AD8223B reference input voltage clamping

I'm working on a design that's going to be using an AD8223B inst amp, however in my breadboard circuit, I can't seem to get the reference voltage to go below about 0.61V.  I'm using it in a single supply configuration with Vs = 12V.  The gain resistor is 3.09kohm.  The input is a differential voltage of approximately 8mV with a common mode voltage of approximately 5V.  The reference circuit consists of a voltage divider feeding an op amp buffer that directly feeds the reference pin on the AD8223.  When I put a potentiometer on the input of the buffer to vary the voltage on the reference pin, I see the voltage clamp at about 0.61V.  Changing the input of the op amp buffer does not result in a lower voltage at the reference pin.  I'm trying to set the voltage to approximately 0.25V for my configuration. 

The datasheet indicates the voltage range for the reference pin as -Vs to +Vs with a 5V single supply, however for my configuration this doesn't seem to be the case.  Is the datasheet correct?  Any help would be greatly appreciated.  Thanks. 

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  • 0
    •  Analog Employees 
    on Jun 22, 2011 6:46 PM over 9 years ago

    Hi Bert,

    It sounds like it may be a current sinking issue.  Let's figure out how much current the op amp needs to sink from the AD8223 so we can verify if the op amp can handle it.  Please refer to figure 31 in the AD8223 for the following calculations:

    The AD8223 has a reference impedance of 60k ohm (the 10 kohm plus the 50 k ohm in figure 31).   This impedance will be between the reference pin and the +IN input's amplifier output.  To get an idea of the current, we need to figure out this preamp's output voltage.  It is going to be the common mode voltage plus 1/2 of the gained up differential voltage, plus an extra diode shift due to the AD623's input architecture.   Common mode voltage is 5V.  1/2 of gained up differential voltage is 8 mV * 31 * 1/2 =  124 mV.  We'll say the diode drop is 0.65V.   So a pretty good estimate for the output voltage of the +IN preamp is:  5 + 0.124 + 0.65 = about 5.8V.   5.8V/60k ohm = about 100 uA.

    From what I can tell from the LM358 datasheet, it looks like it has difficulty sinking much current below about the 0.6V you are seeing.  So I think is what may be the issue.

    We have a wide selection of op amps that can easily handle the 250 mV requirement while sinking the AD8223's current.  Harry can probably give a better recommendation than I can, but something like the AD8567 comes to mind.

    Matt

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  • 0
    •  Analog Employees 
    on Jun 22, 2011 6:46 PM over 9 years ago

    Hi Bert,

    It sounds like it may be a current sinking issue.  Let's figure out how much current the op amp needs to sink from the AD8223 so we can verify if the op amp can handle it.  Please refer to figure 31 in the AD8223 for the following calculations:

    The AD8223 has a reference impedance of 60k ohm (the 10 kohm plus the 50 k ohm in figure 31).   This impedance will be between the reference pin and the +IN input's amplifier output.  To get an idea of the current, we need to figure out this preamp's output voltage.  It is going to be the common mode voltage plus 1/2 of the gained up differential voltage, plus an extra diode shift due to the AD623's input architecture.   Common mode voltage is 5V.  1/2 of gained up differential voltage is 8 mV * 31 * 1/2 =  124 mV.  We'll say the diode drop is 0.65V.   So a pretty good estimate for the output voltage of the +IN preamp is:  5 + 0.124 + 0.65 = about 5.8V.   5.8V/60k ohm = about 100 uA.

    From what I can tell from the LM358 datasheet, it looks like it has difficulty sinking much current below about the 0.6V you are seeing.  So I think is what may be the issue.

    We have a wide selection of op amps that can easily handle the 250 mV requirement while sinking the AD8223's current.  Harry can probably give a better recommendation than I can, but something like the AD8567 comes to mind.

    Matt

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