Op Amp Instrumentation Amplifier INA128, how to use

[dstockman]

Op Amp Instrumentation Amplifier INA128

I want to increase voltage coming out of a wind turbine/PV control box that ranges from 0 – 70 mV and feed it into a Phidget or Arduino board that requires 0 – 5 volts so I can record the wind and sun output. I am a total nubie to electronics, but am hoping someone can point me in the correct direction.

It appears a TI INA128 might do what I am hoping for. By varying the resistor between pins 1 and 8, I can choose the appropriate gain. Then I just need to power the device. Am I understanding this correctly? Is there a better Op Amp for this purpose?

If the INA128 is appropriate for this application, please help me understand the rest of the pins.

1 & 8 = resistor
2 & 3 = mVolt in
4 & 7 = power supply
5 & 6 = output voltage (0-5)


Thanks for any help offered.

Doug

 

[ericgibbs]

hi,
Is the output voltage from PV controller 'single ended' ie: one wire and common wire.??

Pin5 on a IA is connected to Vref or 0V, not output.

E.

 

[crutschow]

If the signal is a normal grounded signal and not differential then you can just use a standard op amp configured as a non-inverting amp. Either way the 1NA128 should work for your requirements.

 

[dstockman]

Thank you very much for replying. Yes, there is a common lead for all the controller box outputs (wind, sun, and battery voltage). Do I then just bring in the positive lead from the controller and bring out the positive output to the phidget and use the common lead at the phidget level, or do I need to hook the common from the controller to the INA128 at pins 2 & 5? Sorry for such basic questions. I just want to make sure I get this correct.

I assume the resistor should be a 1% or better tolerance to improve accuracy.

Should I consider placing an opto-isolator in the circuit to reduce the chance of destroying my Phidget? Do you know where I could learn more about this and recommend an opto-isolator to use? If this should be a separate post, I will be happy to do that.

Thanks very much for your help!

Doug

 

[ericgibbs]

hi,
The 0 to 70mV signal is analog, so after amplification it will be 0 to +5V, still analog. You can buy analog onto-isolators if required, HCNR200.

Do you have a dual power supply for the INA128.?

 

[alec_t]

I don't think an opto-isolator is needed between the amp and the phidget. Current-limiting resistors and clamping diodes perhaps?

 

[dstockman]

re: power supply:
My ignorance is probably coming through again, but I thought I could use the Phidget's 12 V DC wall-wart power supply to supply the INA128, or get the 5 volt off the Phidget-provided buss/power. It looks like the INA128 can accept 2.25 - 18 volts. Would either of these work? I assume you need this information to better answer my previous pin connection question? Once I get closer to figuring this out (with your kind help), maybe I can draw a crude schematic that would ensure I understand this.

re: the opto-isolator
(quote: I don't think an opto-isolator is needed between the amp and the phidget. Current-limiting resistors and clamping diodes perhaps?)
Is there a downside to using an opto-isolator? Ericgibbs part number supplied is < $5 US. If the wiring is straight forward, it seems like a solution that might work. Again my ignorance may be coming through. I suspect resistors and clamping diodes may be cheaper. Would this be easier to wire than an opto-isolator?

I really appreciate all your help! I hope to get battery voltage, wind current and PV current out of my controller for the system that powers my house. I am learning Java to code the Phidget so that I can save the system data to see how my system is performing over time (and if I solve some Java hurdles, also have a real-time display of system performance).

Thanks again!

Doug

 

[ericgibbs]

hi,
The INA128 is designed for a dual supply voltage, which would be essential as you have a single input voltage of 0mV to 70mV referenced to 0V.

A 0mV thru 70mV signal range could be amplified to 0V thru +5V using a standard Op Amp, powered by a single supply.

There are rail to rail OPA that work from a 5V supply, so the output would be from close to 0V thru close to +5V.
If you want to use a OPA of this type, let me know and I will post a circuit.

 

[dstockman]

Eric:

Please post the circuit you are talking about. If fairly simple I might be able to understand it and build it. Thanks again for all your help.

Doug

 

[ericgibbs]

Eric:

Please post the circuit you are talking about. If fairly simple I might be able to understand it and build it. Thanks again for all your help.

Doug

hi Doug,
Look at this option, it uses a mid range specification rail to rail OPA, OPA350

You can get single, dual versions of this OPA in 8 pin DIP packages.

https://uk.farnell.com/texas-instruments/opa350pa/op-amp-highspeed-rri-o-dip8-350/dp/1097443

E.

