trying to understand High Impedance on Input

[cjc38]

New member...I have searched everywhere and am struggling with the precept of high impedance input being very susceptible to circuit noise thus requiring a pull-up resistor (internal or external) for microcontroller pins. Doesn't high impedance mean high resistance/Ohms? If that is the case wouldn't the input require a higher current to change the states instead of being influenced by low power noise?. I understand the function of the pull-up (down) resistor for current limiting and stable state. Here are the 2 articles that reference the situation.


and
ATmega8 protostack simple demo | YourITronics
paragraph 4. Any help would be great. Thanks

 

[audioguru]

A high current is needed for lower value resistors.
A very low current is needed for higher value resistors so interference pickup occurs with high value pull-up or pull-down resistors.

 

[kpatz]

High impedance inputs use voltage as a trigger. Very little current passes through the input. If left floating, it will tend to float to a voltage in between Vdd and ground, right in the middle of the high/low thresholds, causing erroneous triggering. Since the current is very low, electrical noise in the vicinity can be easily picked up by the input. The pull-up or pull-down resistor "pulls" the input either high or low, and is strong enough to overcome noise pickup but weak enough so that the input can be changed by something driving it (e.g. a switch).

CMOS circuits use MOSFETS and have high-impedance inputs. TTL circuits use bipolar transistors which have low-impedance inputs. With TTL, it's current rather than voltage that triggers the input. This makes TTL more immune to noise without pullups but more power is drawn and more heat is generated.

 

[cjc38]

Thanks Kpatz. The last paragraph in your reply really was what I was getting at. It seems backwards to me. I am thinking of it as that a high impedance (high Ohm?) input would require a higher current to activate a state change like you mentioned about the TTL . I must be viewing the term "high impedance" from the wrong standpoint. I would think that the TTL would be a HIGH impedance input as it would require a higher current to overcome the 'high impedance' (high resistance?) of the input. Thanks for your patience

 

[mneary]

A higher impedance requires only a low current to develop a significant voltage.

 

[crutschow]

Per Ohm's law, V=I x R. Thus for a given voltage, the higher the resistance (impedance) the smaller the current.

 

[ke5frf]

I am thinking of it as that a high impedance (high Ohm?) input would require a higher current to activate a state change like you mentioned about the TTL . I must be viewing the term "high impedance" from the wrong standpoint.

What you are doing is confusing voltage controlled devices with current controlled.

As was explained, a high impedance device (ex. MOSFET or CMOS) is controlled by voltage levels. A bipolar junction device is a current controlled device. The emitter-collector current is controlled by the base-emitter current. This is how it amplifies a signal, a small current at the B-E shapes a higher current at the E-C.

In a Field effect device, there is a high impedance (resistance) between the gate-source (analogue of base-emitter). When a voltage is present, an electric field occurs. This field redistributes the charge carriers in the P or N material, creating a channel for current flow or closing it off, depending on the voltage and configuration of the device.

It is key to remember that a voltage is potential energy, and a high input impedance creates a potential difference equal to the applied voltage. In FE devices, the potential is nearly infinite, so there is little current draw at the G-S. thusly, a 5 volt signal will switch a typical MOSFET on or off without much current flow from G-S. This makes MOSFETS particularly suited for low power digital circuits.

BJTs work differently. They have a lower input impedance at the B-E junction, similar to a forward biased diode, which is what they really are. This requires a current limiting resistor at the base of the transistor to prevent too much current. A BJT is current controlled in this matter between its cut-in and saturation limits.

Low impedance means more current flowing.
High impedance means voltage controlled.

All of this relates to Ohm's Law.

 

[Robert_G]

Good explanations, good thread!

 

[cjc38]

Thanks ke5frf. Thanks to everyones replies it was obvious that MOSFET's have high impedance inputs and BJT's have low one's. That part I knew. What I didn't know is why they were called that. After realizing that the inputs of the microcontroller were FET based, your explanation made sense only after I researched FET and especially MOSFET function. without getting into OHM's law I discovered that the reason they are called high impedance inputs is that there is an insulating layer attached to and under the Gate keeping it from contact with S-D thus no current flow but as you explained the voltage acting on the charge on the Gate creates an electric field allowing current flow from Source to Drain. Thanks to all of you for clearing this up....now I can move forward with the project with a better understanding of the circuit component selection. Take care all and thanks again