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converting negative voltage to positive voltage using LMP 348

mohammed bilal
Prodigy 50 points
Other Parts Discussed in Thread: DAC0800, TLC7225

Hi,

      I am using DAC0800 which is giving a negative voltage. I tried to convert it to positive by using inverting amplifier but the voltage is getting decrease like if am getting  0v for min. input of DAC and -5V for max. input of DAC after converting 1.9V for min. DAC input and 5V for max. DAC input. Is it possible to get a 0V for min. input of DAC after converting?

This is our first project.Please do help.

  • 0
    TI__Mastermind 43870 points

    Hi Mohammed

    Can you tell us more about what speed of conversion you need, and what is the ultimate output voltage range you are looking for? It may be possible to get the DAC0800 output converted to the range you are looking for, but there may also be other TI DAC products that are more suited to your application.

    Also, if you could send the schematic of what you currently have prototyped and what type of IC packages you can work with it would help us recommend a circuit to achieve what you are looking for.

    Best regards,

    Jim B

  • 0 in reply to Jim Brinkhurst1
    Prodigy 50 points

    Hi Sir ,

                 Thanks for your replay. I am attaching  the schematic diagram of our DAC4341.DAC.doc circuit .The output voltage range what we are looking for is 5V for max. DAC input and 0v for min. It would be helpful for our project if conversion speed is within 100u sec. 

  • 0 in reply to mohammed bilal
    TI__Mastermind 43870 points

    Hi Mohammed

    Here are some recommended changes to your current circuit  to achieve 0V to 5V Vo range. These first few items can be found by careful reading of the LM348-N and DAC0800 datasheets:

    • Change LM348-N supplies to +12V and -12V, this older bipolar op-amp cannot be operated from 5V single supply.
    • With Vref = 12V and Rref = 5kohm, Ifs = (12V/5000ohm x 255/256) = 2.3906 mA
    • To calculate feedback resistance for 5V full scale, we have 5V/2.3906mA = 2091.5 ohms. I suggest using a 1.8kohm fixed resistor plus a 500 ohm variable resistance.

    Information on general op-amp usage can be found in numerous applications notes, including this one: (http://www.ti.com/lit/an/snla140a/snla140a.pdf)

    • This circuit does not have any offset adjustment. Therefore the output voltage will not be exactly 0V at Zero Scale DAC setting. If that is needed, use a resistor ladder with trim adjustment to fine tune the voltage of the positive feedback terminal of the LM348-N. Connect the resistor ladder from +12V to -12V, with adjustment range of perhaps +/- 10 to 20 mV around 0V depending on the accuracy of your supplies. For optimal input bias current cancellation, the equivalent impedance of the bias ladder network should be equal to the feedback resistance on the negative feedback at around 2.09 k ohms.
    • If offset adjustment isn’t needed, it would still be good to put 2.09 k ohms resistance to ground on the positive feedback terminal (rather than 0 ohms) to cancel out input bias current.

    You may also want to consider a DAC like the TLC7225. It is a voltage output DAC with parallel inputs. It may more directly meet the needs you have.

    I hope this is helpful.

    Best regards,

    Jim B

  • 0 in reply to Jim Brinkhurst1
    Prodigy 50 points

    Hi Sir,

               Thanks for your answer.It was very helpful for our project.

    Best regards ,

    Mohammed Bilal Tamkin