How to amplify several hundreds of nV?

Dear Bruce,

Now the amplified output has improved with the bias current you suggested, I can see on oscilloscope the waveform is much stable compare to the one without resistors. But it is not a satisfied result yet as the output now is in range of Volt. As I know the output voltage induced (V= - nBA) from the coils  sensor, which is used to detect mT magnetic field, should be in range hundred of nV. When this signal is amplified 10000 times, the output should be in range of mV.  Could you pls check my schematic? I haven't done much on it. I just followed the one in the datasheet. -15V is supplied to pin 4 and +15 V is supplied to pin 7. 

Could you please advice me what should I do to improve my results? I wonder is there anything to do with the offset voltage (50uV) as stated in the first page of datasheet? Below is the schematic I copy from datasheet with the resistors at the input sides.

Thank you.

Regards,

Cheatra.

Cheatra,

Sorry but I do not see a diagram. It would also help if you could provide an oscilloscope picture. Can you describe how this is not meeting your expectation. The amplitude is larger than expected but is it the correct wave shape? Is it the correct frequency? Please provide more information on what you expect versus what you are getting.

Regards, Bruce.

Dear Bruce,

Opp copy paste error. Now I already uploaded the schematic here. I don have the oscilloscope wave form with me here. I can say that, the frequency the wave shape have met my expectation. Only the amplitude of the output signal from amplifier is larger than the expected result.

I will upload the oscilloscope waveform later.

Thank you.

Regards,

Cheatra

Dear Bruce,

Below is the waveform from the oscilloscope. Sorry for late update because here we have one week holiday. The input signal from function generator with the frequency of 900Hz is supplied to Helmholtz coils generating magnetic field. Then the planar coil sensor is used to detect the magnetic field. Finally, this induced voltage is amplified 10000 times. The problem that i am facing now is the amplitude. The Helmholtz coils and the amplified circuit are placed inside the magnetic shield chamber.

Could you please advice me what should I do in order to improve this?

Thank you.

Regards.

Cheatra.

Cheatra,

I do not have a clear understanding of your problem. As I read your various posts, you seem to think that your input signal to the INA129 is larger than expected. Is that correct. Do you have other ways to measure and validate the magnitude of the input signal.

The circuit diagram you provided is incorrect. You show both inputs connected directly to ground. I trust that this is not the case in your actual circuit. What is the value of the gain-setting resistor, RG, connected from pin 1 to pin 8. This should be approximately 5 ohms. Have you checked the value of this resistor with an ohm-meter? You should try connecting the amplifier to a known signal source to see that it has the expected behavior and gain.

Regards, Bruce.

Dear Bruce,

Since the limitation of the tool we have, we can't measure the uV input signal. The reason why I have doubt because of the output signals after 10000 amplifying from several planar coil sensors with different number of turns are almost the same, around 12V.

The circuit diagram I got it from page 10 of datasheet, figure 3. I have tried with the first and second one from figure 3, all gave me the same results. The value of the gain resistor RG is 5.1 Ohms. And I also have tried to connect the small signal from function generator to the amplifier pin 3 while pin 2 was grounded, then the result was right according to gain setting value.

Could you please advise me on how to connect the output from the coil sensor to the amplifier in the correct way? I use the coil to detect the magnetic field B in the middle of Helmholtz coil and then from the sensor I directly connect to pin 2 and pin 3 of amplifier as shown in previous figure. Your help is really appreciated. 

Thank you for your time of sharing.

Regards.

Cheatra.

Dear Bruce,

I have checked the amplifier. It works well. And I also have found out the problem is with the probe connected from the sensor to the amplifier. These probes will generate a loop with induce the voltage along the line and add up to my signal. I verified it by changing the length of the probe and the area (formed by the two probe) of the probes, then the output voltage is changing. 

May I ask you one more thing, Do you know any probe or circuit that won't induce any voltage while being measured or used in magnetic field? I have tried Agilent 6 1/2 Digit Multimeter probe, it can help to reduce the output voltage but the probes still interfere in my signal even i place the probes in the magnetic chamber.

Regards.

Cheatra.

Cheatra,

I'm not sure I understand your question. I believe that the basic nature of the probe is this issue. The presence of a coil in a magnetic field is sure to create some disturbance in the field. A smaller probe will likely create less of a disturbance. I believe that the only contribution to the disturbance created by the electronics would be due to input impedance or input bias current of the amplifier.

Regards, Bruce