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OPA2376-Q1: Lowest V+ that can achieve unity gain

Part Number: OPA2376-Q1
Other Parts Discussed in Thread: OPA2376, LM7705

Tool/software:

Hi Team,

As title, below is our test result.

As you can see, our V+ that we measured was 0.3mV and Vout was 1mV. May you share your expertise why I get this result?

In addition, what is the minimum value of V+ that we recommend?

Regards,

Roy

  • Roy,

    The input voltage in rail-to-rail input/output op amps may get all the way to its rail BUT output CANNOT - see below. In the case of OPA2376-Q1 at 25 deg C, the linear range of the input voltage extends all the way to the negative rail but the output can only get linearly within 50mV above negative rail (see conditions of AOL) - this has nothing to do with the input bias current, IB.

    The minimum value for the input voltage at room temperature is the same as negative rail (in single supply application Vin>/=0) BUT at high temperature Vos will increase for Vin=0.  For this reason, in order to minimize offset you should increase the input common-mode voltage above ground at higher temperature.

  • Hi Marek,

    Thank you for your comments. Still has some questions

    1. According to attached datasheet spec, I am not sure how 0.3mV and 1mV generated?

    2.If we want to let our output = 0V, we should add Vos on IN+, is it correct? In datasheet, it's 25uV?

    Regards,

    Roy

  • The OPA2376 max IB at 25C is +/-10 pA but the input bias current doubles for every 10 deg C increase in junction temperature so it could be at 300pA at hot temperature - see Fig 6-13.

    2. There is no way to drive the output all the way to its negative rail using only its input.  For OPA2376 output to swing to 0V would require using a negative supply voltage like LM7705 (negative charge pump) or connect 10k pull-down resistor between the output and -5V - see below.

  • Hi Marek,

    Thank you.

    For 1st question, the datasheet shows the IB may reach out to 300pA when 125C junction. But in our test, we operate in amb at 25C. The junction wouldn't be high.

    For 2nd question, may you help share why the architecture can let Vout = 0V when Vin = 0V?

    Thank you.

    Regards,

    Roy

  • 1.  Only at 125c junction temperature OPA2376 IB can be anywhere close to 300pA - see Fig 6-13. 

    What you measuring is the rms noise of the 1M input resistor.  Thermal noise spectral density of 1Mohm resistor is 126.5nV/rt-Hz (see equation below).

    The total integrated thermal noise over bandwidth is given by following formula:

    Using above formula to calculate 1M resistor noise = 126.5nV/rt-Hz*(5.5e6)^.5 = ~296.7uVrms or ~0.3mVrms where OPA2376 BW = 5.5MHz

    2. For the Vout to swing all the way to ground, Vds would need to be zero but this would require no current, IQo, in the output stage (open-loop operation).

    Below please see MOS transistor characteristics showing that for Vds=0, IQo would need to be zero.  However, there is some minimum quiescent current in the output stage that cannot be turn off by external means.  This leads to a minimum output voltage swing of Vout_min = IQo*Ron where Ron is define by slope of the curve in the ohmic region - see below.