HD3SS3212: HD3SS3212 VBIAS

Hello Jovnas,

Biasing voltage is only required when there is a common mode voltage higher than 2V on the line.
The purpose of biasing is to make sure the I/Os of the pins see a voltage that is within recommended operating condition of the device.

If there is no risk of common mode voltage exceeding 2V in your application, then VBIAS is not needed. 
If there is a risk, then VBIAS should be 1.8V and pulled up using a 1k resistor.

Thanks,

Nir 

Hi Nir and thank you for the quick reply.

The datasheet states "Because the switch requires a bias voltage, the designer must place the capacitors on one side of the switch. If they are placed on both sides of the switch, a biasing voltage should be provided".

The datasheet also states "In the case where the common mode voltage in the system is higher than 2 V, the coupling capacitors are placed on both sides of the switch".

I interpret this as a bias voltage is required in two cases:

1. AC coupling is added to both sides of the switch.

2. The common mode voltage is > 2 V.

Regards

Hello Jovnas,

AC coupling on both sides and VBIAS are required when common mode voltage is > 2V.
In any other case, you AC couple one side without any VBIAS.

Thanks,

Nir 

Hi Nir.

As I stated above, my application requires me to AC couple both sides. And according to HD3SS3212's datasheet, I must then add a bias to the HD3SS3212.

So back to my original question: Can you clarify that it is it ok to connect as Figure 6, with 100 kOhm resistors and using ground as VBIAS?

In e2e.ti.com/.../hd3ss3212-ac-coupling-capacitors-and-vbias-resistors-for-the-lines-exceeding-common-mode-voltage-of-2v they write "I recommend using a relatively weak bias resistor for VBIAS. 200Kohms to ground, or a bias voltage between 0 - 2V should work ok".

In e2e.ti.com/.../hd3ss3212-q1-common-mode-voltage-for-hd3ss3212-with-tusb501 they write "Placing AC coupling on all sides of HD3SS3212 is possible while biasing to ground with weak pull down resistors".

In e2e.ti.com/.../hd3ss3212-how-to-determine-the-pull-up-resistor they write "100k would be the recommended resistor".

Regards

Hello Jovnas,

Yes, to your original question that is fine if you do so. 
I suggested 1k, since that is what the datasheet recommended.

Though, it is related to common mode voltage concerns, which is not relevant here.
That is a miss on my part.

Thanks,

Nir 

Thank you Nir.

I did not understand what the RBIAS component was when I read the datasheet, since Figure 8 does not have any VBIAS.

But I find it confusing that you and the datasheet recommends 1 kOhm when several of your collegues have recommended 100-200 kOhm.

I also find it weird that the datasheet states "the switch requires a bias voltage" and that it also states that it is ok to use ground as VBIAS.

I would prefer if my concerns were answered before I make the PCB. Getting it theoretically correct is much better than experimenting with it on a PCB. Do you have a collegue that knows more about the inside of the HD3SS3212.

Regards, Jovnas

Hello Jovans,

After reading the other responses and the datasheet, I retract the 1k pull up value suggestion, because that is way too strong of a pull. 
I believe the RBIAS value in the datasheet is a typo, since such a pull up value doesn't really makes sense in high speed signaling. 
Using a 100k to 200k resistors is more appropriate.

The "the switch requires a bias voltage" statement from the datasheet is misleading. It refers to a different kind of "biasing".
It talks about the Host/Controller biasing on the I/Os of the mux, or in other words just the signals it is sending to its inputs. 
In the same paragraphs it mentions "the switch is biased by the system/host controller". 

The VBIAS with a resistor pull up, is what we call external biasing.
This is something we strongly recommend when common mode voltage exceeds 2V for this part.
The point of this biasing is to make sure the mux sees a voltage level of 0V to 2V on the I/Os.
You can pull down the line to ground, so the mux sees 0V, or pull it up to a VBIAS of 0V to 2V. 
 
External voltage biasing is only needed to counter high common mode voltage.
It is not needed when you AC couple both sides.
It is fine if you still do it, but it is not something we recommend. 

Hopefully this clarifies your concerns.
If not, fill free to follow up with further questions. 

Thanks,

Nir 

Hi Nir.

You wrote "External voltage biasing is only needed to counter high common mode voltage". But the datasheet states "Because the switch requires a bias voltage, the designer must place the capacitors on one side of the switch. If they are placed on both sides of the switch, a biasing voltage should be provided".

I must disagree with your interpretation, I think the datasheet is quite clear when it states that a bias voltage is needed when using AC coupling capacitors on both sides.

Regards, Jovnas

Hi Nir.

Thank you. I will then add pull-down resistors to ground on one side of the HD3SS3212.

Our application connects to other transceivers that have built-in AC coupling so we can't control where they are placed.

Regards, Jovnas