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Original question:

OP07C: About spply voltage

OP07C: Single supply operation

Krasimir Krastev
Prodigy 30 points
Part Number: OP07C
Other Parts Discussed in Thread: OP07,

Hello, my problem with this amplifier is maybe that I don't understand single supply operation. I'm trying to power the OP07CP with a 9V battery. If I split the supply equally with a voltage divider it works. But if I want to directly connect the battery to the Vcc+ and Vcc-, and use the (-) of the battery, also the op amps Vcc- as ground, I get a constant output voltage between 8 and 9 V, and I can't amplify signal. I attach a photo for clarity.

  • 0
    Guru 140200 points

    Hi Krasimir,

    the right schematic violates the common mode input voltage range of OP07. The input voltages of OP07 should stay 2V away from the supply rails. Please read the datasheet.

    Kai

  • 0
    TI__Guru 55461 points

    Hello Krasimir, 

    I am simply modifying your attached circuit from above in Tina simulation. There are many ways to bias and compensate the thermocouple sensors. Typically, if you want to measure accurate temperature, TCs will require cold-junction compensation - see the attached application note. 

    Also, you can measure temperature using RTDs, Thermistors, PN junction diodes, TCs temperature sensors. 

    If you need additional help, please let us know. 

    snoa544a.pdf

    snoa663b.pdf

    Regards,

    Raymond

    Simplified Thermocouple Input circuit 12102019.TSC

  • 0 in reply to Raymond Zhang1
    Prodigy 30 points
    Hello,
    thank you very much for the assistance. However, since I'm using a microcontroller I plan to simply amplify the signals from the thermocouple and cold-junction sensor and do all the compensations and linearizations programwise. Also I decided to split the battery supply to 9V, 2V and 0V, and use 2V for ground, because I have to power an Arduino board as well. I have another question: Is it possible to use the offset pins of OP07CP and a potentiometer to create a precise offset instead of nulling it as it is originally intended? Because I want to offset the thermocouple output entirely above ground. If not, then I guess I have to include a precise voltage reference at the input, right ?
    Regards,

    Krasimir

  • 0 in reply to Krasimir Krastev
    TI__Guru 55461 points

    Hello Krasimir,

    You can use the positive voltage to null out the input Offset-voltage. See the Figure 2 in Section 10.1 of OP07C datasheet. I think that this is what you meant, see the attached image. You can use 2V bias to do the Offset-voltage null. The small offset voltage is inserted between Offset N1 and Offset N2, see the functional Block Diagram in Section 9.2. 

    http://www.ti.com/lit/ds/symlink/op07c.pdf

    Since you are doing temperature computation via MCU, you can substrate or compensate much of temperature related errors through software. As you indicated, all associated errors have to be measured above the ground, and within the measurement resolution of your ADC. 

    Regards,

    Raymond

  • 0 in reply to Raymond Zhang1
    TI__Guru 55461 points

    Hello Krasimir,

    I forgot to mention that if you are concerned about the variation of 9V batteries as the input voltage and temperature accuracy over time, it will be a good practice to place a zener in series with a resistor from 9V battery. And generate the 2V within your zener voltage with resistor divider. This way, you will have stiffer 2V to work with, and perform a better input offset-voltage nulling operation over time. 

    Regards,

    Raymond 

  • 0 in reply to Krasimir Krastev
    Guru 140200 points

    Hi Krasimir,

    you might also want to read this:

    sbaa274.pdf

    Kai

  • 0 in reply to kai klaas69
    Prodigy 30 points

    I want to thank you both for your cooperation. However I have concluded that this amplifier is not suitable for battery powered applications and I have considered other options. I wish you happy holidays and a wonderful time with your families!

    Best regards,
    Krasimir

  • 0 in reply to Krasimir Krastev
    Guru 140200 points

    Hi Krasimir,

    thank you very much and all the best to you as well!

    Kai