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Need to modify SBOA233- voltage-to-current converter circuit for- 0 to 1024uA

Krishna Kumar1
Prodigy 250 points

Other Parts Discussed in Thread: INA826, OPA188, TLV6003, OPA189

Hi,

I want to modify the circuit in SBOA233 to the below requirements

1. Vinput= 0- 12V

2. Iout= 0 to 1024uA

3. Compliance voltage 12V.

I tried modifying the circuit for the above ranges, by replacing the OPA and INA, but could not get successful results.

Kindly guide me for the new changes.

Regards

Krishna

  • 0
    TI__Genius 10850 points

    Hello Krishna,

    Thank you for your post. I have done this for you already for:

    • VIN = 0-10V
    • IOUT = 0-1024uA

    https://e2e.ti.com/support/amplifiers/f/14/p/860834/3194420?tisearch=e2e-quicksearch&keymatch=sboa233#3194420

    That was your initial request. I understand that requirements change, so we encourage our customers to try to make these modifications themselves to fit their application. The only way you are going to learn how to do this is to try modifying the circuit yourself following the SBOA233 design. The design is written such that we share all the calculations so you can make modifications as needed. For instance, I followed this design to make the modifications for you the first time.

    You may use the same devices I used in the original solution (OPA188, INA826). You will need to change the supplies, though, since you know want operation up to 12V in. Change the supplies to:

    • OPA188: single supply 15V
    • INA826: dual supply +/-15V

    In the SBOA233 design, you need to follow Step 2 on page 2 to calculate the new gain you need to operate at your new conditions:

    • Vsetmin = ILmin * Rset = 0uA * 100k-ohm = 0V
    • Vsetmax = ILmax * Rset = 1024uA * 100k-ohm = 1.024V
    • G = (Vinmax - Vinmin) / (Vsetmax - Vsetmin) = (12 - 0) / (1.024 - 0) = 11.7V/V

    11.7V/V is the new gain you need in order to get 0-12V in, 0-1024uA out.

    The gain equation for the INA826 can be found in the product datasheet: G = 1 + (49.4k / Rg). Solve for Rg when G=11.7. You will get Rg = 4616-ohm. Change the gain resistor to this in your circuit. 

    Please follow these steps, read through the design and make the changes necessary for your new requirements. 

    Here is the old file that you can make changes to:

    1462.0-10V in, 0-1024uA out.TSC

    -Tamara

  • 0 in reply to Tamara M Alani
    Prodigy 250 points

    Dear Tamara,

    Thank you for your continued patience in explaining the design in detail. I could achieve the required result. Thanks a lot for the same.

    I actually got this document SLAU507 ( http://www.ti.com/lit/ug/slau507/slau507.pdf ) which is the same as SBOA233 but fully in detail, after I posted the question to you. I could also arrive at the same values as per your calculation shown above.

    There is something more in this document, which mentions about the nonlinearity in the op-amp output swings.(Pg 19 of SLAU507). The OPA188 which we would be using has higher non linearity range of upto 0.5V or 500mV which is very high as mentioned in the open loop gain range.

    So I am on the hunt for an op amp which gives the least non linearity and is better than OPA188.

    TLV6003 is one such I could find. If you have a better suggestion please let me know.

    Regards

    Krishna

    #ArthurKay

  • 0 in reply to Krishna Kumar1
    TI__Genius 10850 points

    Hi Krishna,

    Happy to help. You may also take a look at the OPA189 with a open-loop gain range of 170dB spec'd at 300mV from either rail.

    The datasheet can be found here: http://www.ti.com/lit/ds/symlink/opa189.pdf

    -Tamara