# Needed urgent help.wheatstone bridge with op amp

Discussion in 'Homework Help' started by nataholick, Mar 7, 2010.

1. ### nataholick Thread Starter New Member

Mar 7, 2010
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im currently working on a project that requires the construction of a wheatstone bridge connected to an op amp(LM 358). the gain of the op amp is to be 20times with R1=R2=22K, R3=R4=220K. I understand that the Vout is calculated by the (+) and (-) signal from the wheatstone bridge. I need to know what are the values for the resistors(R1, R2,R3,R4) on the wheatstone bridge.would appreciate all help. thanks!

2. ### t_n_k AAC Fanatic!

Mar 6, 2009
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If those are the values for a differential amplifier configuration wouldn't the gain be 10 times?

3. ### nataholick Thread Starter New Member

Mar 7, 2010
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yes.But 20 is the ratio between the 2 output (-)&(+) from the wheatstone bridge and the LM358 op amp.which makes me really confused. since the op-amp is an inverting with subtractor on the top. =(

4. ### t_n_k AAC Fanatic!

Mar 6, 2009
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The differential amplifier simply amplifies the bridge difference output signal.

If the bridge output magnitude is 10mV from +ve to -ve midpoints then the amplifier output magnitude would be 100mV since the differential input voltage magnitude would be 10mV. Whatever the polarity of the difference signal this would be inverted at the output.

Last edited: Mar 8, 2010
5. ### t_n_k AAC Fanatic!

Mar 6, 2009
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Maybe a diagram would help ...

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6. ### nataholick Thread Starter New Member

Mar 7, 2010
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thanks for ur help! i will try out by building it out on the circuit board