Floating supply design for INA128.

Thread Starter

Cookieman10101

Joined Jul 18, 2019
18
Hey all,

I'm trying to use an INA128 and a sense resistor to measure current from a supply that is beyond the voltage rating of the the amp. I've come up with this circuit, but I haven't tested it yet.

Floating Supply.JPG

I need the op-amp supplies to follow the high voltage input because that input could be anywhere between 1v and 48v. I'm planning on converting VOUT to a digital signal (ADC) and opto-isolate it from an MCU for control of another circuit. I'm wondering if anybody can give any input on whether or not there is a better solution, or if this just wont work at all. Thanks!
 

MisterBill2

Joined Jan 23, 2018
8,723
The problems that will appear are several. When the V+ voltage drops the amplifier operation will change as it goes below the zener voltage. In addition, zener noise will be strong at the power input connection because f no filtering. A small supply with a decent isolation transformer and a rectifier/ regulator scheme can provide much better performance.
 

Thread Starter

Cookieman10101

Joined Jul 18, 2019
18
Are you suggesting to completely isolate the supplies from the inputs? Would the amp still work? I was reading somewhere that the inputs to an op-amp need to be in some way referenced to its supplies (I know this is technically an IN-amp). If isolation would work, I could probably substitute for a small isolated DC-DC converter and a linear regulator like this. I know the pins for this particular DC-DC converter don't quite match the symbol, bear with me.

Floating supply INA128.JPG
 

crutschow

Joined Mar 14, 2008
27,172
One problem is that VOUT will be floating at the V- input (the Ref input), and not referenced to ground.
So that voltage will change with the +48V which, I assume is not satisfactory.

I suggest you use a high-side current sense amplifier such as this.

If you want to roll your own, below is the LTspice simulation of an example circuit using a low power, rail-rail op amp:

1613669871324.png
 
Last edited:

AnalogKid

Joined Aug 1, 2013
9,248
Still won't work.

The common mode operating voltage range for that IC does not include the positive rail. That is, the input voltage on the non-inverting input must be at least 1.4 V (typ) *below* V+ to keep the amp within its linear operating range.

Bottom line - wrong part for the job. Maxim and Linear Tech male lotsa high-side current monitor chips, many with the shunt resistor built-in.

ak
 

Thread Starter

Cookieman10101

Joined Jul 18, 2019
18
Thanks guys, I knew there would be a better part out there for this, but I couldn't find it. High side current sense amplifier it is!
 

MisterBill2

Joined Jan 23, 2018
8,723
The circuit in post #3 will work, but certainly any isolated power source or supply of the correct voltage and adequately stable will work. You do need to look at the manufacturer's application literature to know what the inputs require in the way of reference common connections.
 

crutschow

Joined Mar 14, 2008
27,172
The circuit in post #3 will work, but certainly any isolated power source or supply of the correct voltage and adequately stable will work.
That doesn't solve the problem of the output voltage to the 48V ground being offset by the floating negative supply.

And the supply can't be fully floating,
There has to be a reference to the 48V ground.
 

MisterBill2

Joined Jan 23, 2018
8,723
Vout is,referenced to the local power supply common terminal. Like I mentioned before, the TS really needs to study the application notes for the device. The folks who created the amp know how to use it and get the best results.
 

Thread Starter

Cookieman10101

Joined Jul 18, 2019
18
Vout is,referenced to the local power supply common terminal. Like I mentioned before, the TS really needs to study the application notes for the device. The folks who created the amp know how to use it and get the best results.
I started reading the applications notes for the INA128 and INA828 and I haven't found exactly what you are talking about. Could an example be provided? If all else fails I could just test it.
 

MisterBill2

Joined Jan 23, 2018
8,723
I started reading the applications notes for the INA128 and INA828 and I haven't found exactly what you are talking about. Could an example be provided? If all else fails I could just test it.
Usually app notes cover the power supply considerations and the input common mode considerations. Also, they often address input protection methods and ways to handle voltages above what is usually found.
 
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