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INA851: Quiescent current with asymmetric supplies

Lucy Fanelli76
Prodigy 80 points
Part Number: INA851

I would like to power an INA851 with +10V, -5V, and drive the input with a common voltage of 0V. So the input common mode will not be the midpoint of the supplies. The output common mode is 2.5V. Can I expect more quiescent current draw than the stated maximum of 9mA with this configuration? Is there any other downside of asymmetric supplies?

Thanks,

Lucy

  • 0
    TI__Mastermind 49965 points

    Hello Lucy,

    Assuming the INA851 is unloaded, in quiescent condition, VIN=0V, and Gain = 1V/V, the quiescent current will be the maximum 9mA.

    The current consumption will change if the output stage is loaded since the output will drive the load.  The input stage will also consume slightly more current when set on a GIN>1V/V as a function of the input differential voltage, as current has to flow through the external RG resistor and the internal gain resistor network:

    Lucy Fanelli76 said:
    Is there any other downside of asymmetric supplies?

    Please check the input and output range of the device using the INA851 Input-Output Range Design Calculator

    The INA851 is similar to standard instrumentation amplifiers, the main difference is that incorporates a fully-differential amplifier output.  Each amplifier inside the INA has it’s own input and output swing limitations.   These limitations combine into an overall input differential and common mode voltage versus output swing range limitations. This relationship can be complex, therefore, we offer an excel calculator to verify the device is inside the input/output linear range.  

    For example with Front Gain = 1V/V, output stage gain, BE Gain =1V/V. Vs+=VCLAMP+=+10V; Vs-=VCLAMP-=-4V; (clamps disabled), the device will offer the highest voltage range when input common-mode voltage (average of the IN+/IN-inputs) is centered around VICM=+3V; allowing a maximum fully-differential signal of ±9V.  However, if the input-common mode signal is centered at 0V (GND), the maximum allowed signal is allowing a fully-differential signal of ±3V.  See plot below, you can adjust the required input common-mode per your application.

    Regarding the INA851 Input voltage range Calculator (excel) tool:

    You can download the excel tool on the link below and verify the amplifier is in range:

    INA851 Input-Output Range Design Calculator

    The tool consists of two sheets:

    On the first sheet, “INA851_Vin_Vout_tool”:

    The User fills the highlighted cells in blue with required voltages: VS+, VS-, Front-End Gain, Back-End Gain, Input differential voltage, input common-mode Voltage, VOCM voltage, VCLAMP+ and VCLAMP- (if Clamps Enabled). The Excel provides resulting VOUT+, VOUT-, and VOUT differential voltages.  The tool highlights in red any voltage out of range, and displays warnings in red if the device is outside of linear range.

    Similarly, the second sheet,  the tool provides an Operating Range Chart provides with the valid input & output ranges for this Instrumentation Amplifier. Use VICM to scroll bar to adjust the input common mode voltage.

    Attached is a pdf document with instructions:

    INA851 Input and Output Range Design Calculator Instructions.pdf

    If you have additional questions, let me know the load the INA851 is driving, the input and output stage gains, the clamp voltages (if using the clamps). the maximum differential voltage required, and the input common-mode voltage range.

    Thank you and Regards,

    Luis

  • 0
    Prodigy 80 points

    Hi Luis,

    The ranges all check out OK in the excel sheet. My question is, if all things are equal, does the device consume more current if the supplies are asymmetric versus if the supplies are symmetric? If I'm using the same gain and driving the same load, and my input voltage common mode is 0V, will the device consume more current on +10V/-5V than it would consume on +10V/-10V.

    Thanks,

    Lucy

  • 0 in reply to Lucy Fanelli76
    TI__Mastermind 49965 points

    Hi Lucy,

    In quiescent condition, with the output un-loaded, when the device is in the valid linear range, and 0V differential input signal, the current consumption will not exceed the max quiescent current spec while using asymmetrical supplies.  The current consumption will increase with differential input signals; AC input signals and/or while driving any load.

    Thank you and Regards,

    Luis 

  • 0 in reply to Luis Chioye
    Prodigy 80 points

    Hi Luis,

    Do you know in terms of asymmetrical vs symmetrical, whether asymmetrical consumes more current than symmetrical? There will be a varying input differential voltage up to 0.8V, the gain is 5, the output common mode is 2.5V, the load is 14k. The input common mode is 0V. I know that the current consumption will exceed the stated maximum with these conditions because of the load and the gain. But only in terms of the power supplies-does asymmetrical consume more current than symmetrical?

  • 0 in reply to Lucy Fanelli76
    TI__Mastermind 49965 points

    Hi Lucy,

    Lucy Fanelli76 said:
    Do you know in terms of asymmetrical vs symmetrical, whether asymmetrical consumes more current than symmetrical? There will be a varying input differential voltage up to 0.8V, the gain is 5, the output common mode is 2.5V, the load is 14k. The input common mode is 0V. I know that the current consumption will exceed the stated maximum with these conditions because of the load and the gain. But only in terms of the power supplies-does asymmetrical consume more current than symmetrical?

    Using asymmetrical supplies will not result in higher current consumption.

    Lucy Fanelli76 said:
    If I'm using the same gain and driving the same load, and my input voltage common mode is 0V, will the device consume more current on +10V/-5V than it would consume on +10V/-10V.

    The setup using +10V and -5V will consume slightly less current than when using +10V/-10V. The reason the +10V and -5V setup will consume slightly less current is because the total potential across the voltage supplies (VS+) - (VS-) is a total supply of 15V on the asymmetrical setup; while on the  +10V/-10V setup the total voltage across the supplies is 20V.  That said, if the total potential is kept the same across the supplies, using symmetrical or asymmetrical supplies will make no difference.

    Thank you and Regards,

    Luis