Measurement and Instrumentation: cram pack

Formula sheets, the key method steps for every topic and 10 practice problems, with full solutions at the back.

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1Formulas and methods

Wheatstone bridge & strain gauges

Divider voltages: V_a = V_sR₂/(R₁ + R₂), V_b = V_sR₄/(R₃ + R₄)
Output: V_ab = V_a − V_b
Balance: R₁/R₂ = R₃/R₄
Slide-wire bridge: R/R_x = l/(100 − l)
Thévenin resistance: R_th = R₁∥R₂ + R₃∥R₄
Gauge factor: ΔR/R = GF·ε
Bridge output: quarter V_s·GF·ε/4, half V_s·GF·ε/2, full V_s·GF·ε
Small change, one arm: V_ab ≈ V_s·ΔR/(4R)
Stress: σ = E·ε

Method

Treat the bridge as two voltage dividers across the same supply.
For balance, set R₁/R₂ = R₃/R₄ and solve for the unknown resistor.
For an unbalanced bridge, find V_a and V_b from the dividers and subtract; use the Thévenin equivalent when a meter loads the output.
For strain gauges, convert strain to ΔR/R = GF·ε and use V_s·GF·ε/4, /2 or ×1 for a quarter, half or full bridge.

Data acquisition & sampling

Nyquist criterion: f_s > 2·f_max, f_N = f_s/2
Alias frequency: f_a = |f − k·f_s|, k = round(f/f_s)
Step size: 1 LSB = V_FSR/2ᴺ
Quantisation error (rounding): ±½ LSB
Truncating ADC code: D = ⌊V_in/LSB⌋
Bits for a resolution r: 2ᴺ ≥ span/r
Ideal SQNR, full-scale sine: SQNR = 6.02·N + 1.76 dB
Data rate: channels × f_s × N bit/s
Multiplexed ADC: f_s = 1/(n·(t_c + t_set))
UART: T_b = 1/baud; 8N1 = 10 bits per byte; bytes/s = baud/bits per frame
Baud rate divider: baud = f_clk/(16·D)

Method

Write down what is given: sampling rate, signal frequencies, number of bits, input range, baud rate.
For sampling questions, compare each frequency with f_s/2; a higher frequency appears at f_a = |f − k·f_s| with the nearest multiple of f_s.
For resolution questions, find 1 LSB = V_FSR/2ᴺ, or the smallest N with 2ᴺ ≥ span/resolution.
For data rates and UART links, count everything that is sent: channels, samples per second, bits or whole bytes per sample, and the start, parity and stop bits of every frame.

Op-amp circuits

Inverting amplifier: V_out = −(R_f/R_in)·V_in
Non-inverting amplifier: V_out = (1 + R_f/R₁)·V_in
Summing amplifier: V_out = −R_f(V₁/R₁ + V₂/R₂)
Ideal op-amp: V₊ = V₋, I_in = 0

Series & parallel resistors

Resistors in series: R_eq = R₁ + R₂ + …
Resistors in parallel: 1/R_eq = 1/R₁ + 1/R₂ + …
Two in parallel: R_eq = R₁R₂ / (R₁ + R₂)
Ohm's law: V = I·R
Power: P = V·I = I²R = V²/R

Method

Start at the end of the network furthest from the terminals.
Combine resistors in series by adding them, and resistors in parallel with 1/R = 1/R₁ + 1/R₂ + …
Redraw the circuit after each step.
Repeat until one resistor is left between the terminals.

Thévenin & Norton

Thévenin voltage: V_th = V_{open circuit}
Thévenin resistance: R_th: all sources off, seen from a–b
Norton current: I_N = V_th/R_th
Load current: I_L = V_th/(R_th + R_L)
Maximum power transfer: R_L = R_th, P_max = V_th²/(4R_th)

Method

Remove the load and mark the two terminals a and b.
Find the open-circuit voltage between a and b: this is V_th.
Switch off all independent sources and find the resistance seen from a–b: this is R_th.
Put the load back on the equivalent circuit: V_th in series with R_th.
For the Norton equivalent, convert: I_N = V_th/R_th, with R_N = R_th in parallel.

