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### Topic: INA118 instrumentation amplifier (Read 2188 times)previous topic - next topic

#### lcruz007

##### Dec 02, 2010, 05:21 amLast Edit: Dec 02, 2010, 06:04 am by lcruz007 Reason: 1
Hello,

I'm currently trying to make the INA118 (instrumentation amplifier) work, but the amplifier is not working whatsoever.

http://img403.imageshack.us/img403/8508/circuit.png

The oscilloscope measure states that the signal is just 200mV, which doesn't coincides with my calculations. That's a gain of 50. There's a small voltage difference with respect to the voltage reference, which could perfectly be amplified.

What am I doing wrong?

#### RuggedCircuits

#1
##### Dec 02, 2010, 05:53 am
I don't follow your voltage divider values. R2 and R1 together should give:

5 * R2/(R2+R1) = 0.0234V, not 0.22V as you write

Similarly, R5 and R6 combine as:

5 * R6/(R5+R6) = 0.0495V and not 0.4545V as you write.

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#### lcruz007

#2
##### Dec 02, 2010, 06:15 amLast Edit: Dec 02, 2010, 06:22 am by lcruz007 Reason: 1
The 0.022 is the approximate voltage I get by the multimeter,   0.023390067 is not rounder. And as I stated in the schematics, r3 and r4  should give a voltage ~ 0.045V... and the other input should be around 0.05V according to my multimeter.

Is that right no?

Remember that I won't get an exact number as the resistors have a tolerance of 5% in the first place.

#### RuggedCircuits

#3
##### Dec 02, 2010, 06:27 am
Quote
The 0.022 is the approximate voltage I get by the multimeter,   0.023390067 is not rounder.

Yes, but your schematic says 0.22V, not 0.022V.

Quote
And as I stated in the schematics, r3 and r4  should give a voltage ~ 0.045V

No, they give a voltage ~0.455V.

You are also not really working within the allowable common-mode range of this device. If you look at the "Linear Input Voltage Range" parameter on the datasheet you see that it expects the minimum input voltage (at pins 2 and 3) to be about 1V above ground.

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