HIGH SPEED, HIGH COMMON MODE IMMUNITY COMMUNICATION INTERFACE

Team May12-05

Chendong Yang

Mengfei Xu

Advisor: Nathan Neihart

Client: RBC Medical Development

STATEMENT OF PURPOSE

Design a high speed low voltage differential signaling communication interface that would be capable of a high common mode immunity and high speed transmission.

Why is LVDS used?

LVDS signals have the characteristics of low-noise and fast data rate

Why is high common mode voltage immunity needed? When the reference voltage changes from

transmitter and receiver, it can cause errors in output signal. It is even important when transmitting patient critical signals which need to be uninterrupted.

Vgound1=10v

Vgound2=0v

Normal system

Our System

FUNCTIONAL REQUIREMENTS

1. System is capable at least 20M bps 2. At least 10 Volts Common mode voltage

immunity 3. TTL/CMOS input level signals in and same

signals out only with some amplitude modification.

4. The clock and associated circuitry will be electrically isolated from the rest of the system

NON-FUNCTIONAL REQUIREMENTS

1. Stability, The output of the system should be stable. 2. Budget restrictionLess than $3000.

3. Easy to useThe whole system should be easily setup and tested.

CONCEPTUAL SKETCH/ BLOCK DIAGRAM

Transmitter Comparator

V_pp = 300mVf = 10 MHz

USB Cable

SerialSignal

CLK

Same DataOut

CLK

Search in the current electronic market to see if we can find existing transmitter and receiver system that meet the requirement.

Market Search

Market Search ResultThere are no such communication system existing in the market. We need to build up our own boards with proper driver and comparator chips in the market, then test the results

PLAN

SCHEDULE SEMESTER I

SCHEDULE SEMESTER II

OUR CHOICE

Transmitter:DS90C031 from National Semiconductor 1. Rising/Falling time: 1.5 ns

2. input switching frequency support: excess 77 MHZ

3. ~$3

4. ±350 mV differential signal

Receiver:LT 1711 from Linear Technology1. High common mode rejection : 65-75dB

2. Rising/Falling time: 2ns

3. Power supply range : 14 V

4. Maximum input toggle frequency : 100 MHZ

5. ~$5

TRANSMITTER PCB DESIGN

schematic

layout

Software: CadSoft EAGLE PCB design software

ACTUAL TRANSMITTER PCB

SMA connector

USB Cable

TRANSMITTER TEST

Test environment 1. Input signal:1-10 MHZ 3 Volts Vpp square wave with 1.5 Volts Vos2. Power supply : 5v3. Equipment used : function generator, oscilloscope, power supply and different kinds of cables4 load:100 ohm

V+

V-

gnd

Schematic

LIMITATION ON TRANSMITTER TEST

The equipment in the lab which we can access can only provide 10 MHz square wave for testing. The test frequency is much less the maximum capacity of the Transmitter which could go above 77 MHz.

The soldering skill we have not meet the industry equipment, the system could increase the mechanical stability with professional soldering.

No Pspice model are provided by the company, we can not do the Pspice Test for Transmitter

RESULTS(LVDS OUTPUT FOR TRANSMITTER)

Output: two differential square wave with 320mv Vpp, and 1.24v Vos. the receiver could decode this signal

V+

V-

RECEIVER PSPICE SIMULATION

Differential square wave, Vp-p = 300mV, frequency = 10 MHzVdd = 11 V, Vss = 0V, and Vlatch = 0V.

RESULT I

Input: Differential Square wave, 300mVp-p, 10M Hz Common mode Voltage = 10 Vdc

• Top: Vout

• Bottom: Vin+(Green) Vin-(Pink)

RESULT II

• Input: Differential Square wave, 300mVp-p, 10M Hz• Common mode voltage changes linearly (PWL) from

0V to 10V

• Top: Vout

• Bottom: Vin+(Red) Vin-(Black)

RESULT III

• Input: Differential Square wave, 300mVp-p, 10M Hz• Common mode voltage changes sinusoidally from 0V and

10 V with 10M Hz.

• Top: Vout

• Bottom: Vin+(Blue) Vin-(Pink)

RECEIVER PCB DESIGN

Software: CadSoft EAGLE PCB design software

ACTUAL RECEIVER PCB

TEST I:

Vin +(Ramp signal): Vp-p = 600mV, common = 6 V and frequency = 10kHz;Vin -: DC = 6V;

Yellow: V+ inputBlue: V- inputPink: outputGreen: ~output

RESULT II

Vin +: DC = 6V;Vin -(Ramp signal): Vp-p = 600mV, common = 6V and frequency = 10kHz;

Yellow: V+ inputBlue: V- inputPink: outputGreen: ~output

RESULT III

Vin + (Square signal): Vp-p = 600mV, Common = 6V and frequency = 10 kHz;Vin - (Square signal): inverted Vin +.

Yellow: V+ inputBlue: V- inputPink: outputGreen: ~output

LIMITATION ON RECEIVER TEST:

Due to the limitations of equipment in the lab, we cannot change the common mode voltage of the differential signals linearly or sinusoidly as we simulated in Pspice.

We manually change the common mode voltage from 1 V up to 10 V, and there is not significant distortion on output shape or frequency. We can still distinguish the outputs correctly.

WHOLE SYSTEM TEST

Input: Square wave, Vp-p = 3V, Frequency =1MHz;

Yellow: InputBlue: Output

Input: Square wave, Vp-p = 3V, Frequency =10 MHz;

Yellow: InputBlue: Output

LIMITATION ON WHOLE SYSTEM TEST:

We cannot swing or change the common mode voltage transmitting through USB cable manually.

But we did common mode voltage test in the receiver test separately, it is sufficient to show that our designed system has the ability to transmitter 10 MHz and tolerate common mode voltage up to 10 V change.

TOTAL COST

Item Approx. Cost

Transmitter Board ~$50

Receiver Board ~$50

Unexpected Cost(i.e. damaged board, chips, electronic)

~$20

Design Poster ~$20

Total Cost ~$140

QUESTIONS?