INA125: Resistance of LoadCell to low?

Hi  Fabian,

Can you send me your schematics?

Enclosed is the INA125 Tina simulation. If you are unable to figure it out, please let us know.

/cfs-file/__key/communityserver-discussions-components-files/14/INA125-Bridge_5F00_Amp-Strain-Gauge-03302020.TSC 

Best,

Raymond

This is the schematic, do you see any problems?Resistance of loadcell (measured between V_supply_Loadcell and GND ist 1kOhm)

Hi Fabian,

what voltages do you measure between the input pins (pin 6 and pin 7) and GND?

And what about pin 2 (/SLEEP)? Is the INA125 activated at all?

Kai

INA is activated (3,3V ). The 2.5 reference voltage can be measured. Or can 2,5V also be measured even is INA is off?

The voltage I measured on pin 6/7 is 1,2V when loadcell has no load.

Hi Fabian,

and what happens when you give a load to the load cell? What are the voltages then?

Kai

When I give load on the load cell on pin 6 and 7 is an difference arround 0.01 to 0.02V when I measure with multimeter.

On pin 10 and 11 the output voltage stays the same 0.04V(measured with multimeter). Another thing I realized is that the AD Converter of Microcontroller

(STM32)is not workin very well with the reading of the gain path (pin 10,11). It only measure around 0.013V, maybe the INA has some damge so that output current is to low for ADC of microcontroller?

Hi Fabian,

so, at pin 6 and pin 7 you measure a voltage of 1.2V + 0.01V and 1.2V - 0.01V, e.g.?

Also: Can you disconnect the STM32 from pin 10 and pin 11 for a test? Is the INA125 doing well then?

Kai

Hi Fabian,

Thanks for Kai's help in troubleshooting the issues. 

Can you check out the following components?

At the Wheatstone bridge, what is the total current from 2.5V reference. You mentioned an resistance of 1kOhm. Was the total or each bridge is 1kOhm?

Since your voltage reference is configured at 2.5V, if your total resistance is 1kOhm, the reference voltage is driving 2.5V/1kOhm=2.5mA. You mentioned that your voltage at pin 6 and pin 7 is measured at 1.2V. If the bridge is perfectly balanced, you should measured at 1.25V. 

Possible issue 1:

My speculation is that you may have 1kOhm on each Wheatstone branch, and your total resistance is 500 Ohm (1kOhm||1kOhm), or you are driving 5mA  out of voltage reference of 2.5V. This may be too much for voltage reference to handle. 

Possible issue 2:

What is your output load at pin 10 and pin 11? Your output current may be too high, see the attached image. It has be less than +/-2mA. If the output current is greater than that, INA125 may go into current limiting mode. 

These are two possible causes based on the data you provided so far. Please let us know. 

Best,

Raymond

Hi,

I made some measurements:

Full Bridge resistance: 1.05kOhms

Current in Full Bridge: 2,35mA

Voltage V_ref = 2.5V

V_in+ =1,167

V_in- =1,167

V_out (PIN10/PIN 11)= 0,048V

When I disconnect pin 10/11 (I scratched of the trace on pcb )the value on pin 10/11 stays the same.

Values without load:  V+=1,167 and V-=1,167

Values with load: V+=1,165  V-=1,169

Hi Fabian,

do you get a signal if you increase the gain of INA125?

Kai

Hi Kai,

if I insert 10kOhm resistor in the gain path the Output voltgae is increasing to 0,049V ->Delta is 0,001V

Best regards

Fabian

Hi Fabian,

and if you furtherly increase the gain? And don't forget to load the load cell!

Kai

Hi Fabian,

Have you had a chance to resolve your problem? 

If you have further issues, please let us know.

Best,

Raymond

Hi Raymond,

I odered INA125 in DIP Package to Breadboard with an STM32 Nucleo Board

and same configuration. It still dosent work.  I try to solve it and when this dosen't work

I' will post a picture of my test.

Hi Fabian,

Please send us the picture. 

I am running out of ideas why your INA125 circuit is not working. In other words, if you have differential voltage presented at the input, the output will be generated, which it equals to Gain*differential voltage. 

Also, please make sure V+in, which is pin 6 is greater in voltage than pin 7 or V-in. I recalled that you are running single voltage rail. I did see that you had differential Values with load: V+ = 1,165  V- = 1,169. If this is the case, please swap the input leads. 

Best,

Raymond

Hi Kai,

if I increase gain by 10K resistor nothing changes.

Hi,

I also run out of ideas. I found a video on youtube. In this video pin 4 abd 5 get connected. In datasheet this isn't the case. What do you think about this?

Hi Fabian,

have you verified that the voltage at pin 6 is greater than the voltage at pin 7, as already asked by Raymond? Also, decrease RG to 1k and below.

And, of course, you can connect pin 4 to pin 5 as shown in figure 6 of datasheet to generate an output offset voltage. But we first tried to fix your problem which you mentioned in your original post: "The Gain value doesn't change."

Kai

Hi Fabian,

I hope that your INA125 issues have been resolved. If you still have issues, please let us know.

Best,

Raymond

Hi Fabian,

does it work now? :-)

Kai

The DIP Package works fine, but SMT still dosen't work. I will continue testing  on thursday

Hi Kai and Raymond,

thanks for your ideas. I gave it another try today with an 1k resistor. According to the given formula the gain results in 64 which suddenly worked. Maybe I connected Vin+ and Vin- correctly this time.

However, w/o an resistor, I still don't see the Gain = 4 and I still have the default Vout = 0.046V (which I also see with the 1k resistor). When I add the 1k resistor what happens is that it does not increase the Vout until it is larger thant the default 0.046V.

Current setup:

V+ = 5.0V

SLEEP=3.3V

V- = GND

Vref,out=2.5V

Vin+ = 1.19V

Vin- = 1.19V

RG = 1.19V

V0/Sense = 0.47 (default without load)

Do you have any ideas?

Thank you!

Hi,

I don't understand why this tells my experience? But I think that means that the reference circuit in the data sheet can't work

with Gain factor of four.

.

Hi Fabian,

I agree with Raymond. Also, why haven't you tried much higher gains?

Kai

Higher gains would cause to great voltage on the input pin.

Hi Fabian,

what amplifier do you want to take now?

Kai