# Operational amplifier INA114 help

Discussion in 'The Projects Forum' started by ayane, Oct 5, 2009.

1. ### ayane Thread Starter Active Member

May 3, 2009
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0
hi, i am new to operational amplifier. I want to know whether this chip is able to sense an input current of 1.3A?.

The specs state power supply input current is 2ma.

If it is not able, any recommendations on any operational amplifier, i have scouted farnell and rs but could not seem find any at the moment.

2. ### t06afre AAC Fanatic!

May 11, 2009
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NO! Your chip can not sense 1.3A. But it can very well measure a small voltage drop over a (small) resistor. Then measuring current you "break" the circuit, and insert a small resistor that do not cause to much measurement error. Then you measure the voltage over this resistor. For a 1.3A current the resistor may be a low as 0.1Ohm, or less.

3. ### ayane Thread Starter Active Member

May 3, 2009
39
0
thanks..i had attached my circuit. The circuit supply is from a solar panel that i had constructed.

my Vin supply has a range of 0.2A to 1.3A ( max)..

i set the gain of the instrumentation amplifier to 126. by adding Rg= 400ohms.

I set the R sense to 0.1 ohms. Actually i want to sense a 0.6A current from the supply. the voltage across the R sense resistor will be 0.6 X 0.1 = 0.06V.

The vout of my instrumentation amplifier will be 7.56V. The input supply to the amplifier from my voltage regulator is approx 12.7V.

The vout of my instrumentation amplifier is feed in to the comparator LM393 at the inverting input. For the non-inverting input, the reference is set by 10kohm/ ( 10kohm +10 kohm) = 1/2

Than 1/2 * Vin = 1/2 * 12.7v= 6.35V.

So when the Vout of my instrumentation amplifier is higher than 6.35V , the led will light up..

I use a 1Kohm resistor to limit the current to the led.

Will it work?

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4. ### t06afre AAC Fanatic!

May 11, 2009
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I see some problem. I do not think the INA114 like a input voltage higher than the supply voltage. Also I do not think the INA 114 is made for single supply use. All in all as your circuit is now, I do not think it will work. But your plan is not bad. Most of your thinking is almost correct.
Last note. Then using single supply opamps. The negative input rail can not have a decoupling cap. Tie it directly to ground

5. ### beenthere Retired Moderator

Apr 20, 2004
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The INA114 is a dual supply instrumentation amp. Unless you have one, it may be hard to acquire right now. TI shows all the INA and OPA (Burr-Brown) line of amps as active, but with a 20 week lead time.

As a dodge, can you place the sensing resistor after the load with one end to ground? That eliminates the common-mode problem and allows you to sense the current produced voltage across the resistor with a garden variety op amp. That way, you see the actual current in the load.

6. ### ayane Thread Starter Active Member

May 3, 2009
39
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sorry for my newbie questions. I thought the input voltage to the IAmplifier is 0.06V. Although this will not be constant.

k sure i will remove the capacitor..

Any other intrumentation amplifiers suggestions to replace this 1? thanks..

7. ### ayane Thread Starter Active Member

May 3, 2009
39
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can i use a AD623 single supply instrumentation amplifier?

8. ### t06afre AAC Fanatic!

May 11, 2009
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Your questions was a typical newbie, but not bad or dumb so you shall not be ashamed about your questions. The INA114 is a instrumentation amplifier. It amplifies the product V+ minus V-. This product is equal to 0.06 volt.
But V+ and V- are almost equal to the Vin voltage

9. ### ayane Thread Starter Active Member

May 3, 2009
39
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hmm, i still do not get it for why V+ and V- is almost equal to Vin?

My V+ is 12.7V..If i switched the amplifier to AD623 or AD8223 which is a single instrumentaion amp. will it solved the problem?..

To beenthere:

Hi actually, i want to sense whether the solar panel is producing 0.6A to load ( which is my laptop)..

Acutally i connecting this circuit to laptop so that it can work parallel with the battery. Hence prolonging the usage time of the laptop.

I set the voltage to about 12.7v because current move from high voltage to low voltage hence i do not want to charge the battery. ( or else i can follow the laptop adaptor and set it 18.5V

. My laptop battery is 10.8V but with full charge it is 12.6V.

10. ### t06afre AAC Fanatic!

May 11, 2009
5,939
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That is not so trange because they are measured in respect to ground. The Ad623 can not be used for supply voltages larger than +/- 6 volt or 12 volt(single supply)

11. ### MikeML AAC Fanatic!

Oct 2, 2009
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Look up the data sheet for a ZXCT1009. It is a much simpler/cheaper way of sensing current in a high-side current shunt resistor and converting it to a ground-referenced voltage reading.

12. ### Ron H AAC Fanatic!

Apr 14, 2005
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Do a search for "high side current sense amplifier". They might be easier to use than instrumentation amplifiers.

EDIT: I missed Mike's post. There are several manufacturers of high side current sense amplifiers.

13. ### tibbles Active Member

Jun 27, 2008
249
3
hi all,
i think ayane is showing how it should be done, he's spent some time researching his project and explained his aims clearly.
just a point, ive been looking at a laptop power board schematic on a repair,and there is at least a couple of diodes and a sense resistor in the laptop power circuit, coupled with the voltage drops on your circuit,
i reckon about 4 volts will be lost.

14. ### ayane Thread Starter Active Member

May 3, 2009
39
0
hi, thanks i realised the problem, the input voltage is higher than the supply voltage. It cant be used for a single supply.

Even if i add a voltage divider to lower the input voltage the error will be very large. Resistor have tolerance even if 1 percent.

The current sensor are only able to sense a very small amount for the ZXCT1009 max nA.

Maybe if theres any current sensor that can sense around 0.2A to 0.3A?

15. ### ayane Thread Starter Active Member

May 3, 2009
39
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i did some research. Even if i change the amplifier to ADM4073 a high side current sense amplifier ..will the problem where by the voltage input exceed the supply voltage?.i felt it will..correct me if i am wrong..

i notice it able to sense a voltage drop of typically 150mv which is 0.15V across the shunt resistor.

in my case, the maximum drop will be 0.13V..not very sure the way i think is correct...

16. ### tibbles Active Member

Jun 27, 2008
249
3
i know im gonna get shot down here, but couldnt he just put a small voltmeter across the sense resistor.
d

17. ### beenthere Retired Moderator

Apr 20, 2004
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Or an ammeter?

18. ### ayane Thread Starter Active Member

May 3, 2009
39
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hmm it seems like a good idea.never think of that.ya but i doing as a project that has to be submitted..

Acutally, i thought of using hall effect current sensor just that the wide range of dc current is quite high. 0-50 plus A..

Which is found from farnell.

19. ### Ron H AAC Fanatic!

Apr 14, 2005
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657
You don't understand the datasheet. The output scale factor is 1mA out per 100mV in. For example, if you have a current sense resistor of 0.1Ω with 1A through it, you will have 100mV across it. If you then connect a 1k resistor from Iout to ground, you will get 1mA*1k=1V on the output, i.e., you get 1V on the output for each amp through the sense resistor.
Other manufacturers have similar parts. I suggested previously that you search for "high side current sense amplifier".

20. ### MikeML AAC Fanatic!

Oct 2, 2009
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You misread the ZXCT1009 app note. It can be used to sense currents from mA to 1000's of A. I am using it with a 1mOhm shunt to measure 50A.

You need to reread the app note. Pay attention to the formula for Iout vs the voltage drop across the shunt.