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XScopes Analog Front End

The signal conditioning of the input is a simple voltage divider, a non-inverting amplifier, and a level shifter. The OpAmp selected is a TL064, which has a GBW of 1MHz, a slew rate of 3.5V/µs, and is low cost. The simulation below shows the front end circuit:

Sorry, you need a Java-enabled browser to see the simulation.

Right on the input there is a spark gap to help keep ESD events out of the amplifier. The high impedance of the input also helps to keep the circuit protected.

An AC/DC coupling switch is present; the AC path is done with a high voltage 0.1uF capacitor.

Let’s calculate the transfer function. At the V+ pin of the Opamp, we have:

$$V^{+}=V_{in}∙{180k}/(820k+180k)= V_{in}∙9/50$$

This voltage is amplified by the OpAmp, using the non-inverting amplifier circuit:

$$V_{Op}=V^{+}∙(1+{20k}/{180k})= V^{+}∙(1+1/9)= V_{in}∙9/50∙10/9=V_{in}/5$$

Then, the voltage needs to be shifted so it can be applied to the microcontroller, using the superposition theorem:

$$V_{out}=V_{in}/5∙{3k}/(3k+3k)+2.048V∙{3k}/(3k+3k)$$

And finally:

$$\bo V_{\bo \out}=V_{in}/10+1.024V$$

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Last Updated on Monday, 06 January 2014 21:51  

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