INA118: TLC5916 led driver and INA118 instrumental amplifier-drop voltage measuring issue

K Kamel

Part Number: INA118
Other Parts Discussed in Thread: LM317

Hello,

I have only one issue i don't no the reason of it, i am connecting the leds with the led driver, the led driver is supplied by a 5 volts voltage regulator "LT323A" ,the 12 volts over the leds, then a 2 channels analog multiplier "max337" then to the instrumental amplifier "INA118", the amplifiers is working as expected when i supply it with another 12 volt supply but to connected to the sam 12 volts rail it doesn't give the right expected value.

I have tried different things like supplying the amplifier with a separated 5 volt voltage regulator, i have changed the main 5volt regulator by "LM317" adjustable regulator but its not working with the same 12volts of the circuit.

I took it step backward and I made one branch as a basic circuit without using a led driver and i didn't get the right result from the amplifier (12v, 3 leds series, 660ohms resistor to set the current 18 mAmp).

i uploaded the schematic

thanks for you help and regards.

over 3 years ago

0 Raymond Zhang1 over 3 years ago

TI__Mastermind 47736 points

Hi K Kamel,

I think that you missed the input common-mode voltage requirements in this part, see the link below.

The datasheet did not present 12V single supply rail in input Vcm vs. output voltage, but the plot should be similar to Figure 5.

I go ahead and rearrange your circuit (did not spend time to duplicate exactly what you had). Here is the captured image and Tina simulation. I can't explain the "different power supply" scenario, and it should not have worked either, unless your configuration is somehow falling within the input Vcm requirements, see the attache image above.

/cfs-file/__key/communityserver-discussions-components-files/14/INA118--LED-I-Sensing-06112020.TSC

If you have additional questions, please let us know.

Best,

Raymond

0 K Kamel over 3 years ago in reply to Raymond Zhang1

Prodigy 50 points

Hello sir,

thank you sir for your explanation, i will try to say what i have understood to make sure that i get it right, "input common mode voltage" means that the 2 signals getting to the instrumental amplifier, in this case"INA118" the 2 signals should be in the range from ((V-)+1.1 volt) and ((V+)-1 volt) which means that the 2 signals should be between (0"GND" + 1.1 volt ) and (12"VCC" - 1 volt ), did i explain it right?

Thank you again for your explanation and you effort.

0 Raymond Zhang1 over 3 years ago in reply to K Kamel

TI__Mastermind 47736 points

Hi K Kamel,

Yes, normally you place the input common mode voltage in the middle of power rail, if you have large input signal swing.

If the input common voltage requirements are violated, the op amp will not behave linearly with respect to the input.

Best,

Raymond

0 K Kamel over 3 years ago in reply to Raymond Zhang1

Prodigy 50 points

hello,

ok, thank you for your clear explanation.

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INA118: TLC5916 led driver and INA118 instrumental amplifier-drop voltage measuring issue