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May I use IA in this way?

I want to turn a formula into circuit by using OP AMP or IA.The formula is Vout=(Vref-Vin)*Gain.So,i constructed a circuit using AD8577 by taking advantage of IA's differential amplification capability.I attach the block diagram of my design.All signals are DC voltages.Obviously,variable Vref is controlled by the DAC output.Vin means external signal input.And the Gain is 28,the default setting of AD8557.

When i set the DAC output a bit greater than the input signal(Vref>Vin),the voltage diffrence is normally amplified but with large voltage fluctuations.It is low frequency noise,and i cannot deal with it. I doubt whether IA can be used in this way,because i found most IAs are used in bridge applications.

Would you like to share your opinions with me and give me a help?Thanks for your attention in advance..

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