Analog Voltage Measurement

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Saravanan0301

Joined Jan 25, 2016
1
My requirement is to measure DC voltage in range 20 to 80 using a microcontroller.

Its is stated in the microcontroller datasheet that "The maximum recommended impedance for analog sources is 2.5 kΩ." and the analog input range of the microcontroller is 0-5v.

I am restricted to do this by using voltage divider and op amp. Here scaling down of voltage is done, is it possible to have the gain of op amp less than unity.

I am an absolute beginner so pls provide me a circuit and explain what is the role of op amp (since scale down of voltage can be achieved by voltage divider itself).
 

hp1729

Joined Nov 23, 2015
2,304
My requirement is to measure DC voltage in range 20 to 80 using a microcontroller.

Its is stated in the microcontroller datasheet that "The maximum recommended impedance for analog sources is 2.5 kΩ." and the analog input range of the microcontroller is 0-5v.

I am restricted to do this by using voltage divider and op amp. Here scaling down of voltage is done, is it possible to have the gain of op amp less than unity.

I am an absolute beginner so pls provide me a circuit and explain what is the role of op amp (since scale down of voltage can be achieved by voltage divider itself).
Voltage divider first to bring the highest expected voltage down to 5 volts, then an op amp as a simple buffer (non-inverting input, output tied back to inverting input).
 

Papabravo

Joined Feb 24, 2006
12,681
To answer the original question. In the inverting configuration an opamp can have a gain less than one or greater than one. In the non-inverting configuration the gain must be greater than or equal to one. You need to pay careful attention when operating from a single supply what inversion means.
 

Sensacell

Joined Jun 19, 2012
2,504
I don't see why you would need an opamp?

If you make a simple 2 resistor voltage divider, set the lower R to 2.5 K ohms (satisfies your min source impedance) and choose the upper resistor to scale the input appropriately.

If what you are measuring has a very high impedance, you might use much higher resistor values and buffer the voltage into your ADC input with a simple follower / buffer to satisfy the minimum signal source impedance, as Bordo suggests.
 

ErnieM

Joined Apr 24, 2011
8,007
Sensacell is mostly correct, though the source resistance for a voltage divider is actually with both resistors in parallel (which is the thevenin equivalent). If you can tolerate drawing the 1.6 ma from the 80 volts then an op amp is not required.

If you need draw less then you need to scale the resistors higher and use the op amp. The amp keeps the source resistance the micro sees down to near zero.
 
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