OpAmp Specs (INA126)

[ADWSystems]

It's funny how you can think you understand something and then read something else and be clueless. I'm looking at the INA126 Instrumentation OpAmp (https://www.electro-tech-online.com/custompdfs/2012/01/ina126.pdf). The power supply range is +/- 1.35 to +/-18V. The Input Common Mode Voltage Range is listed at +/- 11.5 with Vs=+/-15V. I am interpreting the Input Common Mode Voltage Range as V- +2.5 to V+ -2.5. Is that correct?

If so, then you lose the top and bottom 2.5V, can you power it from +/-1.35V?

 

[ericgibbs]

You should consider what the result of having say an Vin CMV of 0v, when the power supplies are +/-1.35V.????

Do you know what CMV means with regard to an OPA or IA.??

EDIT:
LTspice sim of the 1NA126 with +/-1.35V supplies and a CMV ~0mV & a 50mVpk 100Hz Sine input

 

[ronsimpson]

Page 4 bottom right picture; shows input range & output range at +/-5V supply.
Page 4 bottom left picture; shows input & output range at +/-15V supply.

There is a input range that is independent on output (2.5v from supplies).
It appears the input range depends on the output at that moment in time. If you will only use the output in the range of 0 to 1 volt then the input range is (fill in the blank).

 

[ADWSystems]

You should consider what the result of having say an Vin CMV of 0v, when the power supplies are +/-1.35V.????

Without taking into consideration input common mode voltage range, I would expect the output to be 0. Using the Vi CM, then I would say that you can't run the chip with Vs=+/- 1.35V. The minimum difference between V- and V+ would need to be 5V and at than minimum would allow 0V of input range. I don't think that alligns with the insert portion of the Page 4 lower right graph.

Do you know what CMV means with regard to an OPA or IA.??

I thought I did. I thought it was the voltage range for the inputs. Looking at an LM358 is 0 to V+ -1.5. The common mode input range is smaller than the allowable supply voltage range. So that made sense.

 

[ericgibbs]

Look at my edit in post #2.

CMVin , means the voltage on the two input pins of the OPA, which is Common to both pins,,, the Vin signal input is superimposed on the CMV.

So you should consider the case where the CMV is 0V, with the Vin signal on one pin swinging above and below 0V by a few tens of mV.


Also as Ron says in his post, check out the d/s images on page #4.

 

[ADWSystems]

Not trying to be difficult, but I'm not following you. I see the edit in post 2, but can't quite understand how it's working.
I see the charts on pg4, but can't wrap my head to adjust the charts base on a change in Vs instead of Vo.

In the circuit in pg4 lower left. I see the Vcm, and I also see V d/2. What is V d/2?

 

[ericgibbs]

Not trying to be difficult, but I'm not following you. I see the edit in post 2, but can't quite understand how it's working.
I see the charts on pg4, but can't wrap my head to adjust the charts base on a change in Vs instead of Vo.


In the circuit in pg4 lower left. I see the Vcm, and I also see V d/2. What is V d/2?

hi,
No problem.
Look at page #8 of the d/s, it explains the CMV formula at low supply rails.
[near the bottom of the page]

 

[ADWSystems]

With Rg=10k, then the low input limit is -(Vin+)?

Lots of talk in the d/s but it's just not sinking in.

Besides freshing and increasing my understanding, the circuit I'm after is to display an amperage on a voltmeter. I've been asked to modify a existing system to display an amp reading on a digital voltmeter display. The display currently has a two position rotary switch to select between two voltage points. The first change will be to change the two position switch to a three poisiton. Then remove one resistor from the circuit and add two; one precision and one standard but still adding to the original value. Across the precision resistor I was going to connect a diff amp with the right gain to display the voltage difference with the correct gain to display the "amperage" on the meter. In this case the voltage difference would be at most 1V.

But as I type this, if I can add the on the return line (off appropriate wattage), then all I need to do is read the voltage between the load and the resistor; as the other side of the resistor will be ground. In this case the maximum voltage reading (input to Vin+ would be 1V, with Vin- connected to ground).

Something tells me neither option will work with the INA126, but I can't exactly put my finger on why.

 

[ericgibbs]

hi,
If I follow correctly, you are/were considering a High side current shunt, using IA across the shunt to measure the current.
The problem as you have seen is the CMV for the IA, it will not work at/close to the supply voltages of the IA.

If you post a sketch of the IA and shunt & switching, I could check it out in simulation.

 

[ADWSystems]

You can ignore the switch, it just selects which point to display. I'm not sure shunt is the right word. I use shunt when it is in parallel, in this case the current sense resistor is in series with the load. The load is known to be 0-250mA. the choices I have is (a) between the source and the load, or (b) between the load and ground. If a 1 ohm resistor is in series with the load the differential voltage will be 0-250mV. For (a), Vin+=5V and Vin- would vary (4.75-5). For (b) Vin-=0 and Vin+ would vary (0-250mV). If a 0.1 ohm resistor is used then the voltages also drop by 10. For either configuration (a) or (b) could the INA126 detect and work with it?

What datahseet parameter defines and OpAmp as having a rail-to-rail input versus not?

 

[ericgibbs]

A current shunt resistor is normally in series with the load current.
It can be in the High or Low side of the current path,.
For High side there are OPA's that will accept a CMV equal to the OPA power supply.

If you plan to use a ready built LCD voltage indicator as a Current indicator, take care with regards to the supply requirement of the LCD module, if you choose the High side method.

The voltage drop across the Rshunt is used as an indication of the current flowing thru the shunt, the shunt is normally a precise value and must be rated to carry the full load current, self heating of the shunt due to the load current should be minimal.

With regards to dual or single supply operation, it is normally stated in the d/s for the device.

 

[ADWSystems]

For High side there are OPA's that will accept a CMV equal to the OPA power supply.

Oops. Lost me again.

If you plan to use a ready built LCD voltage indicator as a Current indicator, take care with regards to the supply requirement of the LCD module, if you choose the High side method.

It is a premade (and currently installed) display. Why does it matter is it is high side or low side? The opamp will be "adjusting" the voltage and "isolating" the display from the voltage supply.

The voltage drop across the Rshunt is used as an indication of the current flowing thru the shunt, the shunt is normally a precise value and must be rated to carry the full load current, self heating of the shunt due to the load current should be minimal.

With regards to dual or single supply operation, it is normally stated in the d/s for the device.

Spec'ing Rshunt is the easy part.

What parameter in the datasheet makes an opamp rail-to-rail, or not?