2Practice problems

1.Wheatstone bridge & strain gaugesMedium

Find the Thévenin resistance R_th of the bridge shown, seen from the output terminals a and b.

V_s = 6 VR₁ = 150 ΩR₂ = 200 ΩR₃ = 20 ΩR₄ = 300 Ω

R_th = Ω

2.Wheatstone bridge & strain gaugesHard

A bridge with 350 Ω arms is excited with 2.5 V and has four active gauges, two in tension and two in compression (full bridge). The gauge factor is 2.12 and the strain is 1500 µε. Find the bridge output V_o in mV.

V_s = 2.5 VGF = 2.12ε = 1500 µε

V_o = mV

3.Data acquisition & samplingMedium

A sensor measures pressure over a span of 200 kPa, and its output fills the whole input range of an ADC. The reading must resolve 1 kPa. Find the smallest number of bits N the ADC needs.

span = 200 kParesolution = 1 kPa

N =

4.Data acquisition & samplingHard

A microcontroller with an 8 MHz clock sets its UART baud rate as f_clk/(16·D), with D a whole number rounded to the nearest value. The target is 115200 baud. Find the percentage error of the real baud rate (positive when it is too fast).

f_clk = 8 MHztarget = 115200 Bd

e = %

5.Op-amp circuitsMedium

The op-amp is ideal and runs from ±15 V supplies. Find the output voltage V_out.

V_in = 1.5 VR_a = 10 kΩR_b = 2 kΩR₁ = 1 kΩR_f = 47 kΩ

V_out = V

6.Op-amp circuitsHard

Two ideal op-amp stages are connected in cascade: an inverting amplifier (R₁, R_f1) followed by a non-inverting amplifier (R₂ to ground, R_f2 in the feedback). Both run from ±15 V supplies. The input is V_in = 0.6 V. Find the output voltage V_out.

R₁ = 10 kΩR_f1 = 47 kΩR₂ = 100 kΩR_f2 = 10 kΩV_in = 0.6 V

V_out = V

7.Series & parallel resistorsMedium

Find the equivalent resistance R_eq seen by the source V₁.

V₁ = 6 VR₁ = 4 ΩR₂ = 2 ΩR₃ = 8 ΩR₄ = 20 Ω

R_eq = Ω

8.Series & parallel resistorsHard

Find the current I_R1 delivered by the source, which flows through R₁.

V₁ = 36 VR₁ = 5 ΩR₂ = 2 ΩR₃ = 3 ΩR₄ = 12 ΩR₅ = 4 ΩR₆ = 4 Ω

I_R1 = A

9.Thévenin & NortonMedium

Find the Norton current I_N: the current that flows from a to b if the terminals are shorted.

V₁ = 15 VR₁ = 4 ΩR₂ = 8 ΩR₃ = 8 Ω

I_N = A

10.Thévenin & NortonHard

Find the Thévenin voltage V_th seen from terminals a–b.

V₁ = 24 VI_s = 4 AR₁ = 20 ΩR₂ = 5 ΩR₃ = 12 Ω

V_th = V

3Solutions

1.Wheatstone bridge & strain gauges

Answer: R_th = 104.5 Ω

Wheatstone bridge: two voltage dividers share one supply V_s. The left divider R₁–R₂ gives V_a = V_s·R₂/(R₁ + R₂) and the right divider R₃–R₄ gives V_b = V_s·R₄/(R₃ + R₄); the output is V_ab = V_a − V_b. The bridge is balanced (V_ab = 0, no current in the meter) when R₁/R₂ = R₃/R₄, that is R₁·R₄ = R₂·R₃. Seen from the output, the bridge is a Thévenin source with R_th = R₁∥R₂ + R₃∥R₄.

