SN75LVDS83A: CLKIN input structure

Zhou Fang67

Part Number: SN75LVDS83A

Hi Expert,

We are having a problem when connecting SN75LVDS83A to FPGA.

As shown in below capture, if SN75LVDS83A is connected to FPGA with CLKIN & D4. For CLKIN, distortion is found(Green waveform). However, the waveform on D4 pin is normal.

Both CLKIN & D4 are input pin for SN75LVDS83A, why CLKIN signal from FPGA is affected by device, but D4 is not? What is the input structure for CLKIN pin?

Thanks.

Zhou

over 6 years ago

0 Ikechukwu Anyiam over 6 years ago

TI__Mastermind 35130 points

Hi Zhou,

It looks like there is some extra capacitance and something causing reflections on the CLKIN pin. The signal still looks fine for a clock signal though so this shouldn't be an issue. I'll check to see if we have any information the input structure though.

Regards,
I.K.

0 Zhou Fang67 over 6 years ago in reply to Ikechukwu Anyiam

TI__Expert 4250 points

Hi Anyiam,

I have the same suspect, but there is no capacitor & resistor on trace.
Not sure about whether the input structure for CLKIN pin is the same as D4 pin. Because there is no such information in datasheet.

Thanks.
Zhou

0 Zhou Fang67 over 6 years ago in reply to Ikechukwu Anyiam

TI__Expert 4250 points

Is there any update for this problem?

Thanks.

0 Ikechukwu Anyiam over 6 years ago in reply to Zhou Fang67

TI__Mastermind 35130 points

Hi Zhou,

I have contacted the designers for more information, but they haven't gotten back to me yet. I will update you as soon as I have more information.

Regards,
I.K.

0 Ikechukwu Anyiam over 6 years ago in reply to Zhou Fang67

TI__Mastermind 35130 points

Hi Zhou,

I have confirmed with design that CLKIN has higher input capacitance than the data pins, which explains the distortion you are seeing.

Regards,
I.K.

0 Zhou Fang67 over 6 years ago in reply to Ikechukwu Anyiam

TI__Expert 4250 points

Hi Anyiam,

Do we have the input capacitor value for CLKIN pin? Then we can note this in design.

Or is there any P2P device we can replace SN75LVDS83A?

Thanks.

Zhou

0 Ikechukwu Anyiam over 6 years ago in reply to Zhou Fang67

TI__Mastermind 35130 points

Hi Zhou,

We don't have that information but you should be able to measure it in the lab. There are P2P alternatives listed on the product page, but I imagine they probably all have higher input capacitance on the CLKIN pins. There shouldn't be an issue using the signal as is though, since it still looks fine for a clock signal.

Regards,
I.K.

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SN75LVDS83A: CLKIN input structure