Verilog AMS problem in mentor graphics

[sina.parsnejad]

Hi people
I have been trying to implement the following code which is a current mode comparator into mentor graphics ADMS tool; I have been receiving the following error:
Analog DC computation aborted : no DC convergence found in this design
Is there any problem with my code ? The schematic is also attached.


`include "disciplines.h"
module current_mode_comparator_AMS_Verilog_AMS (OUTplus,OUTnega,INplus,INnega);
//inout OUTplus,OUTnega,INplus,INnega;
//electrical OUTplus,OUTnega,INplus,INnega;
inout INplus,INnega ;
output OUTplus,OUTnega ;
current OUTplus,OUTnega ;
electrical INplus,INnega ;
parameter real positive_amp = 30n ;
parameter real negative_amp = -30n ;
parameter real VIN = 0.6 ;

analog begin
I(OUTplus) <+ positive_amp*tanh(1000*(I(INplus)-I(INnega))) ;
I(OUTnega) <+ negative_amp*tanh(1000*(I(INplus)-I(INnega))) ;
V(INplus,INnega) <+ VIN ;
end

endmodule