Supply shorted

R₁∥R₂ = 150·200/350 = 85.71 Ω

R₃∥R₄ = 20·300/320 = 18.75 Ω

Thévenin resistance

R_th = 104.5 Ω

Answer

R_th = 104.5 Ω

2.Wheatstone bridge & strain gauges

Answer: V_o = 7.95 mV

Strain gauges: a gauge changes its resistance with strain: ΔR/R = GF·ε, where the gauge factor GF is about 2 for metal foil. In a bridge with all arms equal to R, one active gauge (quarter bridge) gives V_o ≈ V_s·GF·ε/4. Two active gauges with opposite strains in adjacent arms (half bridge) give V_s·GF·ε/2, and four (full bridge) give V_s·GF·ε. Stress follows from Hooke’s law, σ = E·ε. Load cells state their sensitivity in mV/V: the output at full load per volt of excitation.

Relative resistance change

ΔR/R = 2.12·1500·10⁻⁶ = 0.00318

Bridge output

V_o = V_s·GF·ε = 2.5·0.00318 = 7.95 mV

Answer

V_o = 7.95 mV

3.Data acquisition & sampling

Answer: N = 8

ADC resolution: an N-bit converter divides its full-scale range V_FSR into 2ᴺ codes, from 0 to 2ᴺ − 1. One step is 1 LSB = V_FSR/2ᴺ: the smallest change the converter can show. A truncating ADC gives the code ⌊V_in/LSB⌋; a rounding ADC is never more than ½ LSB away from the true value, its quantisation error. Every extra bit halves the LSB and adds about 6 dB to the ideal signal-to-quantisation-noise ratio SQNR = 6.02·N + 1.76 dB for a full-scale sine. To reach a resolution r over a span, choose the smallest N with 2ᴺ ≥ span/r.

Steps needed

200/1 = 200

Bits

log₂(200) = 7.644

2⁷ = 128 < 200 ≤ 2⁸ = 256

N = 8

Answer

N = 8

4.Data acquisition & sampling

Answer: e = 8.507 %

UART: an asynchronous serial link sends each byte as a frame: one start bit (low), the data bits, an optional parity bit and one or two stop bits (high). The baud rate is the number of bits per second, so one bit lasts T_b = 1/baud and a frame of n bits lasts n/baud. The common 8N1 format has 10 bits per byte. The baud generator divides the clock: baud = f_clk/(16·D) with a whole number D, so the real baud rate differs slightly from the target; an error below about 2 % is usually acceptable.

Divider

D = 8·10⁶/(16·115200) = 4.34 → 4

Real baud rate

8·10⁶/(16·4) = 125000 Bd

Error

(125000 − 115200)/115200·100 = 8.507 %

Answer

e = 8.507 %

An error above about 2 % makes the link unreliable: choose another clock or baud rate.

5.Op-amp circuits

Answer: V_out = 12 V

Ideal op-amp rules: with negative feedback, the op-amp drives its output so that its two inputs sit at the same voltage (V₋ = V₊), and no current flows into either input. Writing KCL at the inverting input with these two facts gives the output.

Voltage at the + input

V₊ = V_in·R_b/(R_a + R_b) = 1.5·2/(10 + 2) = 0.25 V

Non-inverting gain

V₋ = V₊, and the current through R₁ also flows through R_f.

V_out = V₊·(1 + R_f/R₁) = 0.25·(1 + 47/1) = 12 V

Answer

V_out = 12 V

6.Op-amp circuits

Answer: V_out = −3.102 V

Gain of each stage

A₁ = −R_f1/R₁ = −47/10 = −4.7

A₂ = 1 + R_f2/R₂ = 1 + 10/100 = 1.1

Stage by stage

V₁ = −4.7·0.6 = −2.82 V

V_out = 1.1·(−2.82) = −3.102 V

Answer

V_out = −3.102 V

7.Series & parallel resistors

Answer: R_eq = 5.867 Ω

Series and parallel resistors: in series the same current flows through each resistor and R = R₁ + R₂ + …. In parallel each resistor has the same voltage and 1/R = 1/R₁ + 1/R₂ + …. A ladder network is reduced one pair at a time until a single equivalent resistance remains.

