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 = 15 VR₁ = 20 ΩR₂ = 800 ΩR₃ = 600 ΩR₄ = 20 Ω

R_th = Ω

2.Wheatstone bridge & strain gaugesHard

A 350 Ω strain gauge with gauge factor 2.1 is bonded to a steel bar. Under load its resistance rises by 0.588 Ω. Find the strain ε in microstrain (µε).

R = 350 ΩGF = 2.1ΔR = 0.588 Ω

ε = µε

3.Data acquisition & samplingMedium

An ADC with an input range of 0 to 10 V must resolve steps of 10 mV or smaller. Find the smallest number of bits N.

V_FSR = 10 Vresolution = 10 mV

N =

4.Data acquisition & samplingHard

A 24-bit ADC streams every sample over a UART at 115200 baud, format 8N1 (start bit, 8 data bits, 1 stop bit). Each sample is sent as 3 whole byte(s) and the link is busy all the time. Find the highest signal frequency f_max that can be sampled without aliasing, in Hz.

baud = 115200 BdN = 248N1

f_max = Hz

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 = 1 kΩR_b = 100 kΩR₁ = 10 kΩR_f = 10 kΩ

V_out = V

6.Op-amp circuitsHard

The summing amplifier uses an ideal op-amp with ±15 V supplies. Find the output voltage V_out.

V₁ = 1.7 VV₂ = 1.6 VR₁ = 22 kΩR₂ = 10 kΩR_f = 10 kΩ

V_out = V

7.Series & parallel resistorsMedium

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

V₁ = 15 VR₁ = 3 ΩR₂ = 6 ΩR₃ = 4 ΩR₄ = 8 Ω

I_R1 = A

8.Series & parallel resistorsHard

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

V₁ = 18 VR₁ = 12 ΩR₂ = 2 ΩR₃ = 30 ΩR₄ = 4 ΩR₅ = 5 ΩR₆ = 30 Ω

I_R1 = A

9.Thévenin & NortonMedium

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

V₁ = 9 VR₁ = 20 ΩR₂ = 2 ΩR₃ = 20 Ω

V_th = V

10.Thévenin & NortonHard

A load R_L = 6 Ω is connected across a–b. Use a Thévenin equivalent to find the power P_L delivered to the load.

V₁ = 12 VI_s = 1 AR₁ = 3 ΩR₂ = 25 ΩR₃ = 15 ΩR_L = 6 Ω

P_L = W

3Solutions

1.Wheatstone bridge & strain gauges

Answer: R_th = 38.87 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 = 20·800/820 = 19.51 Ω

R₃∥R₄ = 600·20/620 = 19.35 Ω

Thévenin resistance

R_th = 38.87 Ω

Answer

R_th = 38.87 Ω

2.Wheatstone bridge & strain gauges

Answer: ε = 800 µε

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 = 0.588/350 = 0.00168

Strain

ε = 0.00168/2.1 = 0.0008 = 800 µε

Answer

ε = 800 µε

3.Data acquisition & sampling

Answer: N = 10

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

10000 mV/10 mV = 1000

Bits

log₂(1000) = 9.966

2⁹ = 512 < 1000 ≤ 2¹⁰ = 1024

N = 10

Answer

N = 10

4.Data acquisition & sampling

Answer: f_max = 1920 Hz

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.

Bits per sample on the line

3·10 = 30 bit

Highest sampling rate

f_s,max = 115200/30 = 3840 Hz

Nyquist limit

f_max = 3840/2 = 1920 Hz

Answer

f_max = 1920 Hz

5.Op-amp circuits

Answer: V_out = 2.97 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) = 1.5·100/(1 + 100) = 1.485 V

Non-inverting gain

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

V_out = V₊·(1 + R_f/R₁) = 1.485·(1 + 10/10) = 2.97 V

Answer

V_out = 2.97 V

6.Op-amp circuits

Answer: V_out = −2.373 V

Input currents

V₋ = 0, so each input resistor has its full input voltage across it.

I₁ = V₁/R₁ = 1.7/22 = 0.07727 mA

I₂ = V₂/R₂ = 1.6/10 = 0.16 mA

I_f = I₁ + I₂ = 0.2373 mA

Output voltage

V_out = −R_f·I_f = −10·0.2373 = −2.373 V

Answer

V_out = −2.373 V

7.Series & parallel resistors

Answer: I_R1 = 2.143 A

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 = 4 + 8 = 12 Ω

R₂ ∥ 12 = 6·12/(6+12) = 4 Ω

R_eq = R₁ + 4 = 3 + 4 = 7 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 = 15/7 = 2.143 A

Answer

I_R1 = 2.143 A

8.Series & parallel resistors

Answer: I_R1 = 1.296 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 = 5 + 30 = 35 Ω

R₄ ∥ 35 = 3.59 Ω

R₃ + 3.59 = 33.59 Ω

R₂ ∥ 33.59 = 1.888 Ω

R_eq = R₁ + 1.888 = 12 + 1.888 = 13.89 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 = 18/13.89 = 1.296 A

Answer

I_R1 = 1.296 A

9.Thévenin & Norton

Answer: V_th = 0.8182 V

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 = 20·2/(20+2) = 1.818 Ω

R_th = (R₁ ∥ R₂) + R₃ = 1.818 + 20 = 21.82 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) = 9·2/(20+2) = 0.8182 V

10.Thévenin & Norton

Answer: P_L = 8.627 W

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 = 3·25/(3+25) = 2.679 Ω

R_th = (R₁ ∥ R₂) + R₃ = 2.679 + 15 = 17.68 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) = 10.71 + 1·2.679 = 13.39 V

V_th = V_B = V_A + I_s·R₃ = 13.39 + 1·15 = 28.39 V

Connect the load to the Thévenin equivalent

The Thévenin source and R_th in series behave exactly like the original circuit at a–b, so the load sees a simple series loop.

I_L = V_th/(R_th + R_L) = 28.39/(17.68 + 6) = 1.199 A

P_L = I_L²·R_L = 1.199²·6 = 8.627 W

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

Strain

Answer

Steps needed

Bits

Answer

Bits per sample on the line

Highest sampling rate

Nyquist limit

Answer

Voltage at the + input

Non-inverting gain

Answer

Input currents

Output voltage

Answer

Combine resistors from the far end

Current from the source

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

Find Rth with every source switched off

Find Vth, the open-circuit voltage

Connect the load to the Thévenin equivalent

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