amplifier's gain problem

Thread Starter

filip.barisic

Joined Jul 24, 2020
6
Hi,
I am having an issue with an amplifier, more precisely with an instrumentation amplifier INA333. I want a gain to be around 500 and that is the reason I put a resistor of 200ohm between pins 1 and 8. Voltage between two input pins equals around 0.0027 V and output is around 1.06 V, but according to my calculation 0.0027*500 equals 1.35 V.

Does anybody have a suggestion how to calibrate this amplifier?
 

Papabravo

Joined Feb 24, 2006
14,416
Show us a schematic. Words do not suffice.
What kind of resistors are you using? Package and tolerance.
 
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Irving

Joined Jan 30, 2016
1,017
Also how do you know input voltage is 2.7mV. What are you measuring it with? And have you provided an input common mode path?
 

KeithWalker

Joined Jul 10, 2017
1,222
What are you using to make the measurements with? 2.7 mV is very small and is difficult to measure accurately unless you have the right equipment an use the right techniques. If you are using a DC input signal the measurement could affected by thermal offset. An AC signal could be affected by EMI.
Regards,
Keith
 

Thread Starter

filip.barisic

Joined Jul 24, 2020
6
I am measuring everything in labview. Voltage supply (+) is 5 V and V(-) is 0 V, Vref is also grounded.

And this is a qualitative scheme of electric circuit.(ignore the numbers)
1595602033497.png
DC voltage supply for wheatstone's bridge is 2.5 V
 

crutschow

Joined Mar 14, 2008
25,431
[QUOTE="KeepItSimpleStupid, post: 1534946, member: 234053You also amplify the offset voltage.
[/QUOTE]The device has active input offset compensation so has only microvolts of input offset.
 

sparky 1

Joined Nov 3, 2018
336
I could'nt find anything wrong with your set up.
Can you measure the resistance between pin 1 and pin 6 also 8 and 5 should measure 350k
if it measures 145k then it is an INA118 the laser trim operator mixed up part of a the batch.
then Rg = 50 ohms G=500 and you have an INA118
 

Irving

Joined Jan 30, 2016
1,017
And what are the chances of that? Pretty unlikely I'd have thought. More likely to be a fake 333 than a mislabelled 118.

But most likely reason is basic measurement error. Input impedance of DMM v input impedance of op-amp? And the fact Vref is grounded on a single supply system.

The 'error' the OP reports is 0.6mV, or a 2uA differential current in the bridge. That could easily be leakage over the breadboard due to moisture. Depending on the way it's been laid out, equivalent leakage of 1Mohm to +5 on one side of the input and the same to ground on the other would give the error.
 
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