[SOLVED] fully differential opamp veriloga

hello ,
I found difficulty in proving the transient response of continuous time amplifier
attached is the veriloga code , the test bench , and the o/p transient
the transient is differential with no feedback (as OTA) , but when applying the input and fb capacitance , the o/p is no longer differential
please help me

`include "discipline.h"
`include "constants.h"

// model opamp - Non Ideal OpAmp Model


module opamp (voutp,voutn, vin_p, vin_n, vspply_p, vspply_n, vcm );
input vin_p , vin_n , vcm;
inout vspply_p, vspply_n;
output voutp,voutn ;
electrical vin_p, vin_n, voutp, voutn, vspply_p, vspply_n, vcm;


parameter real gain = 1412.5 exclude 0.0;
parameter real pole_freq = 172k;
parameter real rin = 12.0M exclude 0.0;
parameter real rout = 75.0;
parameter real ibias = 0.0n;
parameter real vin_offset = 0.0u;


// parameter real slewp = 20.0M from (0:inf);
// parameter real slewn = -20.0M from (-inf:0);

real c1, r1;
real r_rout,gm_nom, vin_val;
electrical coutp, coutn, vref;


analog
begin

@(initial_step)// or initial_step("dc", "ac", "tran", "xf"))
begin
gm_nom = 1.0;
r1 = gain/gm_nom;
c1 = gm_nom/(`M_TWO_PI * pole_freq * gain);
r_rout = rout;

end

vin_val= V(vin_p, vin_n) + vin_offset;


// ------ Vref is at Virtual Ground

V(vref,vspply_n) <+ 0.5*V(vspply_p,vspply_n);



// ------ Input Stage

I(vin_p, vin_n) <+ vin_val / rin;
I(vref, vin_p) <+ ibias;
I(vref, vin_n) <+ ibias;



// ------ GM stage


I(vcm, coutp) <+ (gm_nom/2)*vin_val ;
I(vcm, coutn) <+ -(gm_nom/2)*vin_val ;



// ------ Dominant Pole.

I(coutp, vcm) <+c1*ddt(V(coutp, vcm));
I(coutp, vcm) <+ V(coutp, vcm)/r1;

I(coutn,vcm) <+ c1*ddt(V(coutn, vcm));
I(coutn, vcm) <+ V(coutn, vcm)/r1;




// ------ Output Stage.

I(vcm, voutp) <+ (V(coutp) - (vcm))/r_rout;
I(voutp, vcm) <+ V(voutp, vcm)/r_rout;

I(vcm, voutn) <+ (V(coutn) - V(vcm))/r_rout;
I(voutn, vcm) <+ V(voutn, vcm)/r_rout;



end
endmodule

any advice ?
I do think that my code is right, but the problem is in the test bench.
should the test bench contain switches (switched cap) , or is it enough as attached ?

The code seems to be right ( except perhaps a typo in the first line in Output stage- (vcm) instead of V(vcm) ).
With resistive feedback, there should not be any problem. However you cannot use an opamp as in your test bench, without any feedback for DC. However, if you just want to see the transient response you can give an initial condition to the opamp input and output (initialize to vcm).

Thank you monsoon
Regards,
Ali