# amplifier's gain problem

#### 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
18,432
Show us a schematic. Words do not suffice.
What kind of resistors are you using? Package and tolerance.

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#### ronsimpson

Joined Oct 7, 2019
2,083
Also show supply voltage(s).
Is the input voltage in the "input voltage range"?

#### Irving

Joined Jan 30, 2016
2,673
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
2,354
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

#### 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)

DC voltage supply for wheatstone's bridge is 2.5 V

#### KeithWalker

Joined Jul 10, 2017
2,354
Sorry, I can't help you there. I specialized in electronic measurements, not simulators.
Keith

#### StayatHomeElectronics

Joined Sep 25, 2008
1,073
Does the reference need to be connected to the middle of the supply range for single supply operation?

#### filip.barisic

Joined Jul 24, 2020
6
Sorry, I can't help you there. I specialized in electronic measurements, not simulators.
Keith
I made this electrical scheme on protoboard and I am just doing data acquisition and visualization with labview

#### filip.barisic

Joined Jul 24, 2020
6
Does the reference need to be connected to the middle of the supply range for single supply operation?
As far as I know for this specific amplifier it does not need to be

#### KeithWalker

Joined Jul 10, 2017
2,354
I made this electrical scheme on protoboard and I am just doing data acquisition and visualization with labview
So, I will repeat; what instrument or device are you using to make the measurements?

#### Irving

Joined Jan 30, 2016
2,673
Yes, I think it does need it
The data sheet says;

8.2.2.9 Single-Supply Operation
The INA333 device can be used on single power supplies of 1.8 V to 5.5 V. Figure 35 shows a basic single-supply
circuit. The output REF pin is connected to mid-supply

#### ronsimpson

Joined Oct 7, 2019
2,083
Yes, I think it does need it
When the two inputs have the same voltage then Vout = Vref.
Vref is often connected to 1/2 supply or to ground.

#### crutschow

Joined Mar 14, 2008
29,815
What is the output when the differential input is 0V?

#### crutschow

Joined Mar 14, 2008
29,815
[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.

#### filip.barisic

Joined Jul 24, 2020
6
So, I will repeat; what instrument or device are you using to make the measurements?
I am using NI Elvis first generation.

#### Irving

Joined Jan 30, 2016
2,673
I am using NI Elvis first generation.
With the all in one DMM/oscilloscope or a voltage i/o module?

#### sparky 1

Joined Nov 3, 2018
717
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
2,673
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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