Definition of: Transducer

Sensor

Signal Conditioning

T: Converts energy from one form to another

S: device (typically transducer) that is used to convert various physical quantities to electrical signals

SC: collection of circuit functions that process a signal in order to change one or more properties (amplitude, frequency etc)

Examples of signal conditioning functions-7 in total

A Bat Flew Down Into BUs

and then Vanished

Amplification/Attenuation Buffering/Impedance Filtering DC offset correction Isolation Balenced to Unbalenced Conversion Voltage/Current conversion

Amplification/Attenutation: What do each do What kind of circuit is needed to do this What is amplification often used for What is attenuation often used for

Increasing/decreasing the power of a signal

An active circuit is needed

Amping up signals from sensors that produce weak outputs

Reducing signals that are too large to be processed properly

Buffering/Impedance matching:

What are they

When are they used

Buffer: op amp where input impedance is very high, output impedance is very low and gain is unity.

Prevents one stage from loading the next

Impedance matching: using an op amp to present a specified input/output impedance

Needed when two stages in a circuit need different impedances

Filtering: just definition

Seperation of signals on the basis of their frequency content

DC offset correction: what does it do

Example of when it is useful

Adding or removing DC offset from an AC signal

When using an ADC converter as input voltage must be completely positive

Isolation: what is it

Why would you use it

Isolating a signal electrically

ALthough there is an output signal there is no electrical current path between input and output

For safety or to protect components from damaging voltage

Balanced to Unbalanced Conversion: how does it work

Converts two conductors to a differential system

Voltage/Current conversion:

whys this useful

How to do it for large currents

Many sensors produce a current as output, but its generally more convenient to process the signal as a voltage

Pass the current through a low resistance

Circuit diagram vs system diagram

CD: contains topology of circuit component connections, drawn schematically

SD: concerned with relationships between functional blocks within a system

System concept diagram vs system module diagram

SCD: not as detailed, tells you how a system operates

SMD: detailed, tells you how real sub-system parts are connected (included signal types, power levels etc)

Op-amps:

Vout equation

Ideal op amp characteristics

Vout = A(V+ - V-) A = gain V+/V- = non-inverting and inverting inputs

A is infinite

Input impedance is infinite

Output impedance is zero

Non-inverting op amp:

What does it look like

Whats the gain equation

Vin = V+

V- : potential dividor between Vout and ground

G = 1 + R2/R1

R2 is top resistor, R1 is bottom

Inverting op amp:

What does it look like

Whats the gain equation

V+ = ground

V- = Vin through a resistor (R1)

R2 goes between Vout and between R1 and V-

G = -R2/R1

Op-amp limitations: input and output limits

Input limits: there is a max voltage difference that can be applied between V+ and V-

Output limit: output voltage cannot exceed power supply voltages

Op-amp limitations: unwanted phase shift

Each RC network in an op amp can cause unwanted phase shift.

Each network has 6dB/octave or 20dB/decade roll-off and -90 phase shift

If total phase shift is 180, we get positive feedback

Op-amp limitations: how to solve unwanted phase shift

Theres a purpose built rc circuit that rolls off sooner to have a more controlled roll off and max of 90 phase shift

Op-amps: removing DC components

Use capacitor before first resistor and the formula F = 1/(2πCR)

Op-amps: impedance matching for non-inverting op amp (Vin = V+)

Input: resister in parallel with Vin, Sets impedance

Output: Resistor in series

Remember about potential dividor

Op-amps: impedance matching for inverting op amp (V+ = ground)

Input: R1 = input impedance

Output: Resistor in series

Remember about potential dividor

Op amps: creating a buffer with non-inverting and inverting op amps

Non-inverting: connect Vout to V- without any resistors

Inverting: set R2 = R1

Op amps: get attenuation

Use a potential dividor into a buffer to avoid high output impedance

What is slew rate?

Whats the value of slew rate needed for undistorted output?

How quickly an op-amp can change its output voltage

Aω (V/s)

A = peak amplitude

ω = frequency

Biasing input currents:

Why is this needed?

