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DC Differential amplification with gain

Jason Wu59
Prodigy 30 points
Other Parts Discussed in Thread: INA103, INA333, INA131, INA163, TIPD129

Hi,

I have a two resistors (R1 and R2, they are about 5000 ohm) in series with a square wave current passing through (250nA - 500nA). I am looking for differential amplifier to enhance the voltage signal across each of the resistor. After differentiating the voltage across each resistor with 100 gain, I want to subtract the two output signal from the two differential amplifiers.

 I am looking at the INA103. I don't know it is good for my approach since I need very high precised voltage signal. the output signal should have resolution down to 1nV with no gain. And the two differential amplifier should have the exact gain. Do you have any recommendation a amplifier has low noise, high gain accuracy, and high input impedance.

 


  • 0
    TI__Mastermind 25775 points

    Hello Jason,

    Can you please provide a little more information about the application? 

    I also have a few questions for clarification.  For example, even though the title states "DC..." in the text you mention a "...square wave current..." which does not imply that the input is dc.  Also, a gain of 100V/V is stated.  But the next paragraph mentions a '...resolution down to 1nV with no gain."

    Also, please verify that you have +/-15V power supplies and your intention for calibration.  I assume you intend on at least calibrating out the offset voltage.  I mention this because the INA333, which is our most precise instrumentation amplifier with respect to offset voltage, has 25.75uV offset voltage (max).  This would appear as 2.575mV at the output, which is much greater than your 1nV requirement.

  • 0 in reply to Pete%20Semig
    Prodigy 30 points

    Hello Pete,

    Sorry for the confusion. I am kind of new in here. I have been using commercial voltage preamplifier (SR560 or DL1201). Let me give you a bit more background of the measurement, a  square wave current (+/-500nA, f= 0 - 24Hz) is passing a resistor(~5kohm) and voltage drop across the resistor is measured by a voltmeter. The resolution of the voltmeter is down to ~1nV because the resistance change is small.

    Now, a square wave current is passing through two resistors in series and I am looking for two amplifiers to differentiate the voltage across each resistors separately. But the amplifier needs to be ultra sensitive because the voltage drop across the resistor usually is around 100 uV and the resolution needs to be around 1 nV. (without any gain). the two amplifiers need to have same gain if a gain is applied because the signal from each amplifiers will pass through a third amplifier to do differentiating. 

    So it is necessary to do a offset calibration before measurement. I do have dc power supply but up to +/-10.5V but it is not a problem to find a higher power supply. As i know, the impedance of the amplifier is huge, right? so I don't need to worry the current leakage problem. I have a lot of questions to ask since I never use this type of amplifier before.

    Do I need to add a extra components (resistor, capacitor, or op-amp etc.) to lower the noises?

    Do I need a dc or ac power supply for mine purpose?

    How to calibrate the gain to make both of the amplifiers have same (as closed as possible ) gain?

    Will the amplifier cut off the voltage signal and what is noise level of the ina333?

    Jason

  • 0 in reply to Pete%20Semig
    Prodigy 30 points

    Hi, Pete,

    How about INA163 and INA131. The noise level is below 10 nV/(Hz)^.5.

    Jason

  • 0 in reply to Jason Wu59
    TI__Mastermind 25775 points
    Hello Jason,

    Thanks for the additional information. I have been out of the office and will reply in greater detail early next week. On a couple of side notes, please understand that the noise of a 5kohm resistor is ~9nV/rtHz. Perhaps you'd be interested in our TI Precision Labs videos on noise (among other topics). Here is the link: www.ti.com/precision-labs. Also note that if you can perform a 2 point calibration on the system (see TIPD129 Appendix A.3 for more info) you can easily calibrate out the offset AND gain errors.
  • 0
    Prodigy 50 points

    Hi Jason,

    did you consider the following circuit (a classic wheatstone bridge) instead?

    You can use one precision IA (instead of 3)