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LT6203 as Buffer Amplifier

My goal is to implement an analog-to-digital conversion. Therefor I used the LT6203 as an input buffer. The analog signal  is directly applied to the OpAmp (IN+ and IN-) as shown in the attached schematic:

I supplied the LT6203 with +/-5V and have executed single-ended measurements with an input range of -5V to +5V, which were not satisfactory (nothing connected to BufferP and BufferN). While there is nothing to criticise for the results of negativ input voltages, in case of positive inputs, the deviation badly increases by approaching the upper rail. So I applied voltages from an external power supply unit to IN+ and referenced IN- to GND and there are deviations like this:

IN+ = 4.40V     BufferP = 4.4019V     BufferP-to-BufferN = 4.4011V

IN+ = 4.75V     BufferP = 4.7522V     BufferP-to-BufferN = 4.7514V

Is there a posibility to get more precise results (deviations beneath 1mV)  or has anyone an idea what might be the reason of this inaccuracies?

I have also tested other amplifiers, like the LT1819 and the ADA4851, and repeated the same measurement, but the results were even more bad:

LT1819: IN+ = 2.0V      deviation = 1.4mV

               IN+ = 4.4V      deviation = 19.5mV

ADA4851: IN+ = 3.5V      deviation = -7.77mV

                  IN+ = 4.0V      deviation = -396.0mV

Especially the ADA4851 delivers precise results for negative input voltges and for lower positive voltages (up to 3V). It would perform well, apart from the mentioned problems at higher voltages.

Am I doing something wrong or do I have overlooked something? I would appreciate if anyone has some tips, alternatives for more precise amplifiers or an idea what might be the source of error.

  • Hello,

    For the LT6203;
    The BufferP and BufferP-to-BufferN are measured values.
    Is the IN+ value is also a measured value? or this is the set value of your external supply?

    If the IN+ value is a measured value, please try to configure your LT6203 into single ended supply, 5V GND. Then you may apply positive input.
    If the result will still have high deviation w/ respect to your input, please try to use 10V GND single ended supply.
    Please let me know the result if possible.

    For the LT1819 and ADA4851, you have worse deviation when you have more positive input.
    Assuming you have used 5V for your +VS, your positive inputs will be out of your input voltage range.
    For the LT1819, the positive input would be 3.5V and for the ADA4851, the positive input would be 2.8V.

    There are also other Rail-to-Rail Input/Output amplifiers that have low Offset Voltage.
    You may check ADA4807 and ADA4805.

    Thanks ad regards.

  • Hello,

    thanks a lot for your response. 

    Is the IN+ value is also a measured value? or this is the set value of your external supply?

    IN+ is the set value of my external supply.

    I see that my inputs did not comply the input voltage ranges of the LT1819 and the ADA4851. Thanks for the hint, that is the reason for the bad results. Thus, these two amplifiers aren't an option anymore.

    Falling back on the LT6203:     It is striking, that the deviation is about 1mV for higher positive inputs. Has anyone an idea what might cause this error of 1mV? Has it something to do with the input offset voltage, which I found in the datasheet of the LT6203: 

    Or is there another reason?

    Many thanks in advance.

  • Hi,

    Since the +IN value is the set value of the external power supply, I would like you to measure the output of the external power supply vs. the set value. In this case, we can eliminate possible offset caused by external power supply.

    Second, you are correct that Vos will affect your expected result.
    It is also recommended to consider the max value if there is a specified value rather than the typical value.
    For this case, please consider the max values depending on your condition to expect the worst case.

    Thanks and regards.