SN74HCS126-Q1: I/O VOLTAGE LEVEL COMPATIBILITY - SCHMITT TRIGGER BUFFER

Pedaiah G

Prodigy 230 points

Part Number: SN74HCS126-Q1

Dear Sir/Madam,

I am planning to use SN74HCS126QPWRQ1 Schmitt Trigger Buffer to interface Optical encoder switch.

Optical Encoder Switch Voltage Levels:

VCC=5V,

VOH min = 3V

VOL max = 1V

Schmitt Trigger Voltage Levels:

at VCC=4.5V,

VT+ min = 1.7V, VT+ max = 3.15V

VT- min = 0.9V, VT- max = 2.2V

We want to know following clarifications.

1. Can I interface Schmitt trigger with Optical encoder switch without any logic voltage level issue when Schmitt trigger supply is 5V?

2. It is given in Application report ( Link: https://www.ti.com/lit/an/scea046/scea046.pdf) that "the input levels Vih and Vil must be greater than (VT+ max) and less than (VT– min) to insure the part will switch".

As per this statement we shall not interface Encoder switch with Schmitt trigger when I operate at 5V. Can you please suggest us an alternate Schmitt trigger buffer part number which i can interface optical encoder with good Noise margin?

3. For SN74HCS126QPWRQ Schmitt trigger at 3.3V and 5V VCC, what will be the VT+ and VT- range ?

4. Are Schmitt trigger inputs of SN74HCS126QPWRQ1 are 5V tolerant?

Thanks and Regards,

Pedaiah G

over 4 years ago

0 Emrys Maier over 4 years ago

TI__Guru* 97046 points

Hi Pedaiah, and welcome to the forums!

You are correct that the input must cross VT-(min) and VT+(max) for normal operation. This is stated explicitly in the datasheet:

You can use linear interpolation to determine the threshold values between those given in the datasheet. There's a post on how to do this here: [FAQ] What method is best used for estimating specification values between those given in the datasheet?

For example, at exactly 5V VT-(min) = 1V and VT+(max) = 3.5V

At 3.3V, VT-(min) = 0.612V, and VT+(max) is 2.358V

You'll usually want to put in your supply tolerance as well to get the outer limits.

As for the question of input being 5V tolerant -- this is more of a question of system design. The inputs have clamp diodes as shown in the datasheet:

Each input can be driven above the supply voltage, but it will activate the clamp diode. The current through the diode should be limited to 20mA (max):

As for another Schmitt-trigger buffer with different thresholds, I would recommend to look into comparators instead. You can dial in your hysteresis precisely with those.

0 Clemens Ladisch over 4 years ago

Guru 317360 points

Please provide the datasheet or at least the complete part number of the optical encoder. It is likely that test conditions of the VOH/VOL specifications do not apply for CMOS inputs, and that the actual output voltages will work.

0 Emrys Maier over 4 years ago in reply to Clemens Ladisch

TI__Guru* 97046 points

That's a good point Clemens.

The VOH and VOL specs should have an associated test current. It's likely that our input will have such a low input current that the outputs will be much closer to the rails.

0 Pedaiah G over 4 years ago in reply to Emrys Maier

Prodigy 230 points

Hello Emrys maier,

Thank you so much for your quick response and clarifications.

Please find the below link of Optical encoder switch.

Link: https://www.grayhill.com/documents/62H-Datasheet

As per your recommendation, we will look into use of comparator.

We need to know the following clarifications.

1. Can I use Level translator (5V/3.3V) with Schmitt trigger input?

2. If Yes, What is the Part number do you recommend to meet our requirement?

Thanks and Regards,

Pedaiah G.

0 Clemens Ladisch over 4 years ago in reply to Pedaiah G

Guru 317360 points

The specified V_OL/V_OH values are for TTL inputs with much higher input currents. When connected to high-impedance CMOS inputs, the 62H outputs (open-collector with pullup) will be near the rails. The SN74HCS126-Q1 is safe to use with the 62H.

You do not need a separate level shifter; open-collector outputs like this can be made to output a different voltage by simply connecting the pull-up resistors to the desired voltage.

0 Pedaiah G over 4 years ago in reply to Clemens Ladisch

Prodigy 230 points

Hello Clemens,

Thank you for the clarifications.

As per your suggestion, we will use SN74HCS126-Q1 with Optical encoder switch.

Thanks and regards,

Pedaiah G.

Logic

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SN74HCS126-Q1: I/O VOLTAGE LEVEL COMPATIBILITY - SCHMITT TRIGGER BUFFER