SN74HCS245-Q1: Differential mode and common mode input impedance

Mark Philip Felipe

Genius 9705 points

Part Number: SN74HCS245-Q1

Hi Team, seeking for some support.

Looking for the differential mode and common mode input impedance for SN74HCS245-Q1 (Schmidtt Trigger)

Thank you.

-Mark

over 1 year ago

0 Clemens Ladisch over 1 year ago

Guru 271320 points

The Schmitt triggers are not implemented with resistors that are visible at the inputs. The input impedance is the same as for a plain CMOS input.

Logic devices have single-ended inputs; there is no differential mode.

0 Mark Philip Felipe over 1 year ago in reply to Clemens Ladisch

TI__Genius 9705 points

HI Clemens,

What would this input impedance be (same as plain CMOS input)?

0 Clemens Ladisch over 1 year ago in reply to Mark Philip Felipe

Guru 271320 points

For practical purposes, approximately inifite. If you really want to compute a value, see [FAQ] What is the impedance of the device's input pin?

0 Mark Philip Felipe over 1 year ago in reply to Clemens Ladisch

TI__Genius 9705 points

Hi Clemens,

I see the input capacitance is 5 pF, can I just model that as the input impedance of the CMOS or do I need to use the leakage calculation where Rpin = Vpin/Ileakage? Is the 5pF in parallel with the Rpin?

Also, can you please tell me the max power dissipation rating of the input (input impedance) in watts?

0 Clemens Ladisch over 1 year ago in reply to Mark Philip Felipe

Guru 271320 points

The input capacitance and resistance are in parallel. And they are so small/large that you usually ignore them.

The input leakage current is at most 1 µA; the resulting power dissipation is not noticeable.

0 Emrys Maier over 1 year ago in reply to Mark Philip Felipe

TI__Guru* 96711 points

Hey Mark,

If you're looking for a model of the device - we have an IBIS model that includes the input impedance on our website.

https://www.ti.com/product/SN74HCS245-Q1?dcmp=dsproject&hqs=#design-tools-simulation

We also have SPICE models for many HCS family devices that have the input / output impedance. This particular one doesn't have a model, but I pulled up the appropriate part of the SPICE model from another device in the family that might help you out:

.PARAM RA = 240000000
.PARAM RB = 240000000
.PARAM CA = 1e-11
.PARAM CB = 1e-11
RA  A  GND {RA}
RB  B  GND {RB}
CA  A  GND {CA}
CB  B  GND {CB}

0 Mark Philip Felipe over 1 year ago in reply to Emrys Maier

TI__Genius 9705 points

Hi Emrys,

Seeking further support for the customer.

1.) Can you send me the full spice model netlist for other HCS circuits below including the .SUBCKT line and ENDS? The spice netlist below is missing some information.

2.) Can you please send me the max power dissipation (both peak and average at the input pins for the part. I need to compare it with power dissipated from ESD pulses.

Thank you.

0 Emrys Maier over 1 year ago in reply to Mark Philip Felipe

TI__Guru* 96711 points

Mark Philip Felipe said:
1.) Can you send me the full spice model netlist for other HCS circuits below including the .SUBCKT line and ENDS? The spice netlist below is missing some information.

Sure - there are many available on TI.com -- one example is here: https://www.ti.com/product/SN74HCS125#design-tools-simulation

Mark Philip Felipe said:
2.) Can you please send me the max power dissipation (both peak and average at the input pins for the part. I need to compare it with power dissipated from ESD pulses.

No, that's not something we specify. You can find the ESD ratings of the device in the datasheet.

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SN74HCS245-Q1: Differential mode and common mode input impedance