Hello,

I want to read sensors with AD620AN. I am using wheatstone bridge.

However, opamp doesnt give a reasonable output. Therefore, I build a test circuit. I attached the screenshot of my circuit. Is this circuit correct for testing the opamp? How can i properly test the opamp ?

I am using LTSPICE and SPICE file that Analog Devices provided. LTSPICE output seems correct. However, when i build the real circuit, it gives me 2V output.

 

That's not a very helpful circuit. An AD620 doesn't have 99 pins!
If we knew which were inputs and which were output maybe someone could help.

 

Hi TS,
Please post your LTS asc file.
The default 'numbers' for the AD620 power supplies are 99 for +V and 50 for -V
E

Note:
As you have only one power supply ie: +V, the Vout will be approx +1V to +2V

Clip from *.cir
* Node assignments
* non-inverting input
* | inverting input
* | | positive supply
* | | | negative supply
* | | | | output
* | | | | | ref
* | | | | | | rg1
* | | | | | | | rg2
* | | | | | | | |
.SUBCKT AD620 1 2 99 50 46 20 7 8

 

Hi TS,
This is your circuit in LTS, note the +0.9V Vout offset at the start of the plot, when using a single power supply.

Let us know what you are trying to do, then we can help.
E

 

Hi TS,
This is your circuit in LTS, note the +0.9V Vout offset at the start of the plot, when using a single power supply.

Let us know what you are trying to do, then we can help.
E
View attachment 290719

Thanks for the response. I am using that cir file you shared. I am aware that opamp cant go below some limits. In your schematic calculations are also correct 1V.Gain. However when i build the circuit and measure the output it is completely wrong. Do you think that my op amp broken?

 

Hi TS,
Do you have a diagram to post showing the connections from the bridge to the AD620.?

It is possible your AD620 is faulty.

BTW: when simulating, use a longer .tran time, 100mSec.

E

 

The AD620 is an obsolete part number,
if You need a trouble-free replacement that's a little more modern,
and has Rail-to-Rail Inputs and Outputs, ( which makes it much easier to work with ),
try a LM6142 ........
https://www.digikey.com/en/products/detail/texas-instruments/LM6142BIN-NOPB/367288
.
.
.

.

 

My main purpose is this circuit. This sensor is 0-190 ohm. My output is what i expected 1-5V. Do you think ad620 suitsble for this application. Also this circuit doesnt work in real life. It gives strange output which is 0.8v

 

The AD620 is an obsolete part number,
if You need a trouble-free replacement that's a little more modern,
and has Rail-to-Rail Inputs and Outputs, ( which makes it much easier to work with ),
try a LM6142 ........
https://www.digikey.com/en/products/detail/texas-instruments/LM6142BIN-NOPB/367288
.
.
.

.

Thanks i will check

 

Hi TS,
This is what I see with your circuit, are you sure you have pin #1 and #2 the correct way around.?

On your actual circuit?

E

 

yes non inverting tied to 10k-1k1 node. inverting tied to 2k2-sensor node.
here is my schematic and output.
190ohm 1.01V
0ohm-4.95V



hope you can see the dots

 

here is my file btw

 

Hi,
TS,
This is what I see, in simulation,
using your new value for Rg 5488R
E

 

you get same result. do you think this circuit is applicable in real life?
as you can see my purpose is to convert 0-190 ohm into 1-5V. LTSPICE gave expected results.

 

Hi,
Will check it out and get back to you later.
If it works in simulation, it should work in a real circuit, check your component values and wiring, if OK, your AD620 maybe faulty.
E

 

Hi @electrosurfer
Try this option.
I know its inverted, so use a 2nd OPA to invert to the correct sense.
E

 

hi,
This is your original circuit.
It should work using real components.
E

 

Thank you sir.I will check it again. Probably my opamp is faulty.

 

Hi,
OK, let me know how it goes.
Good Luck.
E

 

An AD620 is not an opamp. Instead it is an instrumentation amplifier. Like most ICs, few through hole pins packages are still available but many surface mount packages are available. Moderm intrumentation amplifiers might be rail-to-rail.