Need help interpretting OpAmp spec, supply voltage

In summary, the datasheet for the OPA627 opamp states a maximum voltage of +/-18V, which can be used for a single supply of 36V. However, in order to properly measure the voltage on a motor using an ADC, a mid-rail reference voltage is needed, which cannot be achieved with a single supply and ground as a signal reference. Therefore, an alternative approach must be taken.
  • #1
DragonPetter
830
1
In the datasheet it says the voltage max is +/-18V.
Does this mean that the maximum on Vs+ is 18V, or that the maximum voltage between Vs+ and Vs- can be 18V, or 36V?

I want to power it with Vs+ at 36V and Vs- at 0 volts.The datasheet is here: http://focus.ti.com/lit/ds/symlink/opa627.pdf
 
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  • #2
DragonPetter said:
In the datasheet it says the voltage max is +/-18V.
Does this mean that the maximum on Vs+ is 18V, or that the maximum voltage between Vs+ and Vs- can be 18V, or 36V?

I want to power it with Vs+ at 36V and Vs- at 0 volts.


The datasheet is here: http://focus.ti.com/lit/ds/symlink/opa627.pdf

Yes, it can be powered with a +18V and -18V supply, or your 36V single-ended power supply. How are you going to make your 18V mid-rail reference voltage?
 
  • #3
berkeman said:
Yes, it can be powered with a +18V and -18V supply, or your 36V single-ended power supply. How are you going to make your 18V mid-rail reference voltage?

What do you mean by mid-rail reference voltage? I'm a little confused and worried now.

I attached a drawing of what I plan to do.
attachment.php?attachmentid=29951&stc=1&d=1290110264.jpg
I want to measure the voltage on a motor with an ADC in a microcontroller. The motor voltage ranges from 12V to 36V, and the ADC range is 3.3V to 0V. So I need to voltage divide the motor voltage before it goes to the ADC, but I only care about measuring 12V and up, so I use a zener to use my gain only from 10V to 36V to increase my resolution. I will use a more accurate zener than bzx84C family since the opamp currents might move around the zener knee, but I use it here just as the idea.

So I am hoping: Vout = 0.0091(Vmotor-10V)
 

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  • #4
Also, I'm changing the 10k and 910 to 100k and 12.5k so that I don't unbias the zener and give the proper gain.
 
Last edited:
  • #5
DragonPetter said:
What do you mean by mid-rail reference voltage? I'm a little confused and worried now.

I attached a drawing of what I plan to do.


attachment.php?attachmentid=29951&stc=1&d=1290110264.jpg



I want to measure the voltage on a motor with an ADC in a microcontroller. The motor voltage ranges from 12V to 36V, and the ADC range is 3.3V to 0V. So I need to voltage divide the motor voltage before it goes to the ADC, but I only care about measuring 12V and up, so I use a zener to use my gain only from 10V to 36V to increase my resolution. I will use a more accurate zener than bzx84C family since the opamp currents might move around the zener knee, but I use it here just as the idea.

So I am hoping: Vout = 0.0091(Vmotor-10V)

Won't work. That opamp (as with many opamps) is designed to work with split supplies (or a single supply where the inputs and outputs are running mid-rail somehow, like with a mid-supply reference voltage).

Take a look at the input and output voltage specs in the table in the datasheet. With a +/-15V supply, the input and outputs can go to about +/-11V. That means that you need to keep the inputs and outputs at least 4V or so away from the rails. You cannot run this part (or many opamps) off of a single supply and use ground as a signal reference as shown in your schematic.
 
  • #6
berkeman said:
Won't work. That opamp (as with many opamps) is designed to work with split supplies (or a single supply where the inputs and outputs are running mid-rail somehow, like with a mid-supply reference voltage).

Take a look at the input and output voltage specs in the table in the datasheet. With a +/-15V supply, the input and outputs can go to about +/-11V. That means that you need to keep the inputs and outputs at least 4V or so away from the rails. You cannot run this part (or many opamps) off of a single supply and use ground as a signal reference as shown in your schematic.

Ok, thank you very much Berkeman, Iæm glad I asked. Back to the drawing boards for me, but I will just try to refine this attempt. I'll show my new idea after some more work.
 

1. What is the supply voltage for an OpAmp?

The supply voltage for an OpAmp refers to the range of voltage that is required to power the OpAmp and allow it to function properly. This voltage range is typically specified in the OpAmp's datasheet and can vary depending on the specific model.

2. How do I interpret the supply voltage specifications for an OpAmp?

The supply voltage specifications for an OpAmp can be interpreted by looking at the minimum and maximum values listed. The OpAmp will only function properly if the supply voltage falls within this range. It is important to ensure that the supply voltage is not exceeded as it can damage the OpAmp.

3. What happens if the supply voltage is outside of the specified range?

If the supply voltage is outside of the specified range, the OpAmp may not function properly or may be damaged. This can result in inaccurate or unpredictable output signals.

4. Can I use a different supply voltage than what is specified for the OpAmp?

It is not recommended to use a different supply voltage than what is specified for the OpAmp. This can affect the performance and accuracy of the OpAmp and may result in damage to the device.

5. How do I determine the appropriate supply voltage for my OpAmp circuit?

The appropriate supply voltage for an OpAmp circuit can be determined by consulting the OpAmp's datasheet and selecting a voltage within the specified range. It is also important to consider the requirements of the other components in the circuit and ensure that the supply voltage is compatible with their specifications as well.

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