SN75DP119: Can SN75DP119 support cascading

Hi,

DP119 can support cascading. They say they can detect the signal on the output side, can they please share the scope waveform, and is it a valid signal? DP119 does not have a way to generate the signal internally, so I am curious where the signal is coming from.

Thanks

David

Hi  David

The customer tested a few test cases and found the SN75DP119 output as shown in the figure below:

The EN pin of the SN75DP119 is directly connected to VCC, so the SN75DP119 starts working as soon as VCC powers up.

We used a differential probe to test the output of the first SN75DP119. We noticed that whether we disconnect the input of the SN75DP119 (or tie it to GND) or disconnect the output, we can still detect an output signal at the SN75DP119 output. Therefore, we believe this signal originates from within the chip itself.

yellow is HPD , pink is DP119 output.

We understand that the DP119 is a wideband amplifier. Could it be that some noise is being amplified? The output we observe from the DP119 is unorganized noise rather than a regular sine or square wave. Could you provide us with some troubleshooting directions?

Hi,

Can they share their schematic and layout for review?

Thanks

David

Hi David

we use it as follow schematic.

we suspect the noise is from chip, we use external DC source give 3.3V and disconnect input DP cable and we also can see the noise in output pin.

Could you help check if this issue must use EN to control chip?

Hi,

If you only use OUT1, then EN = VCC/2. Below shows the EN control

EN = HIGH: Device in Normal Mode, both outputs OUT1 and OUT2 are enabled;

EN = VCC/2 (input left floating): Device in Normal mode, 2 nd output is disabled;

EN = LOW: Device in Power Down mode. All outputs are high-impedance; Inputs are ignored

Can they share the actual schematic/layout for review?

Thanks

David

Hi  David

We tested change EN. We can see the noise is removed.

So we would like to know if the chip itself outputs some noise as soon as EN (Enable) is turned on. I noticed that the SN75DP119 does not have an evaluation board available. Could you help us confirm whether it is normal to see noise on the output when the input and output are left floating while EN is enabled?

Thank you very much.

Hi,

We tested change EN. We can see the noise is removed. -> Are they pulling EN low in this case? When EN = LOW, DP119 is in Power Down mode. All outputs are high-impedance and Inputs are ignored, so I am expecting that you see the noise being removed when EN = LOW.

Have they measure the input and the power supply to make sure there is no input or power supply noise?

Can they please share the schematic/layout for review?

Thanks
David

Hi David 

Yes , you are right .

The customer used an external DC-DC power supply for testing and found that the noise still exists, so they ruled out power supply noise from the PCB. They also disconnected the input and output pins, but noise is still being observed on the output pin after the EN (enable) pin is pulled high.

Sch in below:

By the way ,for EN pin , customer use 10k RES connected to the VCC.

Hi,

Do they have power decoupling capacitors on VDD_3.3_eDP? 

Leaving the DP119 inputs un-connected can be a problem if noise is present in the system.  The inputs can have a significant differential "gain" and could possibly be causing some issue.  It is recommended that unused inputs have a fail safe circuit installed (1k pull down on the inverting input) to insure good operation. If they implement the safe biasing, do they still see noise on the output?

Thanks

David

Hi David

We have complete decoupling capacitors on the 3.3V rail as shown in the figure below. As you can see, both of our channels are in active use; there are no unused channels. Let me summarize the experiments we have conducted:

1. The MPU is connected to the DP119 input (software-controlled with no output). Noise appears at the output right after the DP119 is powered up and enabled (EN).

2. We disconnected the input between the MPU and the DP119 to rule out any impact from the MPU, but the noise was still observed at the DP119 output port.

3. We tied the DP119 input to GND, but the noise persisted.4. We disconnected the DP119 output from the display side, and the noise was still present directly at the DP119 output pins.

4. We use external ACDC source to give 3.3V , also have noise,

We also believe this is possible because the DP119 has a wide bandwidth and is relatively sensitive. However, we would like to get an official confirmation on whether having output noise after power-up is expected behavior.

Since we do not have an EVM , we cannot perform cross-verification.If possible, could you please help check with DP119 or test under what specific conditions the DP119 will show no output noise? Looking forward to your reply. Thank you.

Hi,

DP119 is a pretty old device, unfortunately we no longer have an EVM for cross-check. I also went back and looked at its characterization data, and don't see any data reflecting any output noise after power-up. This is also a mature product, so I would expect other customers reporting the same issue and I am not seeing it. 

But with DP119 being a high speed limited re-driver, I think it is picking something up at the input and then amplify it. Have they probe the input the same time they see the output noise?

Thanks

David