SN74LVC16T245: SN74LVC16T245 16-BIT DUAL-SUPPLY BUS TRANSCEIVER

Max Pom

Part Number: SN74LVC16T245
Other Parts Discussed in Thread: SN65LBC175

Hello,

The Item datasheet doesn't mention the output impedance of the driver.

It is common to add a series termination at the output of the transmitter to achieve a total 50ohm, when dealing with relativelly fast signals,

how can I obtain the value?

(Same thing about the SN65LBC175)

Thanks,

Max

over 6 years ago

0 Clemens Ladisch over 6 years ago

Guru 316440 points

What is the required accuracy of this 50 Ω output impedance?

0 Max Pom over 6 years ago in reply to Clemens Ladisch

Intellectual 350 points

Accuracy is not very important, supposed to be relatively close to 50ohm.. if the output resistance is about 20ohm (common value), i'll place a ~30ohm resistor, but if its already close to 50, I will not place one.. some datasheets mention this value.. is ~20 value also a common value for TI buffers?

Thanks!

0 Clemens Ladisch over 6 years ago in reply to Max Pom

Guru 316440 points

The worst-case output impedance is specified in the datasheet (0.55 V / 24 mA = 23 Ω at VCC = 3 V).

The typical output impedance can be derived from the graph in SZZA010 (about 13 Ω; LVC has strong drivers), but there is no guarantee, and there is a temperature coefficient.

If your VCCs are not above 3.3 V, you could use translators with integrated output damping resistors.

0 Max Pom over 6 years ago in reply to Clemens Ladisch

Intellectual 350 points

Hello, could you point on the page on datasheet where you took the 0.5V from? I want to understand how do I derive it in general. Thanks!

0 Clemens Ladisch over 6 years ago in reply to Max Pom

Guru 316440 points

This is from the V_OL specification in section 7.5.

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

TI__Mastermind 45675 points

Hi Max,

These FAQs may also help you:

0 Max Pom over 6 years ago in reply to Dylan Hubbard

Intellectual 350 points

Thanks a lot!
Any way, I think I missed something, because I see a little conflict in these explanation. It says that the typical input impedance of CMOS is 10Mohm. So, the input current (for the 3.3V Vcc example) is 3.3/10M=0.33uA. Than, why using the 0.55V/24mA to calculate the output impedance (which is 23ohm), and not the 0.1V/100uA (which is 1000ohm)? Or can I actually allways assume it's 25ohm? Thanks!

0 Clemens Ladisch over 6 years ago in reply to Max Pom

Guru 316440 points

Why are you mentioning the input impedance? It has nothing to do with the output impedance.

At a lower VCC, the gate voltage of the output transistors is lower, so they have a higher impedance. What VCC are you using?

0 Max Pom over 6 years ago in reply to Clemens Ladisch

Intellectual 350 points

Hello,

Im using VccB=3.3V. My point was that according to the explanations in the FAQ, if I drive a CMOS device, which has a typical 10Mohm input, than my driver's IOH shall be about Vcc/10M, which is 0.33uA. In the datasheet of the device (LVC family), the VOH for such IOH is VccB-0.1 (the dataheet mentions only for 100uA). So, why calculaying the Rout=(3-2.4)/24mA=25ohm, and not Rout=(3.3-3.2)/100uA=1000ohm? Maybe because the calculation is too senstive in such values?

In my case, I chacked the input current of the destination device, and it's about 50uA, so I just want to be sure what series resistor to use (comlimentary to 50ohm). Hope I explained myself, thanks!

0 Clemens Ladisch over 6 years ago in reply to Max Pom

Guru 316440 points

The datasheet guarantees that the output impedance is less than 1000 Ω at any VCC; at larger VCCs, it will actually be smaller (as can be seen by the other VOL/VOH specifications).

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

TI__Mastermind 45675 points

Hi Max,

For LVC I always recommend a 33 ohm in series. This should get you close enough to 50 ohm.

0 Max Pom over 6 years ago in reply to Dylan Hubbard

Intellectual 350 points

Thanks!

Is there any comparison tabe, which matches the recommended series resistor value to device family (LVC etc.)?

0 Clemens Ladisch over 6 years ago in reply to Max Pom

Guru 316440 points

No; you have to derive the output impedances from the respective datasheets.

Logic

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SN74LVC16T245: SN74LVC16T245 16-BIT DUAL-SUPPLY BUS TRANSCEIVER