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INA333: Gain setting issue...

Peter Tseng
Mastermind 6490 points

Part Number: INA333

Dears,

I have some question about the gain setting for INA333.

Below is my testing.

V+=5V, V-=0V, Vin-=0V, Vref=0V, signal source is square wave of 130mV input from Vin+. 

And I found the output gain different with what I set, it always around 2.x times.

Vin (mV) Vout (mV) Rg Gain set Gain actual
130 178 100K 2 1.37
130 230 22K 6 1.77
130 262 10K 11 2.02
130 262 5.1K 21 2.02
130 276 1K 101 2.12
130 276 510R 197 2.12
130 276 100R 1001 2.12

Is there something wrong with my test?

Thanks.

  • 0
    TI__Mastermind 23050 points
    Hi Peter,

    You cannot ground Vin- of the INA in a single supply configuration because the internal op amp is not able to drive its internal inverting input to ground. If you are using single supply you need to have a common mode voltage at the inputs. To maximize your output swing, you would want your input signal to have a common mode voltage at mid-supply, 2.5V in your case. You can use this calculator to help you understand the limitations of the INA333 in your desired configuration: www.ti.com/.../ina-cmv-calc

    Regards,
    Zak Kaye
  • 0 in reply to Zak Kaye
    Mastermind 6490 points
    Hi Zak Kaye,

    Did you mean what I need to do is add 2.5V between Vin+ and Vin-, right?
  • 0 in reply to Peter Tseng
    TI__Guru 72061 points

    Peter,

    In order for the INA333 to operate within its linear input voltage range, you have to lift the input common-mode voltage, and not the differential voltage, above the negative rail or 0V - see Vcm vs Vout graphs below for Vref=0V and Vref=2.5V; this may be done with the input voltage level shift but in order to further assist you, you need to specify desired INA333 gain and provide the circuit showing what exactly drives the input voltage signal.