# What is common mode voltage exactly in current sense amplifier ? how to select a current sense amplifier based on common mode voltage?

I understood the average difference between the input and output voltages is called common voltage .But still i have a doubt when it comes to current sense amplifier. i a m confused like is it the voltage drop across the shunt resistor .which we will feed as input to the current sense amplifier.

i am using 36V 2500 mAh battery pack its maximum voltage is 42 v when the battery is fully charged and its minimum 30V when the battery is fully discharged.

Iam using a 6.6 mohm shunt resistor it will give maximum voltage drop of 100 mv for max 15 Amps of current . is this voltage drop is called as common mode voltage.

The current sense amplifier iam plannin to use is AD8207 whose input supply is 3.3v . so when i search in google about choosing the current sense amplifier i saw that when we use High side bidirectional current sensing amplifier the common mode voltage should be equal to the bus voltage Why. and the AD8207 current sense amplifier whose common mode voltage is equal to my nominal battery to load bus voltage that is 36v but it is not equal to my maximum bus voltage 42 v . so can i use AD8205 as current sensing amplifier in my circuit.

What is bus voltage is it the nominal voltage of Battery pack given to the load or maximum voltage of battery pack given to the load.

So according to my understanding the common mode voltage is the voltage drop between the shunt resistor and it is extremely low compared to the bus voltage and why we need a current sense amplifier whose common mode voltage is equal to the bus voltage .

Battery pack nominal voltage : 36v maximum discharge voltage : 42v minimum discharge voltage : 30v Nominal current : 2.5 A maximum current that can be used in the bus (between battery to load): 15 A shunt resistor value based on 100 mv dropout voltage : 6.6 mohm

What is exactly a common mode voltage? and based on my bus voltage and my common mode voltage.Can i use AD8207 in my application.

based on above battery to load specification can i use AD807 current sensing amplifier with 3.3 v of input voltage and 20 V/V of gain.

What is exactly a common mode voltage?

In your situation it is helped if we put numbers on your diagram: -

The common mode voltage when 10 amps are flowing through the 6.6 milli ohm resistor is: -

$$\dfrac{36 + 35.934}{2} = 35.967 \text{ volts}$$

I understood the average difference between the input and output voltages is called common voltage

No, it's the average voltage of both inputs - it's got nothing to do with "average difference" - that term makes no sense.

when we use High side bidirectional current sensing amplifier the common mode voltage should be equal to the bus voltage

No, the specified common-mode input voltage should be greater than the maximum possible bus voltage (and preferably, by some margin).

So according to my understanding the common mode voltage is the voltage drop between the shunt resistor and it is extremely low compared to the bus voltage and why we need a current sense amplifier whose common mode voltage is equal to the bus voltage

No, it's the voltage that is common to both inputs with respect to ground or 0 volts.

• Yes Sir Thank you so much Apr 21, 2020 at 23:02

For an amplifier, the input common mode voltage range is the range of inputs across which the amplifier will operate as advertised in the datasheet.

It is also important not to exceed the absolute maximum ratings.

Exceeding the common mode range means that the amplifier will not meet it's datasheet specifications and can have very odd effects such as output phase reversal (common in older JFET input amplifiers).

Some devices specify what will happen when operated outside the common mode range, but most do not; for a high side current sense amplifier that is powered from the same supply as being sensed requires a common mode range that goes to the positive power rail.

The AD8207 is specifically designed for this type of application; the common mode range exceeds the power rails (both positive and negative) by a significant margin, so you can use this device in your application if you power it from 5V; the maximum common mode continuous when powered from 3.3V is only 35V for proper operation which is lower than your application is sensing.

Trying to use this device to sense beyond 35V when powered from 3.3V will not damage the device (according to the absolute maximum ratings) but the operation of the part is not guaranteed.

• Thank you so much I understood. But i have one more doubt can i able to give 5v supply and reference voltage also using my battery monitoring IC . electronics.stackexchange.com/questions/492749/… Apr 13, 2020 at 10:33

Your understanding is backwards. The common mode voltage is the average of the two inputs. The differential voltage is the difference between them. If you read Peter's detailed answer you will see that it may not be a good idea to use this amp unless you are able to power it from 5V instead of the 3V you show in your sketch. Also I want to caution you that 15 Amps will cause a 6.6mOhm shunt to dissipate around 1.5W. So if you will have 15 Amps, you may want to choose a slightly smaller shunt resistor, like 4mOhm or 2mOhm.