IBIS - Input/Output buffer information specification files are not models, they contain data used for modelling. IBIS is a specification.

1. EMI Analyst™
EMI ANALYST™ Software Suite
I n t u i t i v e  A c c u r a t e  E f f e c t i v e
RS-422 IBIS files for EMI Analyst™
IBIS – Input/Output Buffer Information
Specification

2. EMI Analyst™
What is an IBIS File?
• IBIS is a standard for describing the analog behavior of digital device
buffers using plain ASCII text.
• IBIS files are not models, they contain data used for modeling. IBIS is a specification.
• Simulation tools interpret IBIS specifications to implement their own models and
algorithms.
• IBIS defines component properties in terms of behavior instead of circuitry.
• Spice models are limited to small signals.
• IBIS covers entire operating range, plus clamping of over- and under voltage
transients, making it possible to model ringing that overshoots or undershoots.
• IBIS does not reflect the product’s interior logic and interactions, only the
behavior of the circuits connected to the device pins.

3. EMI Analyst™
IBIS File Contents
• Header
• Parasitics
• Package R, L, C
• Pin R, L, C
• Inputs
• Test conditions
• Die capacitance
• Power clamp
• Gnd clamp
• Outputs
• Test conditions
• Die capacitance
• Pulldown data
• Pullup data
• Power clamp
• Gnd clamp
• Ramp (rise/fall time)

4. EMI Analyst™
What Is Needed for an EMI Analyst™ Model?
For example, RS-422 circuit
Outputs Out+/Out-
• Output Impedance
• Cpin, Rpin, Lpin
• C_comp
• Rout
• Rise/Fall Time Waveforms
• Rising waveform
• Falling waveform
Inputs In+/In-
• Input Impedance
• Cpin, Rpin, Lpin
• C_comp
• Rin
Out+
Out-
In+
In-
100Ω

5. EMI Analyst™
IBIS Output and Input Models
VDD
Output +
Output -
GND
Input/
Enable
Pins
Rising
Ramp
(V-t)
Pullup
(V-I)
Power
Clamp
(V-I)
Ground
Clamp
(V-I)
Pull-
Down
(V-I)
Falling
Ramp
(V-t)
Internal
Logic
VDD
Out Pin
GND
C_comp
C_comp
Lpin Rpin Cpin
Lpin Rpin Cpin
Lpin Rpin Cpin
Power
Clamp
(V-I)
Ground
Clamp
(V-I)
Internal
Logic
VDD
In Pin
GND
C_comp
C_comp
LpinRpinCpin
LpinRpinCpin
LpinRpinCpin
VDD
Input +
GND
Output
Pin
Input -

6. EMI Analyst™
Pin Parasitics – Rpin, Lpin, Cpin
• Outputs
• Inputs
Out Pin
C_comp
Lpin Rpin Cpin
C_comp
LpinRpinCpin
In Pin

7. EMI Analyst™
Output Model Input Model
DO+, DO- RI+, RI-
Out Pin
C_comp
Lpin Rpin Cpin
C_comp
LpinRpinCpin
In Pin

8. EMI Analyst™
RS-422 Waveform Specs
• Signal voltage: 3.1 V nom. (VD)
• Char. Z: 100 ohms nom.
• Output current: ±20 mA nom.
• Offset voltage: 1.85V (VC)
• VOH = 3.4V, VOL = 0.3V
• Rcvr input differential threshold
voltage: ±200 mV nom.
• Rcvr max. CM voltage: ±7V VCM = 1.85V
VOut-
VOut+
VOH = 3.4V
VOL = 0.3V
Out+
Out-
100 Ω
Receiver
0.3V / 3.4V
3.4V / 0.3V
High
High
Low
Low
51.6 Ω51.6 Ω
9.7 Ω 9.7 Ω
Driver 5V
0V

9. EMI Analyst™
Output Impedance and Input Impedance
Output Z from datasheet
Rout = 61.3 ohms
(5V - 3.1V) / (3.1V / 100)
Input Z from datasheet
Rin = 6.8 k nom.
Input and output impedance change dynamically with signal amplitude
From IBIS model, output impedance is about 95 ohms in operating range

10. EMI Analyst™
Pullup Curve
I-V curve for VOH
Rising
Ramp
(V-t)
Pullup
(V-I)
Power
Clamp
(V-I)
Ground
Clamp
(V-I)
Pull-
Down
(V-I)
Falling
Ramp
(V-t)
Internal
Logic
VDD
Out Pin
GND
C_comp
C_comp
Lpin Rpin Cpin
Lpin Rpin Cpin
Lpin Rpin Cpin

