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I am working on a data acquisition project ,I have to get data from a pressure sensor of 5 pins(similar to a load cell) Using AD620 inst.amplifier.

I have used a gain set resistor = 390 ohm so gain is roughly 130 Output of sensor is 2mV/V in this case 0 to 10mV(fullscale)

sensor details :

  • Pin1 = Vcc
  • Pin2&3 = Sensor output
  • Pin4 = Shield
  • Pin5 = Ground

I want to power circuit using +5 volts.

I have made a circuit on general purpose PCB which is shown below

enter image description here

------Questions-------

  1. Output seems to jump to saturation and again to zero.

  2. Is the circuit Correct or I have messed up something?

  3. Should reference voltage be grounded as I have done here?

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  • \$\begingroup\$ An AD620 is not suitable for powering with +5V \$\endgroup\$ Sep 26, 2016 at 0:03

2 Answers 2

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If you look on page 4 of the data sheet it tells you that the output from the AD620 can swing between -Vs + 1.1 volts to +Vs - 1.2V. Your -Vs is 0V and your +Vs is 5V. This means the output can swing between 1.1 volts and 3.8 volts. Take note that if the output is outside (or close to the edge of) this range it becomes unreliable.

Next, your reference input (pin 5) is specified as having a range of -Vs + 1.6 volts to +Vs - 1.6 volts i.e. a range (in your circuit) of +1.6 volts to +3.4 volts.

You have the ref input tied to 0V and this is wrong if you want your circuit to work. Try connecting it to a node that is centred between 0V and +5V. This will also "position" your output to be at the same "midpoint" when the bridge on the input is neutral balanced (not imbalanced).

I would recommend you use a precision voltage reference for this so that it is reliable in terms of measurement error reduction.

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  • \$\begingroup\$ Thank you Andy !! that was a very good explanation ..i understood where i was getting it wrong !justa follow up question ..What if i power the sensor and circuit with +5V and -5V supply and ground the ref pin ..it should work right ?? \$\endgroup\$
    – Mr.Sky
    Sep 26, 2016 at 14:42
  • \$\begingroup\$ It should work just fine if you did that. However if the bridge is heavily imbalanced then it might look like the same as doing something illegal with the ref pin. \$\endgroup\$
    – Andy aka
    Sep 26, 2016 at 14:57
  • \$\begingroup\$ Thank you !! What does heavily imbalanced mean ? i have a load cell it has a specified range and output ...that should work right ?? \$\endgroup\$
    – Mr.Sky
    Sep 26, 2016 at 15:01
  • \$\begingroup\$ I said it implying that if the load cell had been overstressed it might give a big imbalance reading and end-stop the amplifier. Nothing to normally worry about for a new undamaged load cell. \$\endgroup\$
    – Andy aka
    Sep 26, 2016 at 16:17
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Sensor must be biased to 2.5V with INA followed by unity gain amp with same v/2 reference since Vin input must not swing with 1.6 V of either supply rail.

see data sheet for examples.

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  • \$\begingroup\$ I read the data sheet and did not understand that specific part ,Can you Please elaborate ? Thank you ! \$\endgroup\$
    – Mr.Sky
    Sep 25, 2016 at 20:24
  • \$\begingroup\$ fig 38 and see details \$\endgroup\$ Sep 25, 2016 at 20:25
  • \$\begingroup\$ I was confused about the same image , i read it again and again and couldn't figure out why we use op-amp AD705 also the sensor i am using is not 3k ohm but just 250 ohm, so if i use the given setup would it work ? also i would really love a very brief explanation if possible ..Thank You !! \$\endgroup\$
    – Mr.Sky
    Sep 25, 2016 at 20:34

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