AD623 ADC notes

AD623-Unable to change gain or reference voltage

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I have an air flow sensor (FS1012-1100NG) with expected differential output voltages ranging from -1mV to 20mV and I need to 1) know the precise value (mV) and gain to correlate V with airflow 2) make sure that value has long-term stability (order of days) and 3) end up with a voltage that makes the best use of my 12bit ADC.

I’m using an AD623 with a single supply (3.3V) with the FS1012 connected per the datasheet (see image).

  • pins 1&8 Gain Resistor (I’ve tried 100ohm-1Mohm but no change in output/gain)
  • pins 2&3 TP2+ and TP1+ (I’ve checked these and there is a differential voltage of about 2-3mV at max airflow)
  • pins 4&7 Ground and 3.3V
  • pin 5 Ref (Tested 0-3V using voltage divider, no change in output/offset)
  • pin 6 Output (to digital multi-meter with ref to ground)

What am I doing wrong? Is there a better differential amplifier I should be using? Thank you in advance!

Basic Block Diagram for FS1012-1100

instrumentation-amplifiershareedit  follow  flagedited 2 hours agobrhans11.2k22 gold badges2626 silver badges3838 bronze badgesasked 2 hours agoRyan2122 bronze badges New contributor

  • 1Any INA with programmable gain shud work. BUT Vin MUST be within Vcm range near V+/2 so you do this with Rdivider pullup network. – Tony Stewart Sunnyskyguy EE75 2 hours ago 
  • 1Use an R network pack for best CMRR noise rejection – Tony Stewart Sunnyskyguy EE75 2 hours ago
  • 1Does your AD632 share a common ground with your sensor? – brhans 2 hours ago
  • 1@brhans Yes all grounds are common. – Ryan 1 hour ago
  • 1@TonyStewartSunnyskyguyEE75 I’m focused on getting the hardware correct right now and then will look at the software side. I have an INA126PA and have reviewed the datasheet but I don’t see what you are referring to. Any direction you could give would be awesome! – Ryan 1 hour ago
  • @Ryan, I see that you are following the datasheet’s example of using a differential op-amp before the AD623. I was wondering why don’t you just let FS0102’s diff output go direct to AD623. In other words, no diff op-amp required. And I have the feeling that AD623 is a bit old, and newer and even cheaper Analog Devices ADC modules are around. – tlfong01 44 mins ago   
  • 1@tlfong01 The AD623 is the diff. amp. How would you recommend measuring 0-20mV? – Ryan 34 mins ago 
  • @Ryan, my apologies. I am newbie enough not to know that 20mV is too small for AD623. Let me read the datasheet to correct my wrong idea about ADC. Thank you again for pointing out my careless mistake. Cheers. – tlfong01 29 mins ago   
  • So I googled and compared AD1248 (a common 24 bit ADC) with ADS623, with the following summary: (1) ADS1248 (24-bit ADC): Input range = –2.5V to +2.5V, PGA gain = 1, 2, 4, 8, 16, 32, 64, Accuracy = 24 bits, (2) ADS623 (don’t know how many bits): Input range = -0.15 to +1.5V, Gain = 1 to 1000, Accuracy = 0.35%. My misunderstanding might be caused by the following: (a) AD623’s accuracy is 0.35%, but ADS1248 is 24 bit accuracy, (b) ADC’s input range is much wider than AD623. I must confess I have no experience with instrumentation amplifier. so I might miss something important, eg, noise. – tlfong01 just now   Edit   

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