 

[dstockman]

Eric:

I think I understand this. BTW, I could not view the attached pdf, but could see the image. Are they the same? For the 5 v coming out of the phidget, I use 2 capacitors to smooth out the voltage (I assume). The + output from that goes to pin 7 on the OPA350. The positive output from the PV controller goes to pin 3. Does pin 2 go to ground which also goes to the negative output from the PV controller? Does pin 4 also go to ground (PV neg output)? Pin 6 is where I measure my output voltage (relative to ground)? Does R3 mean 4,700 ohm (4k7)? I assume R4 was initially a 58 ohm resistor, but you are suggesting a 100k pot. I am further assuming this will allow me some adjustment of Vout. Is all this correct? If I have this somewhat correct, I believe I can build this. Let me know how I am doing. Are 8 pin DIP sockets a standard size? I figure I should solder a socket and not the chip, given my skill level.

Thanks again for all your help and being so patient with me!!!

Doug

 

[ericgibbs]

hi,
I have re-attached that pdf here, also added pin numbers to the diagram

4K7 is 4.7K or 4700R,,, the 'K' is often used in place of the decimal point.

The 8 Pin DIP IC sockets are 0.3inch width on the pins and the pins are spaced at 0.1inch.

The 100K pot will enable you to set the gain exactly, most components have some tolerance in their values, so using general purpose resistors may not provide the precise required Vout.

Please ask if not clear.

 

[dstockman]

Eric:

If you are still listening, I have run into some confusion. I found a simple schematic of the Controller circuit that reveals my common lead is positive, while each energy generator line (wind, PV) and battery are negative. I am confused on how to make this work. I was assuming the common lead on the controller was ground, but it is not. Hopefully, this is an easy obstacle to overcome.

Diagram attached as gif.

Doug
View attachment 67904

 

[ericgibbs]

hi,
Looking, let you know.

EDIT:
Do you have low voltage DC supply on the project, say around -5Volts or so.?

Which OPAmp did you actually use on the project.?

EDIT2:
Do you know the resistance value of the Ipv resistor.

E.

 

[ericgibbs]

hi,
This is one option.

Dual supply and two OPA's

E.

 

[dstockman]

I was thinking I could use the 5 volt coming off the phidget. See gif file.

I did not get any parts yet (out of town for work). I was planning on getting the OpAmp you suggested earlier.

View attachment 67915

Doug

 

[ericgibbs]

I was thinking I could use the 5 volt coming off the phidget. See gif file.

I did not get any parts yet (out of town for work). I was planning on getting the OpAmp you suggested earlier.

View attachment 67915

Doug

hi Doug,
Measure the full voltage range across the 1 megohm resistor, let me know the minimum and maximum voltage and its polarity with reference to the 0V line.

E.

 

[dstockman]

Eric:

Your question about the 1mohm resistor confuses me. The only 1mohm resistor I see in these circuits is in the phidget. The phidget will measure the 0-5 volt output from the OpAmp circuit, AND hopefully, provide a stable 5 volt power supply for the OpAmp circuit.

The confusing part for me is that I assumed (incorrectly) the PV controller had the common lead as a ground and the leads from each energy source (wind, PV) were positive. The common is positive and the energy source leads are negative. So how do I amplify the 0-70 mV coming from the controller to 0-5 volt to be read by the Phidget? When I use a multimeter with the black lead on the controller common and the red lead on the PV output, I get a negative mV value. Can I just attach the Controller common to the Vin and the PV lead to the OpAmp negative? But then can I connect the power supply's (Phidget's) negative common lead to the OpAmp's circuit's ground? It seems to me we need to isolate the power supply's negative from the Controllers negative. I am so confused. I have attached a pdf of the 3 potential circuits.

If this is all too confusing, I could place a shunt between the PV and controller and measure the voltage drop across that. I cannot do this for the wind, because that comes into the controller as 3 phase AC wild (which the controller converts to 48 V DC that then gets fed into the battery). I could place a shunt between the controller and battery and then subtract the PV amps from the battery amps to get the wind amps (batter amps = wind amps + PV amps).

Thanks again.

Doug
View attachment 67970

 

[ericgibbs]

hi,
As the the signal is from 0mV down to -70mV, adding a shunt resistor will mean the signal is still negative going.

I posted a solution in Post #15. You need to invert and amplify the -70mV signal to get +5Vout

E.