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WHAT'S YOUR ENGINEERING QUOTIENT?

Problem 1—The figure shows a circuit with an input multiplexer that can reverse the polarity of the sensor as seen by the instrumentation amplifier.

When SEL is high, the positive output of the sensor is connected to the + input of the IA and the negative output of the sensor is connected to the - input of the IA.

When SEL is low, the positive output of the sensor is connected to the - input of the IA and the negative output of the sensor is connected to the + input of the IA.

This arrangement is most often found in circuits where the required gain of the IA is very large. What advantage does this circuit have over one without an input chopper?

ANSWER

Problem 2—Assuming A = B, consider the following operations. Each is apparently legal, but the answer is clearly paradoxical. What is wrong with these equations:

A = B
A2 = AB
A2 - B2 = AB - B2
(A + B)(A - B) = B(A - B)
A + B = B
B + B = B
2B = B (???)

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