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INA849: INPUT BIAS CURRENT RETURN PATH RESISTOR SELECTION

Kiran K
Prodigy 55 points

Part Number: INA849

Hi,

I am designing a amplifier circuit using INA849. The input to the INA849 is coming from hydrophone array of 96 channels as differential signal . It is given in the datasheet that there should be a input bias current return path. The screenshot of the data sheet part is attached below.

How to choose the resistor value and Inductor value for the same.?

Is the Inductor required for this? (Can i use only resistor? )

Will adding these input bias current return path resistor induce any noise to the signal chain?

What is the recommended value of resistor to be used?

The supply is +/-5V with ref as ground.

Thank you in advance

  • 0
    TI__Guru 71711 points

    The L1 inductor is an example of the input of a microphone or hydrophone so in your case it's already there as part of hydrophone array of 96 channels. Thus, all you need to add are R3 and R4 resistors to provide the path for IB current. 

    The value of the resistors needs to be chosen based on your system requirements; any mismatch in the large value resistors combined with maximum input offset current, Ios, of 6nA will induce offset error but in your application you probably do not need to worry about the input offset voltage.  Therefore, the only other concern with high value input bias resistors is their thermal noise - considering that INA849 input voltage broadband noise spectral density is 1nV/rt-Hz, this is an equivalent of thermal noise of 1kohm/4^2 = ~63ohm resistor (1kohm resistor has a thermal noise of 4nV/rt-Hz). Thus, any input resistors greater than 63ohm will dominate the total input noise.  Having said that, choosing too small resistors, which will appear in parallel with the hydrophone impedance, may overload the input signal. Thus, you must choice the resistor values to meet your system input noise performance BUT without overloading the current driving capability of the hydrophone, which would result in signal being distorted.

  • 0 in reply to Marek Lis
    Prodigy 55 points

    Thanks Marek, 

    The impedance value of single element of array of hydrophone is 2Kohm resistance and 20pF capacitance.

    In this case can you suggest value for resistor which can be used.

  • +1 in reply to Kiran K
    TI__Guru 71711 points

    If your 96 channels are connected in parallel, this would mean that the effective resistance of the entire array is around 20 ohm.  Thus, in order for the bias resistors not to dominate the noise, their value would have to be around 50ohm.

    On the other hand, if the channels are multiplexed (one at the time), the input bias resistors could be around 1kohm.