an rs 422 ibis model for emi analyst
TRANSCRIPT
EMI Analyst™
EMI ANALYST™ Software SuiteI n t u i ti v e A c c u r a t e E ff e c ti v e
RS-422 IBIS files for EMI Analyst™IBIS – Input/Output Buffer Information Specification
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.
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)
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-
100
Ω
EMI Analyst™
IBIS Output and Input ModelsVDD
Output +
Output -
GND
Input/Enable
Pins
Rising Ramp(V-t)
Pullup(V-I)
PowerClamp(V-I)
GroundClamp(V-I)
Pull-Down(V-I)
FallingRamp(V-t)
InternalLogic
VDD
Out Pin
GND
C_comp
C_comp
Lpin Rpin Cpin
Lpin Rpin Cpin
Lpin Rpin Cpin
PowerClamp(V-I)
GroundClamp(V-I)
InternalLogic
VDD
In Pin
GND
C_comp
C_comp
LpinRpinCpin
LpinRpinCpin
LpinRpinCpin
VDD
Input +
GND
OutputPin
Input -
EMI Analyst™
Pin Parasitics – Rpin, Lpin, Cpin• Outputs
• Inputs
Out PinC_comp
Lpin Rpin Cpin
C_comp
LpinRpinCpin
In Pin
EMI Analyst™
Output Model Input ModelDO+, DO- RI+, RI-
Out PinC_comp
Lpin Rpin Cpin
C_comp
LpinRpinCpin
In Pin
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 / 0.3VHigh
High
Low
Low
51.6 Ω51.6 Ω
9.7 Ω 9.7 Ω
Driver 5V
0V
EMI Analyst™
Output Impedance and Input ImpedanceOutput 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 amplitudeFrom IBIS model, output impedance is about 95 ohms in operating range
EMI Analyst™
Pullup CurveI-V curve for VOH
Rising Ramp(V-t)
Pullup(V-I)
PowerClamp(V-I)
GroundClamp(V-I)
Pull-Down(V-I)
FallingRamp(V-t)
InternalLogic
VDD
Out Pin
GND
C_comp
C_comp
Lpin Rpin Cpin
Lpin Rpin Cpin
Lpin Rpin Cpin
EMI Analyst™
Pulldown CurveI-V curve for VOL
Rising Ramp(V-t)
Pullup(V-I)
PowerClamp(V-I)
GroundClamp(V-I)
Pull-Down(V-I)
FallingRamp(V-t)
InternalLogic
VDD
Out Pin
GND
C_comp
C_comp
Lpin Rpin Cpin
Lpin Rpin Cpin
Lpin Rpin Cpin
EMI Analyst™
Output Model – Power ClampI-V curve when output pin is pulled above VCC
Rising Ramp(V-t)
Pullup(V-I)
PowerClamp(V-I)
GroundClamp(V-I)
Pull-Down(V-I)
FallingRamp(V-t)
InternalLogic
VDD
Out Pin
GND
C_comp
C_comp
Lpin Rpin Cpin
Lpin Rpin Cpin
Lpin Rpin Cpin
EMI Analyst™
Rising Ramp(V-t)
Pullup(V-I)
PowerClamp(V-I)
GroundClamp(V-I)
Pull-Down(V-I)
FallingRamp(V-t)
InternalLogic
VDD
Out Pin
GND
C_comp
C_comp
Lpin Rpin Cpin
Lpin Rpin Cpin
Lpin Rpin Cpin
Output Model – Ground ClampI-V curve when output pin is pulled below GND
EMI Analyst™
Output Model – Rising Waveform10% - 90% (0.6V – 3.1V): 39 nsec
Rising Ramp(V-t)
Pullup(V-I)
PowerClamp(V-I)
GroundClamp(V-I)
Pull-Down(V-I)
FallingRamp(V-t)
InternalLogic
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
EMI Analyst™
Output Model – Falling Waveform90% - 10% (3.1V – 0.6V): 49 nsec
Rising Ramp(V-t)
Pullup(V-I)
PowerClamp(V-I)
GroundClamp(V-I)
Pull-Down(V-I)
FallingRamp(V-t)
InternalLogic
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
EMI Analyst™
DM Waveform from IBIS FileNeeds to be replaced with RS-422 WaveformCreate data file from IBIS values and import into EMI Analyst™ Source form.
EMI Analyst™
Receiver Input Model – Power ClampI-V curve when input pin is pulled above VCC
EMI Analyst™
Receiver Input Model – Ground ClampI-V curve when input pin is pulled below GND
EMI Analyst™
RS-422 Circuit and Equivalent ModelRS-422 Circuit Model
Equivalent Circuit from IBIS File and Datasheet
Out+
Out-
In+
In-
100
Ω
50m1.49n
0.306p
61.3
50m0.95n
0.146p
Out+
Out-
In+
In-
100
Ω
50m 1.49n
0.429p
6.8k
50m 2.07n
0.306p
6.8k
3.0p
3.0p
1p
1p
Z0 = 100 Ω
100mohm25 mV
Flat Spectrum
3.1V, 1 MHz Trapezoid
VDM
VCM
EMI Analyst™
Predicted Radiated EmissionsRS-422, 1 MHz, 25 mV CM Noise
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=snlm068&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=snlm067&fileType=zip • EMI Analyst: https://emisoftware.com