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AD623 Error in gain

Question asked by ctzof on Dec 6, 2014
Latest reply on Dec 8, 2014 by ctzof



We are designing a new PCB where we use an AD623 to amplify a signal after an optical isolator amplifier HCPL7840. The gain of the AD623 was set to 2.78 through a 56K resistor as RG the amplifier is dual supply with -5V and 5V. I used the calculator tool provided by analog devices here Analog Devices: Interactive Design Tools: Instrumentation Amplifiers:. The maximum input from HCPL7840 is 1.66 V and I supposed to get an output from AD623 at about 4.45 V. Instead I get something near 3.8 V. In the beginning I thought that the amplifier works near to saturation but as you increase the voltage to the input the voltage in the output also rises so that's not the case. I changed then the resistor to 65K and the gain to 2.53 so the amplifier has more space before it goes out of region. The output in that case supposed to be 4.06 but again I got 3.9 V. My question is if this error in gain is normal or if I make something wrong. The AD623 supposed to have a very low error gain and this doesn't compile with the results. In case I am wrong the maximum output of the isolator can be set up to 0.2 V and in this case I want the output of AD623 to be something near 4.45 what can I do then. Attached you can see a picture of my design. Thanks