AD8033, bipolar mode switching

Hello guys am working on photodiode amplifier, and am playing with AD8033. From the datasheet, under section Input Overdrive (Rev D, pg. 16), it states that if the common mode input is within 3V of the positive power rail, the input stage switches to a bipolar mode.
I want to understand, how this switching to a bipolar mode behaves when operated with a single supply of 5V?

In my application, the AD8033 is used with a single 5V rail, as trans-conductance amplifier, with the non-inverting pin held at 2.5V; second AD8033 acts as an integrating servo to effectively AC couple the trans-conductance amplifier.  It appears to power up into 2, linear modes of operation, In one mode, the output is correctly biased around 2.5V, but in the other it is biased at around 3.3V. This problem is not seen when tested with an 8V power rail.

  • 0
    •  Analog Employees 
    on Nov 16, 2020 7:37 AM


    Figure 53 of the data sheet shows the simplified Input Stage of AD8033/34.

    There are two input stages; the JFET input and the Bipolar Input.

    The Input Overdrive Section discussed how the transition occurs between these two input stages and this is applicable for single or split supply.
    The threshold is kept to be +Vs - 3V.
    For +/- 12V supply, the JFET is the input stage up to 9V and for 5V/GND supply, the JFET works up to 2V.
    This can be verified by the Input Voltage Range in the spec table.

    For your application, I would like to request if you can attach your schematic here for us to better understand the problem.

    Thanks and regards.