Hi,
I'm trying to load cell mod my Logitech G27 brake pedal. And I need to amplify the signal from the load cell. But first I have to know how to use INA122PA.

I have bought 3 INA122PA ICs from the internet. However when I try to connect the circuit, the output is weird.

Here is the basic connections from the datasheet:


For testing purpose, I connect the ic as following:
pin1: no connection
pin2: connected to pin3
pin3: connected to pin2
pin4: grounded
pin5: grounded
pin6: (output)
pin7: v+
pin8: no connection

(upper left is pin1, bottom left is pin4, bottom right is pin5, upper right is pin8, correct me if I'm wrong)

the output should be 0 since pin2==pin3 and gain=5

however, no matter what did I connect to pin2 and pin3 (e.g. (Vin+ - Vin-) = 0v, 1v, 0.3v, 0.1v, 0.005v ), the output is always close to v+.
e.g.
v+ = 3.37v, output = 2.84v
v+ = 4.98v, output = 4.44v
v+ = 8.89v, output = 8.33v
v+ = 11.90v, output = 11.33v

I tried with all 3 ICs and all behaved the same.

My question is:
Am I connecting them wrong?
Or is it possible that all 3 ICs are damaged/failed?
Should I buy INA122P/INA122PA from other site / shop to try again?

Huge thanks <3

 

Why do you think that shorting together the two inputs is a good way to get a useful output?

What is the value of V+?

Please show us *your* schematic, including all power and input components and connections. Be sure to include reference designators for all components so they can be discussed clearly.

If there is no DC relationship between the input voltage and the chip power source, the output can saturate, which is what your readings indicate.

ak

 

Why do you think that shorting together the two inputs is a good way to get a useful output?

What is the value of V+?

Please show us *your* schematic, including all power and input components and connections. Be sure to include reference designators for all components so they can be discussed clearly.

If there is no DC relationship between the input voltage and the chip power source, the output can saturate, which is what your readings indicate.

ak

Thanks for the quick reply

Since I have tried to connect the two inputs to the load cell, but found that the output voltage is weird.
So I tried to short two inputs so that the voltage difference of the two inputs is 0.0mV, so the output voltage (e.g. voltage between pin5 and pin6) should be 0v theoretically. However, it still has weird output. It always output 4.4V when power supply is 5V.

The value of V+ is 5v

Here is my schematic:


The IC used is INA122PA,
load cell from taobao(aliexpress): no datasheet or part number, just some specs:
operating voltage: 5V to 10V DC
sensitivity: 1.0 ± 0.1 mV/V

and measuring and logging some more data with voltmeter:
V+ vs V- = 4.97V
V+IN vs V-IN = from 0.0mV to 1.2mV (i tried my best to bend the 20kg load cell)
V+IN vs V- = 2.48V
VO vs REF = 4.43V

 

That's not a Schematic,
that's a nebulous Block diagram.
Schematics are the "language" of Electronics,
one accurate Schematic is worth more than a thousand words.

If all You have is a Block-Diagram,
can You at least provide a description of the Controller-Input-Requirements to
achieve the desired end-results ?
( and any and every other detail possible )

There's nothing wrong with the Chips.
.
.
.

 

Where did you get the amps?

 

Where did you get the amps?

from taobao since they are cheap, like 1usd each

 

That's not a Schematic,
that's a nebulous Block diagram.
Schematics are the "language" of Electronics,
one accurate Schematic is worth more than a thousand words.

If all You have is a Block-Diagram,
can You at least provide a description of the Controller-Input-Requirements to
achieve the desired end-results ?
( and any and every other detail possible )

There's nothing wrong with the Chips.
.
.
.

Sorry about that just learning...

Is this what a schematic looks like?


