• State
  • Locked Locked
  • Replies 2 replies
  • Subscribers 48 subscribers
  • Views 439 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.

Circuit help

derek koonce
Prodigy 160 points
Other Parts Discussed in Thread: INA327, TLV3701

I've been scratching my head for a bit and cannot seem to find the right path. I need an analog solution that uses minimum IC usage for a circuit that does the following.

*  When Vin = 0 to 1.2V, the output remains at 0V

* When the input varies from 1.2V to 5V, the output linearly varies from 0 to 1.5V

* Supply voltage is 7V and ground

* I may have to adjust the 1.2V setting later

I was looking at a rail-to-rail opamp, but cannot get anthing working properly.

My other option is to use a current amp whereby the output current is 0A until I hit the 1.2V input. Then the current ramps up. I have not designed with current opamps yet to really know the right approach for the design.

Any support would be greatly appreciated.

Derek

  • 0
    TI__Guru* 93105 points

    Hello Derek,

    I have developed a circuit that is fairly simple using a TLV3701 low power comparator and an INA327 instrumentation amplifier that has rail-to-rail input and output, and a shutdown pin. Also, the iNA327 may be operated with a gain less than 1V/V which is required to reduce the 0 to 5V input range to 0 to 1.5V. The circuit is shown below.

    The INA327 is kept off until the input voltage, VS1, exceeds the comparator threshold voltage which is set to 1.2V. Then, the comparator output goes high bringing the INA327 out of shutdown and into amplification mode. The input voltage is applied to the INA327 non-inverting input while an offset voltage is applied to the inverting input. That offset moves the output down to 0V when a voltage greater than 1.2V is applied to the circuit input. Some iteration in the divider R7 value and the gain set resistor R2 was necessary to get the right combination of output offset and gain. Some optimization of the circuit would be required for best performance such as adding hysteresis to the TLV3701 comparator. The schematic shown is to present the idea more so than a final design. The product data-sheets provide ore detail about the individual devices.

    The INA327 can't be powered by a suppy with a voltage as high as 7V so a 5V LDO regulator would be needed in the supply line. The LDO would help establish a more constant voltage for the resistor dividers at the INA327 and TLV3701 inputs.

    I hope this idea is of some help to you.

    Regards, Thomas

    PA-Linear Applications Engineering

     

     

     

  • 0
    Genius 5430 points

    Derek;

    Here is an oddball circuit that depends on a single-supply amplifier's not being able to swing below its negative supply-- in this case, ground. You need to generate a +5V supply voltage from your 7V but that should be easy. If you analyze this thing you might recognize it as a difference amplifier with a DC offset. Keep the output loading very light.

    Offset Input SS Amplifier.TSC