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EMK 310

This document explores signal pre-processing within electronic systems, focusing on the implications of non-ideal op-amps. It covers critical concepts such as open-loop gain, offset voltage, input resistance, and frequency gain roll-off. Additionally, it discusses the temperature effects on resistors, voltage references, and the importance of grounding in circuit designs. Various strategies for PCB layout optimization, including separating analog and digital ground lines, are presented to minimize noise and ensure accurate measurements. The analysis also emphasizes calculating maximum bit resolution for accurate system performance.

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EMK 310

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  1. EMK 310 Signal pre-processing

  2. Content • Non-ideal opamps • Open-loop gain • Offset voltage • Input resistance • Frequency gain rolloff • Temperature effects • In resistors • Voltage references • Grounding • PCB layout

  3. Open-loop gain • Closed loop voltage gain equation • Thus with Av=100 000, the ideal value of V0 if R1=10k, R2=50k and V1=1.2V is 7.2V. With the non-ideal equations V0=7.199568V • In a system operating from 0 – 7.2V, what is the maximum bit resolution where the system is still accurate?

  4. Input offset voltage • Input offset included gain equation • Thus if Vx is in a similar range as V1 the results will have a noticeable offset and introduce errors in measurements.

  5. Frequency effects • Transfer characteristic of an opamp • The higher the frequency the lower the gain, thus frequency dependant gain (=bad) need lookup table or some form of compensation.

  6. Temperature effects • Resistor temperature coefficient equation • Can calculate the resistance at specific temperatures by solving for R1 or R2. • Voltage references, voltage references can have a high TC and cause the reference voltage to drift. Can be compensated for by measuring the temperature and adjusting the value in software.

  7. Noise and grounding • Think ground wires can cause voltage fluctuations with respect to the ground potential of different devices, thus causing misreadings. • Minimize the effect of this by separating the digital and analogue ground lines.

  8. PCB layout • Separate grounds • Sandwich analogue and digital lines between a ground line • Run tracks at 90 or 45 degrees • In power supplies add filter capacitors to reduce supply noise • Try isolating sensitive circuitry!

  9. THE END! Thank you!

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