University of Tehran 1

Interface DesignConnections

Omid Fatemi

University of Tehran 2

Typical Interface Design

Connect Compute Convey Cooperate

Sense RealityTouch RealityConnectTransform

Embedded SystemsMicrosAssembler, CReal-TimeMemoryPeripheralsTimersDMA

PC interfacesHCI

BussesProtocolsStandardsPCIIEEE488SCSIUSB & FireWireCAN

University of Tehran 3

Transducer Signal

• varying current or voltage for analog signals• varying duty cycle or pulse widths• micro or milli values to large values• sensor signal will also contain some element of

noise• at some resolution of the signal, the amount of

noise becomes relevant• the signal to noise ratio is often noted as S/N

University of Tehran 4

Problems

• Signals have low values. (low level milli volt signals)

• Sensors are remote to DAQ board long cable

• Electromagnetic interference (EMI) • Non ideal grounding• Thermal noise

University of Tehran 5

Transportation Costs

• If sensor is integrated with the computing system (on-chip), then there is less chance of noise from the signal being transported through the real world over connecting wires.

• External sensors must connect to the computing elements through some sort of wiring arrangement which can create noise.

University of Tehran 6

Noise Source in Resistive Devices

• above absolute zero, all materials have random thermal motion which gives rise to uncertainty in a material’s thermal energy.

• This leads to uncertainty in the dissipated electrical power of a resistor or noise in a signal

University of Tehran 7

Resistor Noise

• findings of J.B. Johnson in 1928• white noise is a combination of all frequencies like white

light• amount of noise increases with resistance and bandwidth

University of Tehran 8

Other Sources

• Electric fields

– Capacitive coupling

• Magnetic fields

– Inductive coupling (close range)

• Electromagnetic wave

– Proportional to loop area and frequency

• Conducted interference

– Ground noise

University of Tehran 9

Review of Capacitive Coupling

VA

IA AI

+ -

1) The creation of a voltage difference from A to Bproduces an electric field in the volume between Aand B. The energy in this field is proportional to V .A

2) As V increases, a current I flows into plate 1. Anequal current flows out of plate 2. Thus plates 1 and2 accumulate electric charges of equal magnitude butopposite sign. The quantity of accumulated (stored)charge depends on several factors:

A A

A

B

University of Tehran 10

Review of Inductive Coupling

AB

V

V

A

B

IA

IB

1) The creation of current I through loop A produces aa magnetic field in the volume surrounding loop A.The energy stored in this field is proportional to I .A

A

2) The area of loop B intersects magnetic flux from themagnetic field surrounding loop A. The quantity offlux intersected depends on several factors:

+-

+-

University of Tehran 14

Digital to Analog Coupling

• fast changing digital signals can capacitively couple noise into neighboring analog signals

University of Tehran 15

Ground Noise

• different ground resistances (milli-ohms) can cause different voltages on ground loops

• separate ground wire is better but costlier

University of Tehran 16

Ignore Noise for Large Signals

• if signal is much larger than the noise and it is a digital signal (resolution is 2), the noise can be ignored

• around a building you can get noise from 1-100 mv in the signal cable

University of Tehran 17

Signal Loss

• voltage from transducer is divided between internal resistance and resistance of the amplifier

• the error increases with small Rdiff and large Verr• this is why high input impedance on an amplifier is

important to get most of the signal

University of Tehran 18

Differential Signals (Balanced Input)• a signal that is the difference between two signals is known as a

differential signal• normal mode is when the signals differ; common mode is when

they both change the same• common mode rejection ratio is the the ratio of an amplifiers

response to normal / common mode signals• For signals below 1 MHz

University of Tehran 19

Differential Amplification

• Common Mode: Two signals change input levels together.

• Normal Mode: Two signals have a differential change

• A differential amplifier has a high “Common Mode Rejection Ratio”

gainrR

RVV

VoutGEE

Cdiff

)(221

rejectionrRR

RG

EE

CCM

12

EE rRR

CMRR

1

University of Tehran 20

Twisted Pairs and ShieldingShielded twisted pair cabling makes noise signals as common mode

A good example of long cabling:

Telephone company

University of Tehran 21

Common Mode Interference Rejection

University of Tehran 23

Single Ended Inputs

• Shield and negative lead are grounded

University of Tehran 24

Floating Signal Shield Grounding

• A shield on a cable should be grounded at the amplifier end only.

• Grounding at both ends generated ground loops• Grounding at amplifier side prevents signal floating near

threshold voltages

University of Tehran 25

The Correct Grounding

University of Tehran 26

Grounded Small Signal Shielding

University of Tehran 27

High Frequency Bypass

• high frequency noise can be bypassed on an amplification stage by using a bypass capacitor

University of Tehran 28

Amplify at the Transducer• If we put a preamplifier to boost the sensor signal and

reduce the source impedance we can improve the S/N ratio

University of Tehran 30

Current Loop

• small current run to detect open circuits

• signal changes current from 4 to 20 milliamps

• can use 250 ohm resistor to change to 1-5V

University of Tehran 31

Analog Multiplexor for Multiple Inputs

University of Tehran 32

50%

50%

t t t t1 2 3 4

V Vin outVin

Vout

time

time

t tpd0 pd1

Note: typically, t = t due to variations incarrier storage times in the transistors,differences in output drive impedancesto L and H, etc.

Propagation delay, t = ( t + t )

pd0 pd1

pd0 pd11

2pd

Propagation Delay

University of Tehran 33

1) Conventional Logic (low to medium speed)

propagation delaysthrough logic elements >> propagation delays

through wiring

--- relatively slow signal rise and fall times

--- circuit size is much less than the wavelengthof the highest frequency signals

--- can safely neglect the parasitic R, L, and C ofwiring when modeling signal propagation

--- can safely use lumped models of circuit elements

t , t > 10 nsr f

Slow Digital Circuits

University of Tehran 34

2) High-Speed Logic

propagation delays propagation delays~=through logic elements through wiring

--- fast signal rise and fall times

--- circuit size is greater than or equal to thewavelength of the highest frequency signals

--- must consider parasitic R, L, and C of wiring whenmodeling signal propagation

--- must use combination of distributed and lumpedmodels of circuit elements

t , t < 5 nsr f

High Speed Logic

University of Tehran 35

--- a noise margin is a parameter that represents themaximum noise voltage that can be present on the inputof a logic gate without affecting the logical level of thegate’s output

--- separate noise margins are usually defined for theL and H voltage levels

V

V

VV

VOHmin

OLmax

IHmin

ILmax

CC

GND

NM

NML

H

VOHmin

VOLmax

VIHmin

VILmax

= minimum high voltage output by a gate= maximum low voltage output by a gate= minimum input voltage interpreted as a H

= maximum input voltage interpreted as a L

Low noise margin, NM = -

High noise margin, NM = -

VOLmaxVILmax

VOHmin VIHmin

L

H

Noise Margins

University of Tehran 37

Summary