ISO1640: I2C rise time is too long and input signal wave issue

hao Su

Part Number: ISO1640
Other Parts Discussed in Thread: PGA900

Tool/software:

Hi:

I use ISO1640 as I2C output isolator for PGA900.

But I find I2C SDA rise time is too long. PGA900 SDA rise time is about 240ns without IS01640 in 400KHZ.

But when I connect the PGA900 i2c to ISO1640, the rise time will be 360ns that is longer than 300ns.

I find there is strong wave on input SDA signal ,like the picture, there is 110ns flat wave part which make the rise time be longer than 300ns.

How can I reduce the rise time?

I have try to change R3 R5 from 100OHM to 0ohm but helpless for this issue.

0 Clemens Ladisch 7 days ago

Guru 298810 points

This is the propagation delay through the isolator; you cannot avoid it.

0 Varun Kumar 6 days ago

Hi Su,

Thanks for reaching out. Please find my inputs below:

The 110ns flat portion in the waveform is a characteristic of the device timing performance, mentioned a tLOOP1 in Datasheet as well:

For more clarity, please refer to section 8.5 which explains this delay and how it is dependent on the device architecture.

Regards

Varun

0 hao Su 5 days ago in reply to Varun Kumar

Thank you , I reduced the pull up resistor value make the rise time meet I2C spec.

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