How to Sense Current Using Op-Amp: Low Side and High Side Circuits Explained

In this post we are going to learn one very interesting and also very much useful topic that is how we can measure the current which is going through any load by just using some normal resistors and one op amp circuit.

We know already that current is one very important thing in every electrical or electronic system because that current is telling us how much power is being used by the load or how much load is getting connected to the power.

But that current is not like voltage because voltage we can measure very easily just by putting two probes across two points but current we cannot measure like that directly.

So we have to use one smart trick for doing that current sensing.

That trick is using one small value resistor which is called shunt resistor and then we use one op amp circuit for amplifying the small voltage drop which is coming across that resistor.

Two types of current sensing methods

Now there are mainly two ways for doing this current sensing work. One method is called low side current sensing, and second method is called high side current sensing.

In the low side method we are putting the shunt resistor on the ground side of the load, that means between the load negative and the system ground.

But in high side method we are putting the shunt resistor on the positive side of the load, that means between the supply positive and the load positive.

Both the methods are correct and both are having their own good points and also some bad points. So depending on what type of circuit we are using and what result we want, we can decide which method we want to use.

What we will explain in this article

So now in this article we are going to explain both these two circuit methods one by one with full step by step detail.

We will see properly how the resistors are connected in the circuit, how the op amp is doing the amplification job and how we are getting some output voltage which is showing us the actual amount of current flowing through the load.

We will also understand what are the good things and what are the bad things in both methods and where we can use low side method and where we can use high side method.

So now let us begin the full explanation without skipping any step.

Understanding Low Side Current Sensing Circuit

So now we are looking at one simple op amp based low side current sensing circuit and we can see that in the image the load is connected at the top and the shunt resistor Rs is connected at the bottom of the load, between load and ground.

This is why we are calling this low side current sensing because the shunt resistor is at the ground side of the load.

Current Sensing with Shunt Resistor

First we see that the load is taking some current from the supply voltage and that current is passing through this very low value resistor Rs.

Here the value of Rs is written as 0.0005 ohm which is extremely small so that it does not drop much voltage and does not disturb the load voltage.

But still it creates a very small voltage drop that is directly proportional to the load current. So we can write this small voltage across Rs as

Vsense = Load Current × Rs. 

That means if current increases then this voltage also increases.

Using R1 and R3 to Send Voltage to Op Amp

But now this small voltage across Rs is very tiny so we cannot directly measure it with microcontroller or ADC or some meter. So we use an op amp to amplify this voltage.

Before giving the voltage to the op amp, we first pass it through two resistors R1 and R3. These two resistors are same value, both are 100 ohm.

So these are acting like input resistors or buffers, just for protection and to create a balanced input for the op amp.

Op Amp as Differential Amplifier

Now we see the op amp is connected in one special way which is called differential amplifier. This configuration helps us to amplify only the voltage difference between the two inputs.

So the small voltage across Rs is appearing between the two inputs of op amp, and then op amp gives output based on the difference of those voltages.

Role of R2 and R4

We see R2 and R4 are both 20k ohm. These are feedback and gain setting resistors. This ratio of R2 to R1 and R4 to R3 decides the gain of the op amp.

Because R1 = R3 and R2 = R4 so the gain becomes just R2 divided by R1 which is 20k divided by 100, that is 200.

So we can say the op amp will multiply the small voltage across Rs by 200 times and give that as the output voltage.

How Output Voltage Shows Load Current

So finally this output voltage of op amp is now a bigger voltage which is directly proportional to the load current. So we can measure this voltage and calculate the load current using formula:

Load Current = Vout / (Gain × Rs)

That means we take the output voltage, divide by the gain (200) and again divide by Rs (0.0005) and we get the current.

Why Low Side and Where to Use

This type of low side current sensing is easy and cheap because ground side is always at low voltage, so op amp does not need high common mode range.

But it has one problem also, that now the load is not directly connected to ground, but through Rs. So we cannot use this in some sensitive ground referencing applications.

Conclusion

So this is one simple but very useful current sensing method using op amp.

We take voltage across low value shunt resistor then we amplify that small voltage using differential amplifier made by op amp and finally we get the output voltage which tells us how much current is flowing through the load.

Understanding Op Amp High Side Current Sensing Circuit

Next we are seeing one different type of current sensing circuit which is called high side current sensing.

In this circuit we can see that the shunt resistor Rs is connected between the supply voltage and the load, not at the ground side like our previous method.

That is why we are calling this one high side sensing because the sensing is happening at the high voltage side of the load.

Current Sensing Through Rs Resistor

Now in this case, the load is receiving power through the resistor Rs.

The value of Rs is shown as 0.2 ohm which is still a low value but not as small as previous one.

When current flows from the supply to the load then this Rs develops a small voltage across it and this voltage becomes equal to Load Current multiplied by Rs.

So that voltage is the indicator of how much current is flowing through the load.

R1 and R2 Act as Voltage Dividers or Buffer

Now this voltage across Rs is applied to an op amp through two resistors R1 and R2. Both are having same value, 200 ohm.

These two resistors are used to balance the inputs and protect the op amp and also they form a simple differential voltage divider. So they help to give proper voltage difference to the op amp input terminals.

Op Amp Drives Transistor for Level Shifting

Now comes the main part, the op amp is not giving direct output to any load or ADC but it is driving a transistor.

That transistor is working like emitter follower or common collector amplifier. So the op amp output is going to the base of the transistor and the emitter of transistor is connected to ground through a resistor Re, which is 2k ohm here.

So what happens is, the op amp takes the differential voltage from its inputs and tries to adjust its output in such a way that the voltage difference across its inputs becomes zero.

While doing this, it also controls the base current of the transistor and that makes the transistor conduct. Because of this conduction, current flows through Re resistor and that creates an output voltage.

Output Voltage Represents Load Current

So the final output voltage appears across the emitter resistor Re. That voltage becomes the actual amplified version of the sensed voltage across Rs.

So this output voltage can be measured and we can calculate the current using the same formula:

Load Current = Vout / (Gain × Rs)

But here the gain is controlled by the transistor and Re, so the actual formula becomes a bit different but idea remains same. We can say that the output voltage is directly proportional to the current flowing through Rs and the load.

Why High Side and When to Use

Now one big advantage of high side sensing is that the load is still connected directly to ground so there is no disturbance in grounding or voltage referencing.

So this circuit is useful for applications where accurate ground is important or where we do not want to lift the ground path.

But the problem with high side sensing is that the voltage at Rs is near supply voltage which can be high. Hence the op amp must have high common mode range to sense that voltage properly.

Also we need to use level shifting using transistor so that the output can come down to lower range.

Conclusion

So this high side current sensing circuit is very useful when we want to measure current without disturbing the ground connection of load.

It uses Rs resistor to create a small voltage then uses op amp to compare and amplify that voltage and finally uses transistor to bring the output voltage down to measurable level.

So this is another effective method to sense current in power supply or battery applications where ground referencing is critical.