# Tri-state buffer wrong output voltage?

I use this Tri-state buffer SN74ABT126N

I supply this Tri-state with the the Arduino Duemilanove 5V pin , both inputs of the tri-state are connected to the digital 1 pin on the Duemilanove.

When digital 1 is low I get high impedance output from the tri-state, but when digital 1 is set to high I only get around 4.5 volt from the output of the tri-state.

The tri-state output is connected to an op-amp with very high impedance input(around 10^12).

Why do I only get 4.5 volt, but not 5v?

The chip has four inputs and four enables. "both inputs" is ambiguous. I suggest posting a schematic. And your sketch (please use code tags).

Are you using Serial?

The output of a TTL gate only needs to be a valid LOGIC “1”. Research will show you that a Logic “1” is never guaranteed to be 5 Volts. A logic “1” is deemed anything berween than 2.2V and 5V. The output resistance when a 126 output is at logical “1” is determined by the output collector resistor.

You have an 85 Ohm resistor in the collector path. If you were to look at it from a voltage divider perspective… anything lower than a 300 ohm path to ground at the output from that point would be enough to get that kind of voltage loss. There seems to be no way you will get a full 5 volts (Same as Vcc) from a TTL output pin in the 126.

No because the serial interference with TX and RX, so I have not enabled serial.
Here is a simple schematic…hehe sorry for my painting skill. And the sketch, just think as the Digital 1 is HIGH all the time.

pwillard: The output of a TTL gate only needs to be a valid LOGIC "1". Research will show you that a Logic "1" is never guaranteed to be 5 Volts. A logic "1" is deemed anything berween than 2.2V and 5V. The output resistance when a 126 output is at logical "1" is determined by the output collector resistor.

You have an 85 Ohm resistor in the collector path. If you were to look at it from a voltage divider perspective... anything lower than a 300 ohm path to ground at the output from that point would be enough to get that kind of voltage loss. There seems to be no way you will get a full 5 volts (Same as Vcc) from a TTL output pin in the 126.

Sounds reasonable, thank you!

Now that your design is shown... its easier to say something additional.

It's not clear which precision op amp you are using... but unless you are using something like an OPA341... a single supply, rail to rail op amp... your output will not reach 5V at the analog input (OP amp output) due to issues with the op am as well.

pwillard: Now that your design is shown... its easier to say something additional.

It's not clear which precision op amp you are using... but unless you are using something like an OPA341... a single supply, rail to rail op amp... your output will not reach 5V at the analog input (OP amp output) due to issues with the op am as well.

The op amp is a LC272ACPE4 Have tested without the op-amp, and the tri-state still outputs around 4.5V And last time I tested this Op-amp it was 100% functional.

check the schematic of the buffer, there are two output transistors, one for low, one for high. you will always have at least 0.2V (looks like 0.5v in your case) voltage drop taken off the supply. That is why there is an accepted range for 0 and 1 as it is never quite literally 0V and 5V (or whatever other voltage is "1")