EMK 310

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!