1. As shown in the recommended operating conditions, the maximum supply voltage is 2.7 V. So there are no specified switching thresholds for 3.3 V. For supported supply voltages, see the recommended operating conditions.

2. The actual switching threshold will be near V_CC/2. But using a DC signal with that voltage would violate the ∆t/∆v limit.

2. This is not an analog device. The output will be either low or high. But an input voltage between V_IH and V_IL will lead to much higher power consumption, can make the output oscillate, and can damage the device. See [FAQ] How does a slow or floating input affect a CMOS device?

Hi,

1) Can you please explain in detail what is the meaning of switching threshold is Vcc/2.

2) I thought inverter switching threshold voltages are Vin_l and Vin_h and not Vcc/2.  For example Vin_l is the max input voltage when the output is logic 1 right. So this is the threshold the inverter switches to logic 1 from logic 0 at the output. Similarly Vin_h is the minimum input voltage when the inverter switches from logic 1 to logic 0. 

30 What is criteria of measuring Vin_l for the inverter. What I mean by that is when we vary input DC voltage of the inverter , what is the value of the voltage we should measure at the output to calculate Vin_l of the inverter assuming Vcc=2.7 V.

Thanks,

Syed

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1. When the input voltage crosses the switching threshold, the output switches.

2. The switching threshold is guaranteed to be somewhere between V_IH and V_IL, but is typcially near V_CC/2. The V_IH and V_IL specifications ensure that you do not try to use invalid input voltages.

3. The output voltages are specified as V_OH and V_OL.

Hi,

My understanding is that the max switching input threshold for logic1 is Vin_l. And the minimum input threshold voltage for logic 0 is Vin_j . In between Vin_l and Vin_h input voltage, the output is in determinate state which means that it can be either logic 0 or 1 .Input voltage should not be in between Vin_l and Vin_h to avoid uncertainty in the output.

1) But when you are saying that vcc/2 is the threshold  and it is in between Vin_l and Vin_h, then what does this mean.

2)When the input voltage is at Vcc/2 then what is the output voltage.

3) When the input voltage crosses Vcc/2, the output switches means what. Does it switch to logic 0 or logic 1.  

Thanks for clarifying in advance.

Syed 

1. When the input goes from V_IH to V_IL, the output goes high. When the input goes from V_IL to V_IH, the output goes low. The datasheet guarantees that the thresholds are somewhere between these limits, but in practive, both are near V_CC/2.

2. It might be high or low or oscillate.

3. Depends on the direction.

Please see below the method used to calculate Vin_l and Vin_h of this TI inverter:

When I have characterized this inverter, I was injecting DC signal at the input. When I vary input signal at the input manually from 0 to VCC volts I have seen the following results:

1) When my input voltage is 0V, of Course the output will be near VCC. But as I increase DC input voltage the output starts dropping and when the output voltage reaches Voh = 0.7 X Vcc, I have recorded the input level as Vin_l. This is the max input voltage I can able use as logic 1 at the output. 

2) Similarly as I increase the input DC voltage in continuation of step1 above, at some DC input voltage I have measured the output voltage V0l=0.3 X VCC. I have recorded the DC input voltage which gives this Vol as Vin_h.

Is this method described above is correct to measure Vin_l and Vin_h of the inverter ?.

3) If this method is not correct. please let me know the right method of calculating Vin_l and Vin_h.  Please note I am not passing through the inverter threshold of Vcc/2 you have mentioned before with this method but still I can able to switch to logic 1 and logic 0 just with DC input voltage value at the input.

Thank you,

Syed

Hi Syed,

This is a fairly common point of confusion. We have an FAQ on the topic here:

[FAQ] Why does my device not switch at VIH or VIL? 

Hi Emrys,

This info is quite helpful.

Two Quick questions:

1) My  logic high and low value at the input of  SN74AUC2G04 inverter is 2.7V and 1.1V respectively. Inverter  VCC is set at = 2.7 V, I know logic 1 input = 2.7 V is OK to switch the output to logic low but the question is regarding input logic low level = 1.1V. Do you think this  logic low = 1.1V can switch the inverter  to logic high at the output reliably as this value is above VIL of the inverter which is 0.7V according to the datasheet. Even If we use 0.3X Vcc = 0.3* 2.7V =0.81V,  1.1 V is higher than 0.81V. Please clarify

2) Just as a side note if we use VCC = 3.3V which is outside the recommended value for this inverter. Is logic low value = 1.4V at the input of this inverter is reliable to switch to logic high at the output. According to your educated guess , is this level of 1.4V too high to allow this inverter to work reliably. What would be the max logic low value you think is reliable for it to work in case if we use this part due to heavy shortage.

Thanks,

Syed

Hi Syed,

You are welcome to use the device as you see fit, but we won't be guaranteeing functionality except what you find in the datasheet.

To be clear, TI cannot recommend using a device beyond the datasheet specifications.

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That being said, if you don't mind operating outside of TI's guarantees, the AUC part will have an input threshold around Vcc/2, typically a little less than that (maybe 45% to 48% of Vcc).

As long as your input is below this value, the input should be recognized as a LOW, and as long as it is above this value, it should be recognized as a HIGH. Typically we will define V_IL as 0.35*Vcc or 0.3*Vcc, and V_IH as 0.65*Vcc or 0.7*Vcc to privide a wide safety margin for operation.

Additionally, holding an input near the threshold can cause other issues -- added power consumption, oscillations, and reduction in reliability among them. I would not recommend operating the device as you have described.

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Have you considered switching to a device that was designed for 3.3V operation? Perhaps the SN74LVC2G04 or the SN74AUP2G04 would work for your application?

Hi Syed,

I see that you have rejected my answer. Can you tell me what is inaccurate or not helpful about my answer so I can help you further?

Hey Syed,

Thank you for clarifying -- you may have accidentally clicked on the 'reject answer' button or it may have been a system issue (recently I have seen this happen a few times). I appreciate you updating this to resolved.

Please let me know if I can be of assistance with anything else in the future.

Hi Emrys,

Just last question. Likewise, If my input is higher than ~ 0.48 * VCC then the inverter senses this as logic 1.

Best Regards,

Syed