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INA826: Vout is not linear

Prodigy 40 points

Replies: 2

Views: 71

Part Number: INA826

Hello all,

I am using the INA826 to convert an differential voltage from the AMC1211-Q1 to an single output voltage to my uC.

Explanation of my circuit:

-"A1" is an analog voltage (0V to 5V) that I convert with the voltage divider to a maximum of 1.9V (the AMC1211-Q1 allows a maximum of 2V on VIN pin);

-"VOUTP1" and "VOUTN1" are subtracted in the INA826 and since I want the maximum input of 1.9V to be 3.3V at the input of my uC I have to apply an gain of 1.73 (1.73 x 1.9 = 3.29V), the gain resistor is 68KOhm (So gain = 1 + (49.4/68) = 1.73)

-I did my calculation based on the maximum input, 5V on voltage divider, translates into 1.9V in "VIN" pin of the AMC1211-Q1, and according to Figure 20 of the AMC datasheet, with VIN = 1.9V, VOUTP = 2.4V and VOUTN = 0.5V for this case. So far so good, I have tested this from 0V to 5V analog input and the differential output of this component is correct according the Figure 20 of the AMC datasheet.

-The Vout pin of the INA826 stops being correct after an analog voltage of around 1.9V (See table bellow for tests results)

So my question is, why isn´t the INA826 output performing like expected after it reaches about 1.9V?

Thank you for your time.

Analog Input before voltage divider [V] AMC1211 VIN pin [V] AMC1211 Output [V] INA826 Expected Output [V] (Vout = 1.73 x (VOUTP - VOUTN) INA826 Real Output [V]
0.78 0.3 VOUTP = 1.6 ; VOUTN = 1.3  Vout = 1.73 x (1.6 - 1.3) = 0.52 Vout = 0.51
1.83 0.7 VOUTP = 1.8 ; VOUTN = 1.1  Vout = 1.73 x (1.8 - 1.1) = 1.21 Vout = 1.23
2.87 1.1 VOUTP = 2 ; VOUTN = 0.9  Vout = 1.73 x (2 - 0.9) = 1.9 Vout = 1.89
3.92 1.5 VOUTP = 2.2 ; VOUTN = 0.7  Vout = 1.73 x (2.2 - 0.7) = 2.6 Vout = 2.18
4.96 1.9 VOUTP = 2.4 ; VOUTN = 0.5  Vout = 1.73 x (2.4 - 0.5) = 3.29 Vout = 2.44

 

 

 

 

 

  • I´m sorry I have forgot the images:

  • In reply to Alexandre Francisco1:

    Hi Alexandre,

    Thank you for your post, I can help you.

    It looks like you are violating the input common mode voltage range vs voltage output range. The VCM v Vout Plot of an INA allows a designer to make sure they are operating within the INA's linear condition. We have an online calculator which allows you to input your specifications and it will display the VCM v VOUT plot: http://www.ti.com/tool/INA-CMV-CALC

    When the inputs to the INA are 0V and 1.9V, with a gain of 1.73V/V, you expect to see 3.3V at the output but the VCM v VOUT plot shows that this is out of the INA826 linear range:

    When the supplies are 3.3V and ground, Gain = 1.73V;V and Vref = 0, at a common mode of 0.95V, your input swing to the INA is 57.8mV to 1.618V, the output swing is 100mV to 2.8V.

    Also, no amplifier will be able to swing to the rail on the output. The amplifier has a output swing limitation and even Rail-to-Rail amplifiers will have a swing within 100mV. 

    If you want to use the INA826 and convert 0-1.9V to 0-3.3V, you may increase your supplies to 5V (I see that this may be available from the schematic you shared) and set the common mode voltage to 1.65V:

    INA826 revised.TSC

    -Tamara