Hello,

I would like to construct a circuit that measures the differential voltage on the load of the circuit depicted below :
I think about something with an op-amp. Could please anyone help?

 

Use an instrumentation amplifier. You can get one in an integrated package or you can build your own with chicklets (cheap individual opamps).

 

Use an instrumentation amplifier. You can get one in an integrated package or you can build your own with chicklets (cheap individual opamps).

As i obeserve, instrumentation amplifiers mainly consist of 3 op-amps. Is there maybe anything with one op-amp?

 

As i obeserve, instrumentation amplifiers mainly consist of 3 op-amps. Is there maybe anything with one op-amp?

You can get one in an integrated package

For example:
FET-Input, Low Power Instrumentation Amplifier datasheet

 

Another possibility is an INA117 high common mode voltage rejection difference amp followed by a simple op amp of your choice:

https://www.ti.com/lit/ds/symlink/i...https%3A%2F%2Fwww.ti.com%2Flit%2Fgpn%2Fina117

 

Thank you for the proposals, but i need to self-design a circuit

 

You will need to look at an IA. There are lots of caveot to worry about. generally IA's measure very low voltage signa;.
Power supply, commn mode range, frequency response, isolation will get you.

Look at Analog Devices and TI.

 

You will need to look at an IA. There are lots of caveot to worry about. generally IA's measure very low voltage signa;.
Power supply, commn mode range, frequency response, isolation will get you.

Look at Analog Devices and TI.

As i mentioned i am not interested right now in choosing an Integrated Circuit but i would like to design one

 

As i mentioned i am not interested right now in choosing an Integrated Circuit but i would like to design one

Why?
You already said you could us an opamp, which is an integrated circuit.
What's the difference between that and an instrumentation amp integrated circuit?

You can design a differential amp using one op amp and four matched resistors, but it will have inferior performance compared to an instrumentation amp.

 

Thank you for the proposals, but i need to self-design a circuit

If you know what you want to do, why are you asking for help and suggestions.

 

Why?
You already said you could us an opamp, which is an integrated circuit.
What's the difference between that and an instrumentation amp integrated circuit?

You can design a differential amp using one op amp and four matched resistors, but it will have inferior performance compared to an instrumentation amp.

Ok, i understand what you mean and i will consider the solution of and instrumentation amp IC. When you say a differential amp using one op amp and four matched resistors you mean something like this? :
If yes, do you know how should i choose the resistors values?
Also in refer to instrumentational amplifier, i want one which is able to measure 0-4V load voltage , and can operate with 3.3 or 5V supply. Also, its output should not be over 3.3V

 

Ok, i understand what you mean and i will consider the solution of and instrumentation amp IC. When you say a differential amp using one op amp and four matched resistors you mean something like this? : View attachment 245799
If yes, do you know how should i choose the resistors values?
Also in refer to instrumentational amplifier, i want one which is able to measure 0-4V load voltage , and can operate with 3.3 or 5V supply. Also, its output should not be over 3.3V

If you want a difference amp all four resistors have the same values. If you want a difference amp with amplification, then Rf and Rg must have the same values and R1 and R2 must have the same values. Under these conditions the output is (Rf/R1)*(V2-V1).
The circuit will always be inferior to a true chip designed for that purpose because you must match all four resistors very accurately, at least 0.1% or better tolerance parts. These will be more expensive than just using an instrumentation amp. Not to mention will requires more parts.
Also notice the non inverting input impedance is R2+Rg and the inverting input impedance does not match it at just R1. The InAmp has built in followers with extremely high input impedance along with a single external resistor that sets the gain.

 

Ok, i understand what you mean and i will consider the solution of and instrumentation amp IC. When you say a differential amp using one op amp and four matched resistors you mean something like this? : View attachment 245799
If yes, do you know how should i choose the resistors values?
Also in refer to instrumentational amplifier, i want one which is able to measure 0-4V load voltage , and can operate with 3.3 or 5V supply. Also, its output should not be over 3.3V

There are plenty of InAmps on the market that can meet those requirements. Here is over 34,000 choices many that meet your requirements. You can refine the parametric search:

https://www.digikey.com/en/products...BcDaIJYDsDOAXATgVwLYFMEoEMU4B7BAAgKwAcQBdAXyA

 

You can't get the performance of an IC IA with discrete parts because performace depends on matching/ratios and temperature. Laser trimming really helps in this case.

 

Peter523 (TS) has not mentioned gain or CMRR requirements so we cannot tell whether the advantages of an integrated instrumentation amp is worth the additional cost, sourcing issues, etc.

@Peter523 what is your required accuracy?

 

If you want a difference amp all four resistors have the same values. If you want a difference amp with amplification, then Rf and Rg must have the same values and R1 and R2 must have the same values. Under these conditions the output is (Rf/R1)*(V2-V1).
The circuit will always be inferior to a true chip designed for that purpose because you must match all four resistors very accurately, at least 0.1% or better tolerance parts. These will be more expensive than just using an instrumentation amp. Not to mention will requires more parts.
Also notice the non inverting input impedance is R2+Rg and the inverting input impedance does not match it at just R1. The InAmp has built in followers with extremely high input impedance along with a single external resistor that sets the gain.

Thank you, with a quick simulation with these values it seems to work. But, of course you are right the circuit is inferion to an IC for this purpose so i may search and select one of this kind.

 

Peter523 (TS) has not mentioned gain or CMRR requirements so we cannot tell whether the advantages of an integrated instrumentation amp is worth the additional cost, sourcing issues, etc.

@Peter523 what is your required accuracy?

My required accuracy is about 3% , for example if the real value is 5V the measured output could be approximately 4.9V or 5.1V

 

Thank you for the proposals, but i need to self-design a circuit

Hi Peter,
This suggests a College assignment, so Thread Moved to Homework.

E

 

Hello again,
finally i will follow the solution with the instrumentation amplifier. But as i know each IC has limits in regard to the range of differential voltage it can measure. For example, can i use the same instr amp IC to measure the differential voltage on different loads (the one with 12V on it, the other with 5V on it etc.)? I found this IC and it refers in page 2(at maximum ratings) :
Could anyone explain please?
Thank you.

 

It means that input signals can be in the range of 0.5 volts below ground to 0.5 volts above Vcc. This suggests clamping diodes on the input terminals which can dissipate some current but not a great deal as the diodes are probably "small" in terms of die area.

It would be handy to know what limits there are on the supply pins. Fragments of a datasheet can cause one to jump to unsubstantiated conclusions.