• Resolved

INA126: Amplifying a very small A/C voltage

Part Number: INA126

Hi, I'm trying to build an EMF detector which can detect very low levels of EMF.

I have a coil which outputs approx 0.2V AC when subjected to very high levels of EMF from a demagnetiser (50Hz?). But I want to amplify the coil's output so that I can easily detect very low levels of EMF with it.

I am using an instrumentation amplification IC (INA126PA) to amplify the 0.2V AC signal and an AC voltmeter to measure the output. I want this to be about 100V AC, but at the moment I am getting OV, with or without a 100 ohm external resistor (RG)

I have included a circuit diagram below. The circuit is currently on a breadboard. My 3V supply is from 2 x 1.5V batteries.

I'm sure I have done something really silly. Can anyone please tell me (simply) what it is?

Regards,

Alan.

  • Hello Alan,

    There are a few things to consider here.

    1. Are you trying to get a 100V peak-to-peak swing or a gain of 100 Volts/Volt?

    In either case you will not be able to truly accomplish this since the max output will be your positive supply voltage (1.5V) minus roughly 0.75V depending on your load resistance. Your negative swing will also saturate at your negative supply (-1.5V) plus 0.8V. The maximum amount of power you can supply to this part is +-18V (dual supply) or 36V (single supply), thus maximum output voltage swing you can accomplish will be 36V peak-to-peak or just below a gain of 36/0.2 = 180 V/V. Overall, consider increasing your dual supply voltage to increase your possible output voltage swing.

    2. ***You need to include input bias current paths for the inputs of the IC. (This is most likely why you're not measuring any output)***

    I ran simulations in TINA-TI and these paths were needed to see a voltage output. These paths are two resistors connecting Vin+ and Vin- to ground. I chose 47kOhms as seen in section 8.2.2.3 of the INA126 datasheet, but you may want to consider other values depending on your application.

    3. You should connect your reference pin to ground and measure the voltage output from Vout to ground.

    If you operate the amp with a single suppply voltage, then you will want to connect Vref to mid-supply power rail. In this case, measuring the output from Vout to Vref will not work.
  • In reply to Peter Iliya:

    Thank you very much Peter. Your advice was spot on. I didn't actually need 100v output (rms), I just wanted enough to give me a good reading on my AC voltmeter. Following your advice, I used 2 x 9v supply with earth as their mid point and put the two 47kOhms resistors in you recommended. This gave me a maximim reading of 16V A/C which is quite adequate. When I then reduced Rg to 10 Ohms the sensitivity to EMF increased dramatically, and though I have yet to calibrate it, I'm sure it will now be fit for purpose. I would like to follow this email up with one with the updated circuit diagram attached so that others can use this to build their own EMF meter. Will your system allow me to do this?
  • In reply to Alan Morris:

    Yes you should be allowed to attach a file or insert an image. Completely up to you.

    Peter Iliya

    Precision Applications

  • In reply to Peter Iliya:

    Hi Peter,

    I've tried to respond with some questions and 2 diagrams, but it won't allow me to insert them as I did in my initial post. Do you know a way round this?. If not I will start a new thread.

    Regards,

    Alan Morris
  • In reply to Alan Morris:

    Use should be able to insert files/media or attachments. Did you click the "Use rich formatting" link at the bottom right of your reply screen? This brings up all of the editing features to your replies.

  • In reply to Peter Iliya:

    Hi Peter,

    I am now using Rich Formatting and have tried sending you the two diagrams by "Paste from Word" which worked before (ctrl C, ctrl V) and then by "Insert File" (as a jpg), but neither appear to be working. I have checked that my PC is working OK, by copying and pasting into a blank page and had no problem. Maybe I could send them attached to an email, or send them by post or start a new thread??

    Best Regards,

    Alan Morris

  • In reply to Alan Morris:

    Hi,

    Please see below, my circuit diagram which is for an EMF detector employing an INA126PA instrumentation amplifier. I've tried it and it works quite well, but after some testing, I now realise that it needs to be more senistive to low A/C voltages in order to get the EMF sensitivity I need.

    Using this circuit I am getting a couple of volts A/C from a very small induced A/C input, which unfortunately I can't measure as my multimeter doesn't go down that low, but since I am using a 10 ohm resister for Rg and getting 1v A/C meter reading in my test, I must assume that the input signal is around 0.1mV. However, this equates to about 1milligauss, and I am looking for readings down to 0.01milligauss, so I really need to multiply the sensitivity by 100.

    I wondered if it is possible to get this degree of amplification using 2 x INA126PA instrumentation amps in series? Or would you recommend a different approach, e.g a different TI amplifier?

    Regards,

    Alan Morris

  • In reply to Alan Morris:

    Hello Alan,

    I can see your circuit. What is your problem?

    Peter Iliya
    Precision Amplifiers
  • In reply to Peter Iliya:

    Hi Peter, looks like you saw my first draft of my message and not the updated version.

    My circuit diagram is for an EMF detector employing an INA126PA instrumentation amplifier. I've tried it and it works quite well, but after some testing, I now realise that it needs to be more senistive to low A/C voltages in order to get the EMF sensitivity I need.

    Using this circuit I am getting a couple of volts A/C from a very small induced A/C input, which unfortunately I can't measure as my multimeter doesn't go down that low, but since I am using a 10 ohm resister for Rg and getting 1v A/C meter reading in my test, I must assume that the input signal is around 0.1mV. However, this equates to about 1milligauss, and I am looking for readings down to 0.01milligauss, so I really need to multiply the sensitivity by 100.

    I wondered if it is possible to get this degree of amplification using 2 x INA126PA instrumentation amps in series? Or would you recommend a different approach, e.g a different TI amplifier?

    Regards,

    Alan Morris
  • In reply to Alan Morris:

    With a 1V peak-peak output and gain = 5 + 80,000/10 = 8005V/V, your input differential signal must be 0.124mV pp, but you must take caution into the INA126's noise and bandwidth.

    What frequencies are you considering inducing into the coil?

    You may be able to just use the INA and another op-amp to satisfy the overal gain as well.