AD623-Unable to change gain or reference voltage

Question

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!

Answer 1

INA126 specs for common mode range is undefined at Vs+=3.3, Vs-=0 yet it says works to Vs=3V I can see the Vcm only shifts down with Vs+ due to high side current source voltage drop. So I take 3.3/5 x100% of the Fig 22 mean voltage of 1.6V. this equals 3.3/5*1.6= ≈1.0V rounded down.

• using 100000x more current than input bias current to minimize offset from input bias current
• we get a 4 resistor solution with no loss of differential input. from Fig 14 Ib≈14 nA so you can make 1V with >1.4mA roughly even 1mA with this low impedance is OK.

thus Rpu=2.3k, Rpd=1.0k I=3.3/ 3.3k=1mA , Vth= 1/3.3 x 3.3V=1.0V. Pullup (pu) to 3.3V, Down(pd) to 0 V pref using lowest error tolerance if using gain=1000 to minimize DC out offset.

Then use a lower Zcm bias resistors for each INA input between Zsource and ZIn(INA)

my preference

Use a 1.0 V LDO with RF cap and RC bias to both inputs at 1.0V. this gives better differential noise rejection and common mode line noise rejection. Using RC=<50ms and R= 50k, C=1uF

Somehow you must to define your nominal input voltage and output if you want unipolar from 0V or bipolar centred around midscale output. TBD

^{simulate this circuit – Schematic created using CircuitLab}

Answer 2

After much trial and error, I've concluded that a minimum offset voltage for both the reference and thermopile "virtual grounds" (aka TP1- and TP2-) needs to be +0.8V-1.0V above ground. The thermopile needs to be considered a voltage source and so the second figure in my OP is the basis for moving forward. I don't understand why this is but it is likely related to the topology of the In-Amp since ref directly tied to the gain shown in the block diagram for INA126. I also suspect there is a "deadzone" in the common mode graphs that is disregarded by using the term Vd/2.

I can say with confidence INA126 is the way to go and without @TonyStewartSunnyskyguyEE75 I wouldn't have come to an answer. Thank you for your help!