Unsteady voltage divider?

Discussion in 'The Projects Forum' started by rougie, Oct 29, 2012.

  1. rougie

    Thread Starter Active Member

    Dec 11, 2006
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    2
    Hello,

    I require a very steady voltage reference of 2.5VDC. So I calculated a voltage divider using a 2.8K(R1-connected to ground) and 820ohms(R2-conmected to Vcc) connected to an approximately 3.34VDC VCC source. Please note R1 resistor value is approximate since I am really using a potentiometer in series to fine tune the voltage divider.

    When I measure the voltage in the middle of these resistors I get an approximate 2.5VDC BUT very unsteady.. voltage varies from 2.489VDC to 2.540VDC... Why does it dance around like this? Voltage is voltage... shouldn't the VR1 and VR2 Vdrops be steady. What's going on?

    I am thinking of using a voltage reference, but does this mean that to keep a very steady voltage of 2.5 VDC, the voltage reference component will be working like mad to achieve this?? Also won't that be bad for the voltage reference in terms of its durability.

    Also, would I be able to use a simple transistor with a feedback configuration and fixed bias to 2.5VDC as a voltage reference... just asking!

    thanks for all comments.

    r
     
  2. ErnieM

    AAC Fanatic!

    Apr 24, 2011
    7,386
    1,605
    If the voltage is unsteady then something is making it unsteady.

    Did you connect something that uses this reference? If you disconnect it does the ref get steady?

    How steady is the 3.34 volt source driving this?

    How steady are the resistors? Pots are notoriously unsteady. If you don't have a fixed value near 820 ohms then use a 2nd 2.8K and see if you get about 1.7 volts out and it is steady.
     
  3. rougie

    Thread Starter Active Member

    Dec 11, 2006
    410
    2
    Hi ErnieM,

    I actually have two voltage dividers, the first is for a steady 2.5VDC and the second one is for a 2.0VDC. These voltages branch off to two input references of a DAC chip TLC7226. The 2.5VDC connects to pin #4 and the 2.0VDC connects to pin #5 which allows for the "AGND bias for positive output offset" configuration as described on p.7-8 of the spec:

    http://www.ti.com/lit/ds/symlink/tlc7226.pdf

    It seems that when I connect the voltage dividers to these pins, the voltage dividers become very unstable!

    Its a little better now that I have added some capacitors in parallel to these inputs ... 0.01uf for pin#4 and 1uf for pin #5... but the voltages still vary around +/- 100 mv around the 2.5VDC and the 2.0VDC marks ??

    I don't know why the inputs of the DAC do this. Oh yeah, one more thing, when I stop counting the 8 bit parallel value, all voltages(the 2.0 and the 2.5VDC) are very stable. When I resume the parallel counting all voltages vary again??

    Would you know why?

    PS, my VDD is 15.5VDC and stable!

    thanks for your help!

    r
     
    Last edited: Oct 29, 2012
  4. Papabravo

    Expert

    Feb 24, 2006
    10,136
    1,786
    Doesn't the impedance of the load affect the divider? If the load impedance changes the divider will be unsteady as sure as the guy upstairs made little green apples. This is why a smart designer who needed a steady reference would never use a voltage divider.
     
  5. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
    4,769
    969
    Yep use true reference IC's
     
  6. rougie

    Thread Starter Active Member

    Dec 11, 2006
    410
    2
    Hello Papabravo,

    little green apples huh! LOL Okay, well, I am waiting for my v-refs to arrive, in the mean time I wanted to see if I could get things running with simple voltage dividers.

    BTW, the only place I see the load impedance values for input references is on p.4 at "ri(ref)" as 2-4K ohms... Do you believe that was my clue?

    Thanks
    r
     
  7. crutschow

    Expert

    Mar 14, 2008
    12,991
    3,227
    You could also add an op amp non-inverting follower to buffer the output of the voltage divider. The op amp output will provide a very low impedance to the DAC input.
     
  8. rougie

    Thread Starter Active Member

    Dec 11, 2006
    410
    2
    Thanks guys!

    Your recommendations are appreciated!
    r
     
  9. wayneh

    Expert

    Sep 9, 2010
    12,094
    3,033
    The problem should lessen if you increase the total current through the divider by lowering the resistor values, so that anything happening with the load is a smaller fraction of that total. The references are a better solution, but if you're impatient to wait for them...
     
  10. rougie

    Thread Starter Active Member

    Dec 11, 2006
    410
    2
    I gather that's what they mean when they use the term "stiff voltage divider"
    right?

    Thanks wayneh!

    r
     
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