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LMH6702: Low voltage Amplifier using LMH6702

? ??
Prodigy 10 points
Part Number: LMH6702

Tool/software:

Hi,

i have a problem about LMH6702.

I want to use Low voltage Amplifier, see the circuit below:

i expected gain_Value as '11 ' and Vout_level as '1~1.1V' 

but, Vout level was always(Vin either 0V or 0.15mV) 4.5~4.7V alike Vcc(5V)

How can i get a Vout value i expected?

Is it a parameter i have to consider real non-inverting curcuit?

And Could you recommend op-amp?

Best Regards.

  • 0
    TI__Intellectual 1485 points

    Hi,

    I see a few problems with your circuit:

    1. The total supply range is 8V to 12V, and the above circuit is only 5V. 

    2. The offset voltage (max) for LMH6702 is 4.5 mV and your DC input signal is 0.15 mV. So you input signal is buried in offset voltage and you won't get accurate output.

    3. Also please keep the input within the valid input common mode range.

    For recommending, I would suggest an opamp with less offset voltage which would result in less error at output side. Also please take care of the supply voltage range & also the input common mode range.

    If you want to see the effect of offset voltage on output voltage in difference amplifier circuit configuration, please have a look at the Shunt Current Sensing tool below-
    https://dev.ti.com/gallery/view/CircuitDesignStudio/Analog_Circuit_Design_Studio/ver/1.0.0/

    Thanks

    Aditya Gosavi

  • 0 in reply to Aditya Gosavi
    Prodigy 10 points

    Hello, Aditya.

    I have a question about your answers:

    1. In LMH6702 Datasheet, Vs MAX LEVEL is -6.75V to +6.75V. If i use Vcc- level as '0V',  Isn't the MAX Vcc+ I can use 6.75V?  

    2. I have a mistake,,, It is 150mV, Not 0.15mV. In this case, Could you give a advice,,,?

    Thanks.

    JEON

  • 0 in reply to ? ??
    TI__Intellectual 1485 points

    Hi Jeon,

    1. A small clarification:

    a. If the max supply level is ±6.75V given, it means Vs+ = 6.75V and Vs- = -6.75V.

    b. If the "Total supply voltage" range is given it means (Vs+ – Vs-) parameter.

    If you open the product page of LMH6702 you can see the total supply voltage range(recommended) as 8-12V. So I would suggest you to use Vs+ = 5V and Vs- = -5V as supply voltage (to make the total supply voltage as 10V which is well within the range).

    2. For 150 mV, this configuration will not work with your supply, because the CMIR (common mode input range) of this OpAmp is ±1.9V (for ±5V supply) which means your Vcm should be at least 3.1V away from the rails. When Vin = 150 mV, the Vcm ~ 75 mV which violates the input common mode range from data sheet.  
    So for this configuration to work, you will have to change the supply to Vs+ = 5V and Vs- = -5, (or ±6).

    3. Also I would suggest to use Rf < 1kΩ (Preferably Rf~500Ω and Rin = 50Ω), as Rf = 100kΩ may cause stability issues in the circuit, due to the OpAmp's inherent cap at the inverting input.

    Thanks 

    Aditya Gosavi