not impacting the performance.

Hi Team,

Can you please suggest the part number of 56nH inductor that can be used, as we are not getting with needed SRF value in both 0402 and 0603 size even we looked for slight reduction of inductance value . Usually we use Less than 10 nH in such applications could you please explain why 56 nH were suggested. While we are checking we found maximum 19nH with SRF 5GHz (with current size, without disturbing layout).

Aswathy

Please advise the part number for the 19nH inductor and we can review.

HI kang,

19 nH - LQW15AN19NG8ZD .. can this be used?

Hi Aswathy,

Please go ahead with this inductor choice. 

19nH of RF choke, along with 100ohm output impedance of the AFE, will create a high pass corner of ~850MHz. With 56nH of RF choke, along with 100ohm output impedance of the AFE, will create a high pass corner of ~250MHz. The goal is to use sufficiently high inductance to move the high pass corner at least a decade away from the main RF band. The corner of 850MHz is roughly 2 octave, and should still be sufficiently well.

-Kang

Thankyou kang.

Hi Hruday and Kang,

We need one more confirmation regarding LMH9135. Please share the maximum PD pin input current.

Hi Nimesh,
   Our validation team is still working to get this data. Will get back to you earliest.

-Hruday

Hi Hruday,

One more doubt we have.