Need help with designing a circuit

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Thread Starter


Joined Jan 19, 2006
Hey I need a little help with designing/drawing a circuit.
Can someone make me a sketch or help me with the solution?

Here is the my problem:
I’m going to make a measurement system for a bridge (AC bridge circuit).
The measure-elements in the bridge can be resistive, for example:
a Pt-100-element for measuring temperature, or a strain gauge for measuring pressure or power,
or it can capacitive so that it can measure air humidity.
The bridge is going to be driven by AC, and shall be square-, triangle-, or sinus-shaped.
The frequency on the AC shall be in the area from 400Hz – 1kHz.
The reason for using AC is because we can reduce influence of the amplifiers DC-error.
When the bridge is driven by AC-voltage, then of course the bridge’s output signal will be AC.
And this output signal must be demodulated. And the method I supposed use, is with Phase Sensitive Detection.

· Make an oscillator circuit to drive/operate the bridge.
· Make a necessary instrumentation amplifier by using op-amps.
· Make a phase shift circuit.
· Make a demodulator.

· Assume that the bridge’s output signal have amplitude ≤ 2mV.
· The signal bandwidth is DC to 10Hz, -1dB with 10Hz.
· The measurement system shall be in the area 1.0V to 5.0V.
· The measurement system accuracy shall be ±0.5% of full scale.
· The measurement system resolution shall be 8 bits.

· Available supply voltage is +12V, -12V and +5V.
· Available op-amps are uA741, OPA27, OPA177.
· Available voltage reference is LM336.
· Available voltage comparator is LM311.
· Available analog switch is DG413DJ.
· Available DA-converters are DA0808, DAC0832.

Thanks in advance!

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