Trouble using LM308 OpAmp

[bbaker]

Hi,
I am building an AC current sensor so sense less than two amps for feeding in to an A/D converter and need to amplify the signal up. The current clamp is CSLA1CD Honeywell Hall Effect Sensor: [url]http://www.scienceshareware.com/articles/measuring-current/csla2cd-current-sensor.pdf[/url] and I do not want to use too many turns so need to work with 0.0033 volts per Amp.

I am using an LM308 for a x10 gain amplifier. Ultimately it will be for amplifying a 50Hz AC signal, but for now I am just trying to test it with DC.

Configuration:
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+Vcc is 4v, -Vcc is -4v. I am using -ve feedback with a 10K/1K resistor (have also tried 10M/1M) to give me the x10 gain.
The input signal I am using is DC and adjustable b/w 0v and 0.4v

The problem:
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The output signal is not being amplified by 10, instead it is following the input very closely - about 0.2v above the input signal.
I know I have the theory right because I can get a x10 gain using a BA728 just fine.

The only thing I can think of is that I need to configure the external compensation but cannot find application notes that help me understand how it should be configured for my application. I put a 20pF ceramic across pins 1 and 8 and it did not do much. I touched the pins and the output rose about 0.3v - so something about the impedance or capacitance certainly makes a difference.

The question:
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Can anyone please help me understand how to use external compensation for OpAmps that do not have an internal capacitor?

Is there a better OpAmp to use for amplifying low voltage 50Hz AC signals?

Thanks in advance!

Bryan

[pebe]

Hi,
You show the 1K resistor returned to ground, but it should also be connected to 0V. Then with the correct compensation, the amplifier should operate OK. But I wonder why you chose an amp requiring compensation when there are plenty available that don't and would simplify your circuit. Even the humble 741 would probably do the job.

The gain from your cicuit is actually 11, not 10.
ie. Assuming the 10k = R1 and the 1K = R2, then Gain = (R1+R2)/R2

[bbaker]

Hi,
Sorry - yes the 1K resistor does go to 0v, my mistake in diagramming it.
Thanks for the advice on the different OpAmp. Is the 741 good for amplifying very low signals at 50Hz?
Thanks,
bryan

[pebe]

Hi,
The LM741 is getting a bit long in the tooth now and is not particularly suitable for very low signals - there are better ones available.

If you are buying new, then I woulod suggest a CA3140 (my favourite) or a low noise one like a TL071.