Desire result:
the INA122PA could amplify the signal from the load cell, with the gain of (5) for testing purpose

Inputs:
0mv <= { (V+IN) - (V-IN) } <= 5mv

Expected output:
0mv <= VO <= 25mv

Current situation:
the INA122PA outputs 4.4V no matter the input signals are

 

hi psam,
Please post details/type number of the load cell.

E

 

hi psam
Why do you want only.?
Expected output:
0mv <= VO <= 25mv

I have Created a LTspice for the INA122 IA, I need more information for the load cells etc.
We can then suggest a final circuit.

This is a test simulation for 5mV signal input.

E

 

hi psam,
Please post details/type number of the load cell.

E

Here is the details of the load cell:
Operating voltage: 5-10V DC
Rated Output: 1.0 ± 0.1mV/V
Zero point(not sure): ± 0.1mV/V
Non-Linearity: 0.05% FS
Hysteresis: 0.05% FS
Non-Repeatability: 0.05% FS
Creep (per 3 minutes): 0.05% FS
Output Impedance: 1000±5% Ohm
Temperature Effect on Zero (per 10°C): 0.05% FS
Temperature Effect on Span (per 10°C): 0.05% FS


link: https://item.taobao.com/item.htm?spm=a1z09.2.0.0.19212e8djLJ5rt&id=578211994809&_u=a1o7b6fbd1fa

 

hi psam,
Looks a simple standard load cell.
Post a sketch showing how you have it connected to the INA122.
Also what resistor value for the Rg INA gain.?
E

 

hi psam
Why do you want only.?
Expected output:
0mv <= VO <= 25mv

I have Created a LTspice for the INA122 IA, I need more information for the load cells etc.
We can then suggest a final circuit.

This is a test simulation for 5mV signal input.

E

Just want to simplify everything at this moment. My idea is: the more component in a developing / unstable system, the harder to make the system work.
Leaving pin1 and pin8 no connection will give a gain of 5V/V. When having a 5V power supply, the max. output of the load cell should be (5V * 1mV/V) = 5mV. Therefore, the theoretic output should 0mv <= VO <= 25mv.

 

hi psam,
Looks a simple standard load cell.
Post a sketch showing how you have it connected to the INA122.
Also what resistor value for the Rg INA gain.?
E

here is how I connect the INA122PA with the load cell and the power source

 

Has no-one mentioned DC PATH TO GROUND?
I bet that the inputs need a dc path to ground to bias them up. Value doesn’t matter too much so long as it is much bigger that the resistance of the load cell.

 

hi gio.
What is the value of RG, also note the load cell connections.

E

 

Has no-one mentioned DC PATH TO GROUND?
I bet that the inputs need a dc path to ground to bias them up. Value doesn’t matter too much so long as it is much bigger that the resistance of the load cell.

I have tried to put a 0.1uF capacitor between +ve and -ve. But still doesn't work. I thought it was just trying to filter out the fluctuations / noises of the input voltage, so I remove it to make everything simply first.

Just to learn more, I would like to ask:
Does putting a capacitor across the power source count as a DC path to ground?
And should I add a capacitor / resistor back?

 

hi gio.
What is the value of RG, also note the load cell connections.

EView attachment 254739

in my testing, Rg=∞, gain=5+(200k/∞)=5

in my final product, Rg will be replaced by a VR, and set to Rg = 201, gain=5+(200k/201)=1000

I'll try to connect the load cell as you showed, and update this comment after trying

edited: still does not work

 

hi,.
What you plan to do with a 25mV output signal.?
Because with a single voltage supply to the INA122 the Vout will be approx 0.7V with no input signal.

E

 

hi,.
What you plan to do with a 25mV output signal.?
Because with a single voltage supply to the INA122 the Vout will be approx 0.7V with no input signal.

E

Just for testing purpose. Since the problem I'm facing now is: the output is always 4.4v when V+ = 5V (even signal voltage different == 0)

 

hi,
If the circuit is wired as per my diagram, for 5mV, RG= o/c and single INA supply, you will see this signal.
E