Question of building a summer/subtractor circuit with a single op-amp and SPDT switch

Hi All, I am building a switchable summer/subtractor circuit using a conventional opamp and MAX4855 SPDT with its integrated comparator.
This is my schematic. VDC1 is a user controllable DC input, it will be used to determine the logic high or low of the comparator, and will be used for the summing or subtraction. VDC2 is a second voltage source with a fixed DC level.
I build this circuit on board, VDC1 and VDC2 are in the correct levels, and the opamp itself was tested before connected with the other signals.However, when I connected everything together, and inject a VDC1, comparator shows the correct 1/0 logic, but the opamp does not work properly. The output of the output is either tied high or tied down, instead of generating the proper sum/subtract signal.
Would anyone give me some suggestion, if there is any practical problem in this cirucit? Or how should I investigate the root cause of the built circuit? Thank you!

What are the resistor values?

crutschow said:

What are the resistor values?

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Hi crutschow, what I am using are:R1=25k, R2=56K,R3=R4=10k, VDC2=0.3V, VDC varies from 1V to 3V.

What are the power supplies?

For the summer circuit, did you intend R1 & R3 to be different values? This causes different weighting factors for your two voltages which you are comparing.

Another thing that can make a difference: VDC2 needs to be able to sink current coming from VDC1. If it is not able to, then you can't be sure of getting a correct summed voltage at the non-inverting input of your op amp.

Are there unseen/unknown resistances upstream from R1 & R3? Diodes, transistors, etc.? Something that might help is to install a resistor at the non-inverting input of the op amp.