Because of the Thanksgiving holiday in the U.S., TI E2E™ design support forum responses may be delayed from November 25 through December 2. Thank you for your patience.

  • State Resolved
  • Locked Locked
  • Replies 3 replies
  • Subscribers 48 subscribers
  • Views 953 views
  • Users 0 members are here
Support feedback
Options
Options
Similar topics

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

Original question:

INA121 with single polarity power supply

INA 121 and INA 122

Bob Edwards
Prodigy 210 points
Other Parts Discussed in Thread: TINA-TI, INA121

I've been using the Analog Engineers Calculator to determine areas of operation (VCM vs VOUT) for the two instrumentation amplifiers listed in the thread title. I have succeeded in building an amplifier from each of the two INA's on prototype boards with a single 5V source, along with the a 2.5V reference voltage and a 3.3V VCM. They work as advertised.

My question concerns interpretation of the data shown at the right side of the app window, which shows the Min and Max Vout and associated input data. It is the latter that I have been unable to interpret, although I understand that the numbers in question result from the solutions of equations. In particular, I do not have a picture of what is meant by the terms: Vin (dif MAX) and Vin (dif MIN), in particular what they tell me about the operating range and how I can use them to predict what will happen in the associated circuit.

I'm attaching a jpeg which shows a typical screen for the INA 121 circuit I'm working with. Any light you could shed on interpreting these values would be greatly appreciated.

Thanks,

Not sure about the image size here, but the input parameters are: INA 121, V+ 5V, V- 0V, VRef 2.5V. Gain 6, VCM 3.3V

Bob Edwards

  • +1
    Guru 140200 points

    Hi Bob,

    a TINA-TI simulation can help to understand what's going on:

    bob_ina121.TSC

    On the left side an input voltage is generated which sits symmetrically on the common mode input voltage of 3.3V. When the "input voltage" (VG1) is 200mV, then the voltage at the +input of INA121 is 3.3V + 100mV = 3.4V and the voltage at the -input of INA121 is 3.3V - 100mV = 3.2V. The differential input voltage of INA121 is still 100mV.

    When the differential input signal is 0V, then the only input voltage reaching the INA121 is the common mode input voltage of 3.3V, which is almost totally suppressed by the instrumentation amplifier. Because the differential input signal is zero, the output signal is also zero. But because VREF = 2.5V, the zero output signal is shifted by an offset voltage of 2.5V.

    When the differential input signal is going more negative than -375mV, the output voltage of INA121 goes into "negative" saturation and cannot emit an output voltage lower than +300mV. When the differential input voltage is going more positive than +267mV, on the other hand, the output voltage of INA121 goes into "positive" saturation and cannot emit an output voltage higher than about +4.1V.

    Sometimes, with very certain sets of common mode input voltage, gain and REF voltage, there are situations, when the output voltage of an instrumentation amlifier behaves weird and is going into saturation at a voltage which is far way from the supply voltage rails. In such a situation an internal section of the instrumenation amplifier becomes hiddenly overdriven. See the red circles:

    This is why so many users think that instrumentation amplifiers are rocket science. But when you carefully consider what is going on internally at the various nodes, the behaviour is no longer mystical Relaxed

    Kai

  • 0 in reply to kai klaas69
    TI__Guru 55381 points

    Hi Bob,

    Thanks for Kai's detailed reply. 

    I captured both graphs and present it in the image below --> please follow the color of markings and shapes. 

    If you have additional questions, please let us know. 

    Best,

    Raymond

  • +1
    Prodigy 210 points

    Hello Kai,

    Thanks very much for your rapid response. It illuminates the situation I was having difficulty with very well. I have a much better understand how the tool draws an envelope around the permissible range of operation and how the right hand side explains how the input voltage form their part of the bracketed permissible linear area of operation.

    Again, many thanks.

    Bob Edwards