12 Replies Latest reply on Nov 24, 2013 4:34 AM by ki.leung

# What means high-z impedance exactly?

Hi, i cant find a note about the impedance of an analog, high z GPIO in the datasheet . Can anyone tell me that? Even in the OPamp datasheet is no info. thanks a lot

• ###### 1. Re: What means high-z impedance exactly?

High-Z means 'high input impedance'. That in turn means that the inputs don't put a load on the output / circuit they are connected to.

• ###### 2. Re: What means high-z impedance exactly?

Thanks for the answer but i mean a value for the impedance, how many ohms?

• ###### 3. Re: What means high-z impedance exactly?

Look into the PSoC3 Family Datasheet to be found here: www.cypress.com/.

On page 75 (english version) you find IIL given as 2nA maximum @ 3V from which you may calculate the input impedance using Ohm's law.

Bob

• ###### 4. Re: What means high-z impedance exactly?

Okay. Thank you

• ###### 5. Re: What means high-z impedance exactly?

Hi Z is a state of all drivers on pin turned off, such that all that remains

is leakage current of the input protection diodes, and the leakage of the

N & P mosfets turned off.

Keep in mind this leakage is a strong function of temp, so spec in datasheet

is just a limiting value at room temp. If you are driving external MOSFET gates

or Bipolar base connections, you need a R to gnd for MOSFET, or R to emitter for

Bipolar to absorb worst case leakage to keep external device from turning on when

a pin is tri-stated. R is computed from worst case leakage and thereshold for external

device.

• ###### 6. Re: What means high-z impedance exactly?

One correction, in case of MOSFET, R connections Gate to Source

for enhancement mode devices.

Regards, Dana.

• ###### 7. Re: What means high-z impedance exactly?

@Dana regarding leakage current and temperature: Datasheet says

Specifications are valid for –40°C

T    A        85°C and T    J        100°C

Bob

• ###### 8. Re: What means high-z impedance exactly?

@Bob, from datasheet, 25 C value (and a note its a characterization spec not a tested

spec) -

• ###### 9. Re: What means high-z impedance exactly?

Oops, I just stuck to the hedline and didn't look for the fine print.

Bob

• ###### 10. Re: What means high-z impedance exactly?

Bob, and others:

It's a little more subtle than that.

The input leakage is a current spec, it can leak to either rail, that's why it's absolute value. The nature of the current is to double every 10 deg C, and it's speced at the limit temperature, TJ= 100 C. Again, it's a current, not a resistance to Vss or Vdd.

There is a difference between "high-Z" and "high-Z analog." "High-Z" is a digital input, it has a small amount of hysteresis (~50 mV) on the digital threshold to prevent noise problems. The hysteresis causes a little bit of charge kick-back to the input pin at the transition. "High-Z analog" disables the hysteresis to yield cleaner ANALOG input waveforms in the case of a high source impedance. This is long standing PSoC practice, goes back to the first chips shipped in 2001.

---- Dennis Seguine

• ###### 11. Re: What means high-z impedance exactly?

@Seg, I understand the doubling every 10 C, classic diode behaviour, but what

about MOSFET leakage which does not follow that behaviour ? Does diode leakage

dominate or Igss and Idsoff in the N and P FETs ?

Regards, Dana.

• ###### 12. Re: What means high-z impedance exactly?

@set. Thanks for information about the effects of the hysterisis of the digital input.