Hello
I have a pressure sensor board, supposed to be 0 to 40kpa. (It is not) Common cheap board using a fixed gain (128*) HX710B serial 24 bit AD. The board works with but has range problems. The Sensor has a 25mv or so offset, 50mv range, and allegedly outputs -20kpa to +20kpa. This would be cool but the range is clipped in the HX710. +/- 3.9*Vref, +/-19.5mv @5V In fact I measured a wall to wall range of -15.2 kPa to +5.3 kPa. So there was clipping on the low end also. Here is the schematic of the board. Where, how, what, can I do to shift the 25mv offset down so I can use the upper end that is getting clipped, or what can I do to fit the entire range into the AD limits? Bridge resistance is reported at 4k to 6k Ohms.

Thanks for the thoughts and help..

 

Probably not good that I am replying to myself....
So I tried an experiment and altered the VCC source to the sensor. With 3.3V on the sensor and 5V to the chip the total range improved to -20.6kPa to 10.8kPa. Still missing 1/2 the high side.

 

Did you draw the schematic yourself?
As pin4 is NC on the sensor as per datasheet of the manufacturer. Also the restores to the HX710B for the sensor seems very low. And AVDD should be decoupled from DVDD with an inline inductor and an capacitor on the DVDD side of the inductor.

 

Did you draw the schematic yourself?
As pin4 is NC on the sensor as per datasheet of the manufacturer. Also the restores to the HX710B for the sensor seems very low. And AVDD should be decoupled from DVDD with an inline inductor and an capacitor on the DVDD side of the inductor.

I did not draw the schematic, I borrowed and added the 710 pin names. I see that the sensor pin numbers were on the wrong side. (now fixed). Yes the sensor data sheet is vague. The pins are reversed, see the bottom of the data sheet in the drawings. Yes, resistors 01A verses 010, 100 or 1k... lol - Does it make a huge difference? at least they are the same. As far as the inductor, I don't know the board comes this way.

 

So I cut the trace at pin 6, it goes under the sensor. I then was able to add a resistor to the bridge circuit. R shift. After a little trial and error, I selected 100 Ohms. This in fact shifted the zero point. The final range -3.42 kPa to +16.98 kPa, or 20.4 kPa The first module I tested had a wall to wall range of -15.2 kPa to +5.3 kPa, or 20.5 kPa. So no real loss of range or resolution. Various values I recorded for O kPa and resistance. 0 ohm, 129,570 : 47 ohm, 82,400 : 75 ohm, 54,690 : 120 ohm, 10,350 : and 100 ohm, 30,015. My output range is 0 to 167,772. 2^24/100. When the bridge was fed with 3.3V I got 31.4 kPa range. I imagine if both modifications were made the sensor could reach a -5 to +25.4 kPa rating, if the sensor works above 20 kPa.