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I'm trying to understand a differential pressure sensing circuit that I have. Have attached rough design sketch as well.enter image description here Sensor used is "10 INCH-D1-MV-MINI" by Allsensors. Output span is 20mV for 10" H2O differential pressure. This output is supplied to instrumentation amp "AD622" having gain set to 100 & REF voltage of +2.5V. Supply rails are +Vs = +15V & -Vs = -4V. This output of op amp is fed to Channel 1 of ADC - AD7718 & Channel 2 of ADC is fed signal which is (op amp output divided by 5 )+`2V. ((ADC_AIN2 = OP-amp-out / 5)+2). Now few doubts i have are,

  1. Sensor output will be 0-20mV for full scale pressure. So, why is op amp AD622 supplied with -Vs = -4V. Why is it not connected to ground. There is REF voltage of 2.5V, which will give final op amp output as 2.5v to 4.5v. Is -4V supplied just in case high and low pressure ports are interchanged and sensor output goes from 0 to -20mV ? But, sensor datasheet does not mention span as +-20mV. Its only +20mV.
  2. Channel 1 of ADC is fed directly from op amp output, that's understood. But, why is channel 2 provided divided output ? Will it give better resolution by reading divided output for particular pressure range?
  3. ADC Vref =2.5V and AINCOM=2.5V and is operated in pseduo differential & bipolar mode ie AIN1 & AIN2 are refrenced to AINCOM. Does this mean that input seen by ADC at channel1 will be 2.5-2.5=0 to 4.5-2.5=2 ie 0 to 2V at AIN1. And at AIN2 (2.5-2.5=0V to 2.9-2.5V=0.9V) If these voltages are correct, what is use of operating ADC in bipolar mode ? Output is not going to be -ve.
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2 Answers 2

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A very simple reason might be that the differential pressure sensor might be wired backwards and this will cause the AD622 problems on the output without a negative supply: -

enter image description here

The output of the AD622 will swing somewhere between the neg and positive supplies but it won't get very close to them. See the box I've marked in red - it's saying that if your neg power supply is 0V, don't expect the output to get any lower than +1.1 volts in normal circumstances.

The smallest magnitude negative supply ought to be about -1.8 volts to be sure of being able to reach 0V cleanly on the output.

I suspect for your 2nd question that this is all about what happens when the pressure is too big and ch1 on the ADC cannot read it any more - at least ch2 can be relied upon for some readings even if they are subject to a few small extra errors due to resistor tolerances.

For question 3 - I'd like to see a circuit diagram.

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  • \$\begingroup\$ For question 2, My sensor is capable of handling differential pressure of 10"H2O for which corresponding electrical output will be 20mV max, which when conditioned is within safe range (+-2.5V) of ADC input. If what i've stated above is true, why do i still need channel 2? \$\endgroup\$
    – user1154659
    Jul 3, 2014 at 8:54
  • \$\begingroup\$ I haven't designed this circuit and clearly neither have you so you should try asking the original designer why there is a facility to measure a potted down version. I can only summise on the information presented. \$\endgroup\$
    – Andy aka
    Jul 3, 2014 at 9:46
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AD622 is not made for rail to rail inputs or outputs. It needs a negative supply if inputs or outputs need to go near ground. I think the AD623 will work rail to rail

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