Combine resistors from the far end

Start at the end farthest from the source and work back towards it. Resistors carrying the same current are in series; resistors across the same two nodes are in parallel.

R₃ + R₄ = 8 + 20 = 28 Ω

R₂ ∥ 28 = 2·28/(2+28) = 1.867 Ω

R_eq = R₁ + 1.867 = 4 + 1.867 = 5.867 Ω

Answer

R_eq = 5.867 Ω

8.Series & parallel resistors

Answer: I_R1 = 5.461 A

Combine resistors from the far end

Start at the end farthest from the source and work back towards it. Resistors carrying the same current are in series; resistors across the same two nodes are in parallel.

R₅ + R₆ = 4 + 4 = 8 Ω

R₄ ∥ 8 = 4.8 Ω

R₃ + 4.8 = 7.8 Ω

R₂ ∥ 7.8 = 1.592 Ω

R_eq = R₁ + 1.592 = 5 + 1.592 = 6.592 Ω

Current from the source

The source sees only R_eq, so Ohm’s law gives the current it delivers. That current flows through R₁.

I_R1 = V₁/R_eq = 36/6.592 = 5.461 A

Answer

I_R1 = 5.461 A

9.Thévenin & Norton

Answer: I_N = 0.9375 A

Thévenin’s theorem: any linear two-terminal network behaves like a single voltage source V_th in series with a resistance R_th. Norton’s theorem gives the same network as a current source I_N = V_th/R_th in parallel with R_th.

Find R_th with every source switched off

Replace voltage sources with shorts and current sources with opens, then look into the circuit from a–b.

R₁ ∥ R₂ = 4·8/(4+8) = 2.667 Ω

R_th = (R₁ ∥ R₂) + R₃ = 2.667 + 8 = 10.67 Ω

Find V_th, the open-circuit voltage

With a–b open, no current flows through R₃, so there is no drop across it and V_b = V_A. R₁ and R₂ form a voltage divider.

V_th = V₁·R₂/(R₁+R₂) = 15·8/(4+8) = 10 V

Norton equivalent

A Norton source is the same circuit seen as a current source in parallel with R_th. Its current is the short-circuit current at a–b.

I_N = V_th/R_th = 10/10.67 = 0.9375 A

10.Thévenin & Norton

Answer: V_th = 68.8 V

Find R_th with every source switched off

Replace voltage sources with shorts and current sources with opens, then look into the circuit from a–b.

R₁ ∥ R₂ = 20·5/(20+5) = 4 Ω

R_th = (R₁ ∥ R₂) + R₃ = 4 + 12 = 16 Ω

Find V_th, the open-circuit voltage

With a–b open, the only path for Iₛ is through R₃ into node A. Find V_A by superposition of V₁ and Iₛ, then add the drop across R₃.

V_A = V₁·R₂/(R₁+R₂) + I_s·(R₁∥R₂) = 4.8 + 4·4 = 20.8 V

V_th = V_B = V_A + I_s·R₃ = 20.8 + 4·12 = 68.8 V

ExamProblems · examproblems.com/courses/measurement-and-instrumentation

Measurement and Instrumentation: cram pack

1Formulas and methods

Wheatstone bridge & strain gauges

Method

Data acquisition & sampling

Method

Op-amp circuits

Series & parallel resistors

Method

Thévenin & Norton

Method

2Practice problems

3Solutions

Supply shorted

Thévenin resistance

Answer

Relative resistance change

Bridge output

Answer

Steps needed

Bits

Answer

Divider

Real baud rate

Error

Answer

Voltage at the + input

Non-inverting gain

Answer

Gain of each stage

Stage by stage

Answer

Combine resistors from the far end

Answer

Combine resistors from the far end

Current from the source

Answer

Find Rth with every source switched off

Find Vth, the open-circuit voltage

Norton equivalent

Find Rth with every source switched off

Find Vth, the open-circuit voltage

Find R_th with every source switched off

Find V_th, the open-circuit voltage

Find R_th with every source switched off

Find V_th, the open-circuit voltage