ADA4350 outputs a small DC voltage.

drive.google.com/.../view

Hello,

Recently designed PCB has a ADA4350 that is connected to a AD4000. Noticed 75mVDC output at pin3 (VOUT1) once the TIA is enabled - pin 16 pulled high. From  the attached schematic, C43 and R22 are removed and R21 is replaced with a solder blob to have an ideal 0-ohm connection to ground (pin 11 is grounded). The feedback paths are normally open according to figure 55 of datasheet. It must be the first stage amplifier that is causing these error voltages, or is it?

Also probed pin 12 after removing R27 and found those 75mV popped up as soon as EN goes high.

Need your expert opinion on how to remove of significantly reduce these DC voltage.

Thanks,

If the attached jpeg is difficult to read let me know and I will try to send a different link of a bigger and better jpeg.



Updated the jpeg file link.
[edited by: Mohi at 11:14 AM (GMT -4) on 18 Jun 2021]
    •  Analog Employees 
    on Jun 18, 2021 7:46 AM

    Hi,

    The attached image is too small and could not see detail.
    Kindly upload another image.

    Thank you!

  • Hello Fdelaram,

    Please use this link from google drive. It is much better.

    drive.google.com/.../view


    Also, to rule out any other possibilities of this 75mV issue, removed all feedback resistors and R27 connecting SWA_IN to SWB_IN. Enabled TIA, probed and found 75mV at pin 12 - the output of the first stage amplifier. When TIA is disabled, pulled pin 16 low, this voltage disappears.


    Thanks,

    •  Analog Employees 
    on Jun 21, 2021 4:23 AM in reply to Mohi

    Hi,

    Thank you for uploading new clearer image and for the detailed problem description as well for the initial isolation.

    What is the signal level of your input during this state? Is it 0V?
    I suspect the output is trying to hit your negative rail which is GND but is limited by Output Voltage Swing.

    Is it possible for you to use split supply? This is to check if the 75mV will still appear on said condition.

    Thanks and regards!

  • I did just that yesterday. Now ADA4350 runs from ±5VDC. However, the output reduced to 45mVDC but never reaches zero VDC.

    I have tried no input or 0ADC input from a Keithley current source; in any case, the minimum output never reaches zero.

    Can you explain "I suspect the output is trying to hit your negative rail which is GND but is limited by Output Voltage Swing." For zero or no input, the output should hit the ground.

    What else can be done to bring the output down to zero?

    Thanks,

    •  Analog Employees 
    on Jun 22, 2021 3:05 AM in reply to Mohi

    Hi,

    Just wanted to confirm few things.
    When you have used split supply of +/-5V and measured 45mV at pin12, is the circuit is still in open loop?

    From the previous setup, where you have used single supply of 5V/GND, you have removed the feedback loop so I am seeing the op-amp to behave as comparator. And since the IN+ is tied to GND, I presume the amplifier would see the offset voltage at IN- and this will make the output saturate to the negative rail which is ground or 0V. However, the Output Voltage Swing Low will limit the op-amp to hit 0V, the minimum it can provide only for single supply seen on table 6 is 0.08V to 4.87V where the 75mV is close to this typical value.

    I suggested to validate this using split supply and I am expecting the output to be close to negative rail, -5V in your case(if open loop).
    However, your output is now down to 45mV which made me think if the op-amp is really in open loop.
    If it is in close loop configuration, the measured output voltage should be the offset voltage which is in uV level only.

    While still checking on possible things that I miss here, can you check the voltage level at IN-?

    Thanks and regards!