INA118 instrumentation amplifier

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Greetings!

I've connected an instrumentation amplifier (INA118) with a gain of 5000 in order to multiply that gain with the input voltage difference. The gain resistor is set to 10 ohms, according to the datasheet:
G = 1+(50kΩ/Rg), ∴"‹ G = 5001.

I'm using a virtual ground as the reference voltage, it's set to 2.5V. The positive voltage is 1.4 above that reference and the negative input is 0.5V below that difference.

Correct me if I'm wrong, but the output voltage shouldn't be (V+) - (V-) * G? So, 5000(1.4V-0.5V)?
If I measure the output voltage with a multimeter and measure the voltage reference (with respect to ground - 0V), the measure I receive is exactly the same. It is not amplifying, Vref = Vout.

What am I doing wrong? Here are my schematics:

http://img228.imageshack.us/img228/9056/ina118.jpg

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Do you think that a gain of 5001 could be a little excessive in this situation?

Tom Pappano
Tulsa, Oklahoma

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tpappano wrote:
Do you think that a gain of 5001 could be a little excessive in this situation?

Well, same thing happens if the gain was just 2 using a 50K resistor. After measuring the voltage in the output and the reference voltage, they are still the same (2.5V).

If I see the signal on an oscilloscope, I just see the DC and not the sine wave from the function generators.

Last Edited: Sun. Nov 28, 2010 - 11:44 PM
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50k would give you a gain of 2. Try that, and it should help you sort out what is going on. Then increase the gain once you see how it works.

Tom Pappano
Tulsa, Oklahoma

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Why are your sin generators referenced to Ground, not to your "Neutral", 2.5 V reference? If they are ref to Ground, then their DC offset should be 2.5 V, not 0 V.

Get rid of one of the sin generators and just use one signal generator to begin with.

Connect it to the V+ and V- on the IN amp.

As Tom said, start with a gain of 2 to get the system working, then adjust it.

I assume this is all a simulation. If not, some other changes would be benficial. The 470 ohms are small, i.e. draw a lot of current. The IN amp Vref pin is typically driven by a voltage follower op-amp. It should be a low impedance driver. (At least on many IN amps, I did not pull this one's data sheet...).

Edit: Yes 470 // 470 is pretty low, but at the cost of high current. Also, for real usage, you want a cap from the 2.5V ref node to ground.

JC

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Thanks to all of you for your suggestions.

Well unfortunately lowering the gain didn't work for me. You can check this other example for instance:

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.