High impedance on high side of inverter

Thread Starter

samyang

Joined Jul 24, 2022
35
Hi,

Can someone please explain what high impedance means? I understand that this means that there is very little current.

Specifically, I am trying to understand what high impedance does to this inverter when it is on the high side ( SN74LVC1G04) I have also attached the datasheet. I currently have it in the schematic for the signal that enters the inverter to go to ground at the low state and 3.3V at the high state, but I understand that because this component has high impedance at the high state, it will never reach 3.3V, so what happens to the signal?


Update: corrected datasheet for part
Thanks.
 

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Last edited:

BobTPH

Joined Jun 5, 2013
5,496
Where did you see anything about high impedance in that datasheet?

Normally, when talking about a gate, a high impedance state indicates it cannot source any current and you need a pullup resistor on the output. But that is not the case with that part, it can both source and sink current.
 

Thread Starter

samyang

Joined Jul 24, 2022
35
Where did you see anything about high impedance in that datasheet?

Normally, when talking about a gate, a high impedance state indicates it cannot source any current and you need a pullup resistor on the output. But that is not the case with that part, it can both source and sink current.
Thank you for your response. I attached the wrong datasheet by accident. I have reattached the new datasheet. I beleive there is high impedance because it does not show the high level output current.
 

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Papabravo

Joined Feb 24, 2006
18,991
This particular inverter has a high impedance input. This means it presents a minimal load to whatever output is driving that input. In addition, it means that output can drive several high impedance inputs without any loss in signal level due to loading.

In addition to there being very little current into or out of the high impedance input the voltage at the input will almost always be at Vcc or GND. the last implication of a high impedance input is that you must NOT leave any high impedance input unconnected. If you do that input will drift around and cause the output to oscillate at a very high frequency causing extreme stress and power dissipation in the part.

The datasheet will have information on what the output will do. In most cases it will be so close to Vcc and GND that you won't be able to measure the difference. Here is a simulation that gives you some idea of what output voltage you can expect with pretty big loads.
1659492402687.png
 

MisterBill2

Joined Jan 23, 2018
12,722
In general, "High Impedance output" means that the circuit output voltage has a high internal source effective resistance. That means that drawing any current will cause a rapid drop in output voltage. A high impedance input circuit means that the input will not draw much current as the input voltage is increased.
As stated with digital devices, a "high impedance" output means it will neither sink nor source enough current to have any effect on connected logic.
 

Papabravo

Joined Feb 24, 2006
18,991
Just to be clear:
  1. The 74LVC1G04 has a single high impedance input.
  2. The 74LVC1G04 has a single low impedance output. It can source or sink 24 mA.
The datasheet link you gave was for an open collector inverter of a different series

There is a part with a 3-state output which is controllable. The three states are HI, LOW, and High-Z (High Impedance). These are:
  1. 74LVC1G125 3-state inverting buffer
  2. 74LVC1G126 3-state buffer
 
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