11. EMI Analyst™
Pulldown Curve
I-V curve for VOL
Rising
Ramp
(V-t)
Pullup
(V-I)
Power
Clamp
(V-I)
Ground
Clamp
(V-I)
Pull-
Down
(V-I)
Falling
Ramp
(V-t)
Internal
Logic
VDD
Out Pin
GND
C_comp
C_comp
Lpin Rpin Cpin
Lpin Rpin Cpin
Lpin Rpin Cpin

12. EMI Analyst™
Output Model – Power Clamp
I-V curve when output pin is pulled above VCC
Rising
Ramp
(V-t)
Pullup
(V-I)
Power
Clamp
(V-I)
Ground
Clamp
(V-I)
Pull-
Down
(V-I)
Falling
Ramp
(V-t)
Internal
Logic
VDD
Out Pin
GND
C_comp
C_comp
Lpin Rpin Cpin
Lpin Rpin Cpin
Lpin Rpin Cpin

13. EMI Analyst™
Rising
Ramp
(V-t)
Pullup
(V-I)
Power
Clamp
(V-I)
Ground
Clamp
(V-I)
Pull-
Down
(V-I)
Falling
Ramp
(V-t)
Internal
Logic
VDD
Out Pin
GND
C_comp
C_comp
Lpin Rpin Cpin
Lpin Rpin Cpin
Lpin Rpin Cpin
Output Model – Ground Clamp
I-V curve when output pin is pulled below GND

14. EMI Analyst™
Output Model – Rising Waveform
10% - 90% (0.6V – 3.1V): 39 nsec
Rising
Ramp
(V-t)
Pullup
(V-I)
Power
Clamp
(V-I)
Ground
Clamp
(V-I)
Pull-
Down
(V-I)
Falling
Ramp
(V-t)
Internal
Logic
VDD
Out Pin
GND
C_comp
C_comp
Lpin Rpin Cpin
Lpin Rpin Cpin
Lpin Rpin CpinNo Rising Waveform provided in this IBIS file, so
use ramp data.
VD = 2.5V, 10% - 90% of 3.1V diff voltage
Rise time = VD / dV/dt_r
= 2.5 V / (4.5V / 75 nsec)
= 39 nsec

15. EMI Analyst™
Output Model – Falling Waveform
90% - 10% (3.1V – 0.6V): 49 nsec
Rising
Ramp
(V-t)
Pullup
(V-I)
Power
Clamp
(V-I)
Ground
Clamp
(V-I)
Pull-
Down
(V-I)
Falling
Ramp
(V-t)
Internal
Logic
VDD
Out Pin
GND
C_comp
C_comp
Lpin Rpin Cpin
Lpin Rpin Cpin
Lpin Rpin CpinNo Falling Waveform provided in this IBIS file, so use
ramp data.
VD = 2.5V, 10% - 90% of 3.1V diff voltage.
Rise time = VD / dV/dt_r
= 2.5 V / (4.6V / 90 nsec)
= 49 nsec

16. EMI Analyst™
DM Waveform from IBIS File
Needs to be replaced with RS-422 Waveform
Create data file from IBIS values and import into EMI Analyst™ Source form.

17. EMI Analyst™
Receiver Input Model – Power Clamp
I-V curve when input pin is pulled above VCC

18. EMI Analyst™
Receiver Input Model – Ground Clamp
I-V curve when input pin is pulled below GND

19. EMI Analyst™
RS-422 Circuit and Equivalent Model
RS-422 Circuit Model
Equivalent Circuit from IBIS File and Datasheet
Out+
Out-
In+
In-
100Ω
50m1.49n
0.306p
61.3
50m0.95n
0.146p
Out+
Out-
In+
In-
100Ω
50m 1.49n
0.429p
6.8k
50m 2.07n
0.306p
6.8k
3.0p
3.0p
1p
1p
Z0 = 100 Ω
100mohm
25 mV
Flat
Spectrum
3.1V, 1 MHz
Trapezoid
VDM
VCM

20. EMI Analyst™
Predicted Radiated Emissions
RS-422, 1 MHz, 25 mV CM Noise

21. EMI Analyst™
References
• IBIS reader: https://www.mentor.com/pcb/downloads/visual_ibis_editor/
• RS-422 driver datasheet: http://www.ti.com/lit/ds/symlink/ds26c31t.pdf
• RS-422 driver IBIS file:
http://www.ti.com/general/docs/lit/getliterature.tsp?baseLiteratureNumber=snl
m068&fileType=zip
• RS-422 receiver datasheet: http://www.ti.com/lit/ds/symlink/ds26c32at.pdf
• RS-422 receiver IBIS file:
http://www.ti.com/general/docs/lit/getliterature.tsp?baseLiteratureNumber=snl
m067&fileType=zip
• EMI Analyst: https://emisoftware.com