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Author Topic: OpAmp + Voltage Divider circuit  (Read 2421 times)
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Hey guys. I posted this question on Y!A but no one has answered it yet... I guess this is a more appropriate place to ask.

Suppose I have a DC power source (a solar array) that can output up to 110V & 40A, and I want to log the voltage produce by the panel throughout the day. Since feeding 110V directly into the analog input pin of my data-logger will damage it, I am going to scale down the voltage to a maximum of 4.5V. Also, I want to minimize power dissipation by the two resistor, so I am going to use 2 high resistance resistor (maybe 1MegaOhm and 40kOhm) // with the panel, so that a tiny amount of current can flow through the resistors.

But, I heard that high resistance voltage divider is not going to give high precision voltage output. My question is - is it possible to use a lower resistance values resistors with an op amp in voltage follower configuration to yield high precision and low power dissipation voltage divider, considering an OpAmp has Infinite input impedance?

According to this site, one of the application is that "A bridge circuit or a voltage divider circuit - maybe where there is a resistive sensor in the bridge or voltage divider - and drawing current when you measure the voltage would be enough to change the voltage."
http://www.facstaff.bucknell.edu/mastascu/elessonsHTML/OpAmps/OpAmp3Note1Buffer.htmlBut I am not sure how exactly do I setup the circuit and what resistor to use. There was another guy who suggested me to use an instrumentation amplifier instead of an OpAmp, but I can't figure out what difference is it going to make.

I guess using an OpAmp also has another advantage - in order to measure the voltage from the voltage divider, I am going to have to connect the ground of the 110V solar panel with the ground pin of my tiny microcontroller. If my voltage divider has infinite impedance, it can effectively eliminate ground loop effect. Can anyone confirm this? Thanks!
« Last Edit: January 27, 2013, 03:59:13 pm by dominicfhk » Logged

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No need for an op-amp at all for a low bandwidth signal like this - stiffen up the mid-point of the voltage divider with a capacitor - this
reduces the impedance of the divider as seen by the ADC as low as you like (but does low-pass filter the signal).

Lets say you use 1M ohm and 47k ohm (about divide-by-22).  The midpoint is about 45k impedance without a capacitor, and
for the timescales involved in charging up the ADC's sample/hold capacitor (12us) even a few nF will bring this down to 10k or
less.   So adding a standard 0.1uF decoupling cap will make the value perfectly fine for the ADC, and will act as a low
pass filter with a roll-off freq of 35Hz, no problem at all with even the most rapidly fluctuating cloud cover!
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Quote
But, I heard that high resistance voltage divider is not going to give high precision voltage output.

Yes. But that can be mitigated effectively and inexpensively.

Quote
My question is - is it possible to use a lower resistance values resistors with an op amp in voltage follower configuration to yield high precision and low power dissipation voltage divider, considering an OpAmp has Infinite input impedance?

Yes.
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Suppose I have a DC power source (a solar array) that can output up to 110V & 40A, and I want to log the voltage produce by the panel throughout the day. Since feeding 110V directly into the analog input pin of my data-logger will damage it, I am going to scale down the voltage to a maximum of 4.5V. Also, I want to minimize power dissipation by the two resistor, so I am going to use 2 high resistance resistor (maybe 1MegaOhm and 40kOhm) // with the panel, so that a tiny amount of current can flow through the resistors.

But, I heard that high resistance voltage divider is not going to give high precision voltage output.

That depends entirely on the resistance of the load it is driving - which in this case is the input resistance of the data logger. If the data logger is one of the analog inputs of an Arduino, then its input resistance of about 100Mohms, and you have nothing to worry about if you use 1M and 40K. I have used 2 x 4.7Mohms in the past.

My question is - is it possible to use a lower resistance values resistors with an op amp in voltage follower configuration to yield high precision and low power dissipation voltage divider, considering an OpAmp has Infinite input impedance?

What I think you mean is: can you use high value resistors in the voltage divider, feeding an op amp in voltage follower configuration, to mitigate the effect of the datalogger input resistance on the voltage divider. Yes you can - but if the datalogger has a high input resistance compared with 40Kohms then there is no need to.

I guess using an OpAmp also has another advantage - in order to measure the voltage from the voltage divider, I am going to have to connect the ground of the 110V solar panel with the ground pin of my tiny microcontroller. If my voltage divider has infinite impedance, it can effectively eliminate ground loop effect. Can anyone confirm this? Thanks!

No, you still need a common ground connection. If you were to use an instrumentation amplifier, then you could tolerate the solar panel ground and the datalogger ground being at slightly different potentials, but only by a few volts. If you really need to isolate the datalogger from the solar panels, you could use a digital isolator chip such as ADUM6401 with an ADC chip on the isolated side.
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