AD620AN opamp problem

Question

Hello, 
 I want to read sensors with AD620AN. I am using wheatstone bridge. 
 However, opamp doesnt give a reasonable output. Therefore, I build a test circuit. I attached the screenshot of my circuit. Is this circuit correct for testing the opamp? 
 I am using LTSPICE and SPICE file that Analog Devices provided. LTSPICE output seems correct. However, when i build this testing circuit, it gives me 2V output.

Achim · Accepted Answer

Hello electrosurfer, 
 with the given parameters, your circuit violates the allowed input voltage range on all inputs. The amplifier will not run in spec, and so it cannot guarantue its intended parameters (like accuracy). 
 Maybe the output comes nevertheless close to the correct value - but it's not guarantued. I cannot recognize, how accurate the datapoints in your simulation are and I cannot judge, what accuracy you require. But if you want a reliable and accurate circuit, you have to run your amplifier in the specified range. 
 For the given circuit this would be easiest achieved with a bipolar supply (e.g. +10V on +Vs and -10V on -Vs). 
 best regards 
 Achim)

ADIApproved · Answer

Hi Electrosurfer, 
 
 Good day! 
 For bridge sensor simulation you may check the link below on how to model bridge sensor in LTSpice. 
 SPICE Model for a Full Bridge Strain Gauge Sensor 
 I hope this helps. 
 Best regards, 
 Franz

Achim · Answer

Hello electrosurfer, 
 the AD620 is not a simple OpAmp, it's an instrumentation amplifier (consisting of several OpAmps internally). For this type of amplifier one has to consider carefully the allowed voltage ranges of all inputs. In your simulation example, the input Vref (0V) is not in the allowed voltage range. 
 You may use the AD instrumentation amplifier diamond plot tool to check your circuit. 
 https://tools.analog.com/en/diamond/#difL=1.99&difR=2&difSl=2&gain=3.5&l=-8&pr=AD620&r=8&sl=-8&tab=1&ty=2&vn=0&vp=10&vr=0 
 For the values you apply it gives the message "Please adjust the Vref value. For the AD620, with supply values of +Vs = 10V and -Vs = 0V, Vref must be within 1.6V and 8.4V" 
 best regards 
 Achim

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AD620AN opamp problem