Selecting current sensing chips for PWM applications

Thread Starter

Gibson486

Joined Jul 20, 2012
313
I usually use an INA139 to measure current. Since it is for a PWM application, I was going to simply filter the output. Simple enough.

However, I also found the INA240 just from reading literature from TI. Aside from having built in gain and a huge common mode voltage spec, it has what is called Enhanced PWM rejection. It looks it has to do with having a step response at the extreme voltage swings, which causes a violation (or close to) of the common mode voltage because the voltage swing over time is just too big. Looking over the graphs, it seems that it is still effected, but it corrects the issue faster than say the INA181, which is about a fraction of the cost. However, it never fully corrects it. Am I thinking about this right?

It seems that the INA139 is still a good chip to use for my application (pulsing a high current LED @ 12V) and that the INA240 (or INA181) is more for space constrained applications that cannot afford the space for a filter and gain resistor or maybe they just need that 80V common mode spec (in the case of the INA240)?

Aside from that, the cost break down is kind of weird...

INA240 is the most expensive (not by much)
INA139 is in the middle
INA181 is the cheapest by a landslide

I find it sort of weird that the INA181 is cheaper than the INA139. Then again, I have learned that lots of this has to do with the fab process and what each vendor wants to try to push the industry to use. Or perhaps I am missing some detail that makes the INA181 unfavorable (aside from the internal gain, which some people may not like and the lower common mode spec).
 

TeeKay6

Joined Apr 20, 2019
377
I usually use an INA139 to measure current. Since it is for a PWM application, I was going to simply filter the output. Simple enough.

However, I also found the INA240 just from reading literature from TI. Aside from having built in gain and a huge common mode voltage spec, it has what is called Enhanced PWM rejection. It looks it has to do with having a step response at the extreme voltage swings, which causes a violation (or close to) of the common mode voltage because the voltage swing over time is just too big. Looking over the graphs, it seems that it is still effected, but it corrects the issue faster than say the INA181, which is about a fraction of the cost. However, it never fully corrects it. Am I thinking about this right?

It seems that the INA139 is still a good chip to use for my application (pulsing a high current LED @ 12V) and that the INA240 (or INA181) is more for space constrained applications that cannot afford the space for a filter and gain resistor or maybe they just need that 80V common mode spec (in the case of the INA240)?

Aside from that, the cost break down is kind of weird...

INA240 is the most expensive (not by much)
INA139 is in the middle
INA181 is the cheapest by a landslide

I find it sort of weird that the INA181 is cheaper than the INA139. Then again, I have learned that lots of this has to do with the fab process and what each vendor wants to try to push the industry to use. Or perhaps I am missing some detail that makes the INA181 unfavorable (aside from the internal gain, which some people may not like and the lower common mode spec).
Ignoring for the moment your actual question regarding "which is best", just what are you trying to do that requires an INA? A schematic would really help.
 

MaxHeadRoom

Joined Jul 18, 2013
18,995
In a PWM application that uses Mosfet drivers for e.g. A very low resistance is often used in the source connections of the mosfets and volt drop input to an op-amp used to monitor it.
There is also Mosfets with built in current sense outputs.
Max..
 

TeeKay6

Joined Apr 20, 2019
377
In a PWM application that uses Mosfet drivers for e.g. A very low resistance is often used in the source connections of the mosfets and volt drop input to an op-amp used to monitor it.
There is also Mosfets with built in current sense outputs.
Max..
I am acquainted with various current monitoring techniques but the use of an INA rather than a simpler opamp makes me curious what is being done (i.e. schematic).
 

Thread Starter

Gibson486

Joined Jul 20, 2012
313
I am not asking which is the best, I am just asking what application you would use this in. I look at the INA240 and say, well maybe I can use a filter or maybe I am misunderstanding something.

As for just using an standard op amp, sure, you can make that argument with lots of applications. The INA139 is a pretty standard part used to measure current. It is one chip and a resistor. Yes, it cannot go rail to rail, but I really do not need it too and there are other INA parts that can do it. If I use a standard op amp, then I need the op amp and and 3 or 4 resistors. Not a big deal, but if I have a circuit that works well and already has data to back up how it works, it makes the design process/reviews easier and quicker. If I were in a industry that made millions of something, then I would actually think about your method. However, I work in an industry where adding $2 to a BOM will never be noticed.
 

TeeKay6

Joined Apr 20, 2019
377
I am not asking which is the best, I am just asking what application you would use this in. I look at the INA240 and say, well maybe I can use a filter or maybe I am misunderstanding something.

As for just using an standard op amp, sure, you can make that argument with lots of applications. The INA139 is a pretty standard part used to measure current. It is one chip and a resistor. Yes, it cannot go rail to rail, but I really do not need it too and there are other INA parts that can do it. If I use a standard op amp, then I need the op amp and and 3 or 4 resistors. Not a big deal, but if I have a circuit that works well and already has data to back up how it works, it makes the design process/reviews easier and quicker. If I were in a industry that made millions of something, then I would actually think about your method. However, I work in an industry where adding $2 to a BOM will never be noticed.
@Gibson486
As I read your first post, it clearly indicates that you are trying to make a choice among various INAxxx parts. You also clearly bring in the issue of cost that you now say is irrelevant. Well, so what--it is certainly okay for you to change your mind.

I have proposed no solution! I merely asked for more detail on how and why you would use an instrumentation amplifier. I have since viewed the INA139 datasheet and see that it is preconfigured for use as a high-side current monitor (although it may perhaps also be used as a general purpose INA) and I assume you intend to use it as a high-side current monitor. In your most recent post (to which I am replying), you seem happy with the INA139 and are not interested in making a choice. Since I know nothing more of your application, I bid you farewell.
 
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