Query related to DS90CR483 - Using 1m cable

Hi Jeyalakshmi,

Are you planning to keep the same 9.1kOhm resistor on the PRE pin for the 1m cable case?

Please note the following in p. 13 of the DS90CR483/484 datasheet:

"The waveshape at the Receiver input should not exhibit over or undershoot with the proper amount of pre-emphasis set. Too much pre-emphasis generates excess noise and increases power dissipation. Cables less than 2 meters in length typically do not require pre-emphasis."

Since a 9kOhm resistor gives you a 50% pre-emphasis, which may lead to overshoot if the cable loading effect is not significant enough to reduce the receiver input signal within a valid operating range. Given a high quality 1m cable, addtional pre-emphasis may not be necessary. I recommend a higher value resistor than 9kOhm or no connection (NC) on the PRE pin.

Thanks,

Michael

Hi Michael,

Thanks for the reponse.

I am measuring the signal quality with 1m cable setup at receiver end. So i need the below details for validating the signal quality.

Receiver Requirement Clarification:

• Need the maximum allowed swing voltage levels
• Need the allowed overshoot and undershoot levels
• Rise/fall time

Thanks,

Jeyalakshmi

HI Jeyalakshmi,

I am a little confused by what you are asking for. Are you asking for specs from the DS90CR484 receiver or are you asking for the output specs from the DS90CR483 transmitter to see if a different LVDS receiver you are using can handle it after transmission over 1m cable?

If we are talking about the DS90CR484, then please see the following:

- Maximum allowed input swing voltage levels: 0V to 2.4V (see p. 3 of datasheet).

- Allowed Overshoot-Undershoot Levels: The waveshape should not exhibit over/undershoot with the proper amount of pre-emphasis set at the Tx (p. 13). However, since the allowed supply noise voltage is < 100mVpp, I advise that over/undershoot < 100mVpp.

- Rise/Fall time (for CMOS/TTL receiver output): CLHT (low-to-high): <= 2 ns; CHLT (high-to-low): <= 2 ns (see p. 6 of datasheet)

Thanks,

Michael

Hi Michael,

Thanks for the reply.

- Maximum allowed input swing voltage levels: 0V to 2.4V (see p. 3 of datasheet). - Is that for single ended signal level for LVDS signal?

Please clarify.

Thanks,

Jeyalakshmi

Hi Jeyalakshmi,

That is a single-ended signal level. An LVDS single-ended signal is centered around a common mode voltage of 1.2V and typically has a 400 mVpp, or 200mV amplitude, range. One of the advantages of LVDS signaling is that there is a large amount of leeway when it comes to acceptable common mode voltage range. The common mode for LVDS is ideally 1.2V, but the TIA/EIA-644 LVDS standard actually allows for +/-1 V Ground Noise variance. Thus, the maximum the receiver input could expect given this ground noise variance is a minimum voltage of 0V and a maximum voltage of 2.4V. See the diagram below for an illustration of this concept:

To add to this, since differential signaling is the difference of (IN+) - (IN-), you should expect a differential signal with an offset voltage of around 0V and voltage swing of approximately 800 mVpp.

Thanks,

Michael