How do you do it?

What do you need to watch out for>

As op amp input impedance isn’t infinite so a small current will flow.

If you have a capacitor at the input, it will accumulate charge.

To solve this place a resistor in parallel going to ground.

Watch out-this forms a high pass filter

Input offset voltage:

Why does it happen?

How do you fix it

An ideal op amp shouldn’t output voltage if the difference between two inputs is the same.

However in practise there is a small internal offset voltage

Most op-amps include a potentiometer to compensate for it

Common-Mode Rejection Ratio equations-2 types and last one in μV/V and dB

CMRR = 20*log(Adiff/Acom) (db) Adiff = differential voltage gain Acom = common-mode voltage gain CMRR = ΔVdiff/ΔVcom CMRR = -20*log(ΔVdiff/ΔVcom) V = voltage signals required to obtain the same change in output voltage

Power Supply Rejection Raito:

What is it

Two equations for it, in μV/V and dB

The extent to which the op-amp rejects fluctuations on the power supply rails

PSRR = ΔVdiff/ΔVPSU (μV/V)

PSRR = -20*log(ΔVdiff/ΔVPSU)

5 things to consider when choosing op-amps

General purpose (price, easy to use) High speed (high frequency) Precision (low offsets, high CMRR) Low power (low standby power consumption) Low noise

How to create a Wheatstone Bridge circuit to get two differential voltages when using a sensor

Diamond shape-Vsupply at top and ground at bottom

Two resistors on top two sides

Balance resistor on one side and sensor on the other

Voltage points are on the points on the left and right

Filter characteristics for n-pole filters-4 types

BBCE

What do the graphs look like

Bessel: Flat start, shallow roll off

Butterworth: flat start, less shallow roll-off

Chebyshev: has wavy start, steep roll-off

Elliptic: has wavy start and finish, steepest roll off

Low pass and high pass circuits-both for RC and LC circuits

Low-pass: RC and LC

High-pass: CR and CL

What does a Sallen-Key filter look like?

Use low pass for example

How to do a high pass?

Combination of inverting and non-inverting op-amps.

Potential dividors into V- (RA/B)

Resistors in series into V+ (R1/2)

Capacitor 1 between Vout and between R1 and R2

C2 goes to ground from between R2 and V+

Swap R1 and C1 etc for high pass

Designing a Sallen-Key filter

What values to choose?

What formulas to choose for butterworth and Chebyshev (RC = …)

R1 = R2 C1 = C2 Use RA and RB in the non-inverting formula K = 1 + RA/RB And get K from the table B: RC = 1/ω0 C: RC = 1/ω0*Cn Cn = normalising factor

Using a op-amp for current to voltage conversion

Whats the formula for Vout for both types?

Inverting Op amp except instead of the first resistor you put a diode facing either away from ground or towards a voltage rail

Ground: Vout = R*IdRail: Vout = -R*Id

Schmitt trigger:

What does it do?

What does it look like?

How to solve?

Detects when the input signal has crossed a certain threshold

Vin = V-

Vout connects to a potential dividor through resistor 3.

V+ = the gap between R3 and the potential dividor

Ask Henry

Peak detector:

What does it do?

What does it look like?

Detects max amplitude or peak of a AC wave

Vin = V+

Vout goes through a diode before splitting into 3.

One goes to V-

One goes to ground through a capacitor

One goes to Vout

For an n-bit ADC how many levels does it have?

What is sampling?

What is quantisation?

2^n levels

S: Recording the amplitude of a signal at specifc

Q: asigning a sample to one of a fixed number of amplitude values

ADC definitions:

What is Full Scale Value

What is Full Scale Range

What is resolution

FSV: High quantisation level

FSR: difference between highest and lowest quantisation levels

Resolution: number of different quantisation levels we have

Using ADC’s with microcontrollers: 3 things to remember

- ADC range is always positive
- FSV determined by MC max voltage input
- Most microcontrollers have a maximum source resistance