SN74AVC4T774: level converter

Layne Jakahi

Part Number: SN74AVC4T774
Other Parts Discussed in Thread: SN74AXC4T774

Hi, guys,

I am trying to use SN74AVC4T774 from TI.

I am using it as a 3.3V to 1.8V SPI level converter. Since it is a high speed SPI and super long cable running the SPI signal, I want to avoid reflection.

So what is the input impedance and output impedance of the A/B port? Then I can calculate series resistor to terminate at the A/B source side.

Thanks a lot,

over 6 years ago

0 Dylan Hubbard over 6 years ago

TI__Mastermind 46005 points

Hey Layne,

These 2 FAQs below should be able to help you with this:

https://e2e.ti.com/support/logic/f/151/t/763609?tisearch=e2e-sitesearch&keymatch=faq%3Atrue

https://e2e.ti.com/support/logic/f/151/t/718814?tisearch=e2e-sitesearch&keymatch=faq%3Atrue

The inputs are high impedance CMOS inputs.

Also, I recommend using the SN74AXC4T774 for this.

0 Layne Jakahi over 6 years ago in reply to Dylan Hubbard

Intellectual 820 points

Hi,

Thanks for the reply.

Usually according my experience, the output impedance of logic level is around 25ohm which matches the examples in your FAQ.

According to the ways to calculate the output impedance, Can I conclude that for 1.8V bank, the output impedance is roughly 56 Ohms (0.45/0.008A = 56.25Ohm) and for 3.3V bank, the output impedance is roughly 58Ohms (0.7V/0.012A = 58.3Ohms).

Can you confirm this information? I will attach the table below.

Thanks

0 Clemens Ladisch over 6 years ago in reply to Layne Jakahi

Guru 317310 points

Yes, you can compute the worst-case output impedance this way. The typcial output impedance will be lower.

Please note that the AVC output impedance is not constant; it is lowered during switching (see AVC Logic Family Technology and Applications (SCEA006)).

Once upon a time, TI wrote reports like Input and Output Characteristics of Digital Integrated Circuits at 3.3-V Supply Voltage (SZZA010). But the AVC/AXC families are newer than that report.

0 Dylan Hubbard over 6 years ago in reply to Clemens Ladisch

TI__Mastermind 46005 points

Hi Layne,

Yes you've done the calculation correctly, and as Clemens mentioned this is worst case. Since there is margin built into those specs, you can probably shave off 15-20 ohms to get closer to the typical output impedance. I'd say 30 ohms would be a good value to work with when doing your matching for 3.3 V side. And 40 ohms for the 1.8 V side.

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SN74AVC4T774: level converter