Hello Guys,
I have some doubts on High Side current measurement. It would be a great help if some expert from this forum can help me to understand. My fundamental is really missing but trying to deep dive in it.

Q1. How the high-side current sensing can detect a load short to ground condition while not in low side measurement. This is not clear to me.

Q2. How common mode voltage of opamp impact the high side current measurement. As per my knowlege common mode voltage of opamp is Vcm = (V1+V2)/2 .




Q3. How the common mode voltage affect the output of below circuit ?
Q4. How to calculate the common mode voltage of this circuit ?
Q5. How to design below circuit ?
Q6. Is common mode voltage lies in +(non inverting ) of opamp ?



Q6. Vo = Ad*Vd+Acm*Vcm (opamp output)
In Above example how VCM value is going to affect the opamp output if we have higher CMRR ??


Thanks in Advance !

 

There are amplifiers designed for exactly that application. To protect the amp from excess differential overvoltage it is common to use series resistors at each input to limit the current to what the internal protection can handle.

 

How the high-side current sensing can detect a load short to ground condition while not in low side measurement. This is not clear to me.

The two circuits you show will both detect a short to ground.
Why do you think the low side doesn't?

Q2. How common mode voltage of opamp impact the high side current measurement. As per my knowlege common mode voltage of opamp is Vcm = (V1+V2)/2 .

The CM voltage must be below the input rating for the op amp.

Q4. How to calculate the common mode voltage of this circuit ?

Determined by the resistor values.
It will be somewhere between ground and the supply voltage.

Q5. How to design below circuit ?

Can't design it without knowing the design requirements, which you have not given.

Q6. Is common mode voltage lies in +(non inverting ) of opamp ?

Question makes no sense.

 

For a high side shunt resistance arrangement the common mode voltage for the op-amp is the supply positive voltage.

 

For a high side shunt resistance arrangement the common mode voltage for the op-amp is the supply positive voltage.

Depends upon the circuit configuration.
For a differential configuration it is less than the shunt voltage.

 

Depends upon the circuit configuration.
For a differential configuration it is less than the shunt voltage.

Consider that the exact value of the common mode voltage, which is defined as the voltage between the two inputs and the amplifier "common" power connection, except where that amplifier has a separate "common mode reference" connection, is close to the supply voltage, because normally the voltage drop across the "100 millivolt current shunt" resistance is very small relative to the supply voltage.
IN a 24 volt, high-side shunt installation, the common mode voltage would be about 23.9 volts. The difference between the two inputs would be 100 millivolts.
So how in the world would the common mode voltage be less than the input voltage across the two inputs???

 

hi mish,
The LM324 is able to operate as a High side current sensor and voltage output.
Example asc.
E

 

So how in the world would the common mode voltage be less than the input voltage across the two inputs???

In a world where the differential circuit looks like the one below:::
For example, if R2 through R5 are equal value, then U1's common-mode input voltage is 1/2 Vcc.

 

The LM324 is able to operate as a High side current sensor and voltage output.

Ah, an example of the dangers of simulation.
It may work with your LM324 model, but that exceeds the common-mode input voltage range (below from the data sheet):

5. The input common mode voltage or either input signal voltage should not be allowed to go negative by more than 0.3 V. The upper end of the common mode voltage range is VCC −1.7 V, but either or both inputs can go to +32 V without damage, independent of the magnitude of VC.

My LM324 model gives 0V output:
If I use a RR input model op amp then I get your result (bottom below).

 

Hi,
That sim circuit is from a number of previous thread posters who have actually built the circuit, and it works as expected.
E

 

Hi,
That sim circuit is from a number of previous thread posters who have actually built the circuit, and it works as expected.
E

So it works even though the common-mode voltage spec for the op amp is exceeded.
Interesting, but not recommended.
I'm surprised you do.

 

I have to side with Cruts on this one. Many years ago, I had my first experience with exceeding common mode range, building a high side current sense circuit. Using LM324, LM358.
We had sold over 100 circuits, and about 2/3 of them worked erratically or erroneously.
We had to replace all 100 devices at significant expense with RRIO opamps to work reliably.
An expensive lesson.

 

Hi sch,
I would agree, for a commercial highside device, an RRIO would be the way to go

But as you know, many TS's state that the LM324 is the OPA that they have and want to use.
E

 

But as you know, many TS's state that the LM324 is the OPA that they have and want to use.

Okay.
But I suspect some LM324s will work for that and some won't.

If you look at the LM324 circuit diagram below, you can see that many of the input stage PNP transistors have zero base-emitter bias when the inputs go to the positive rail.
Problematic at best, for proper op amp operation.

 

An amplifier with Rf=Ri is generally a unity gain amplifier, and if my shunt resistor is a very low resistance then the signal is very small and requires a fair amount of gain for what I would use it for. AND I would not consider an LM324 for use as an instrument amplifier. It is a handy single supply amplifier for many applications though.

 

Hi Bill,
This is what I posted in post #13.

I would agree, for a commercial highside device, an RRIO would be the way to go
But as you know, many TS's state that the LM324 is the OPA that they have and want to use.

Post #7, was posted an example.

E

 

Analog Devices have some op-amps that will work with the inputs at a higher voltage than the positive supply
https://www.analog.com/en/resources...ut-impedance-with-inputs-driven-apart-or.html

 

Indeed!
Originally from Linear Technologies.
They call them Over-the-Top opamps.
Ideal for high side current sensing without accuracy degradation.

 

An LM324 is an Under-the-Top op amp.

 

An LM324 is an Under-the-Top op amp.

It's below average in a lot of ways (apart from noise and distortion)