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An RS 422 IBIS Model for EMI Analyst
The document provides detailed information about the EMI Analyst™ software and the IBIS (Input/Output Buffer Information Specification) file format, which describes the analog behavior of digital device buffers. It outlines the required components and parameters for modeling RS-422 circuits, including output and input impedance, waveform specifications, and various electrical characteristics. Additionally, it includes references to relevant datasheets and online resources for further information.
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An RS 422 IBIS Model for EMI Analyst
- 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 isan 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 FileContents • 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 IsNeeded 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 Outputand 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 ModelInput Model DO+, DO- RI+, RI- Out Pin C_comp Lpin Rpin Cpin C_comp LpinRpinCpin In Pin
- 8. EMI Analyst™ RS-422 WaveformSpecs • 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 Impedanceand 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-Vcurve 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-Vcurve 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 LpinRpin 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 Waveformfrom 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 InputModel – Power Clamp I-V curve when input pin is pulled above VCC
- 18. EMI Analyst™ Receiver InputModel – Ground Clamp I-V curve when input pin is pulled below GND
- 19. EMI Analyst™ RS-422 Circuitand 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 RadiatedEmissions RS-422, 1 MHz, 25 mV CM Noise
- 21. EMI Analyst™ References • IBISreader: 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