SS2TF Error

sysm s R1 R2 C1 C2
A = [-(R1+R2)/(C1*R1*R2) 1/(C1*R2); 1/(C2*R2) -1/(C2*R2)];
B = [1/(C1*R1); 0];
C = [0 1];
D = 0;

[n,d]=ss2tf(A,B,C,D);

After executing, i get:

Error using ss2tf (line 26)
The following error occurred converting from sym to double:
Unable to convert expression containing symbolic variables into double array. Apply 'subs' function first to substitute values for
variables.

Error in sstf (line 8)
[n,d]=ss2tf(A,B,C,D);

I am not familiar with subs, can someone explain?

Answer 1

[madhan ravi]

State space representation should be with numerical values and it cnnot contain symbolic values.

Answer 2

[Paul]

If you want the numerical answer, then don't even bother with the symbolic stuff. Just assign values to the variables R1, R2, C1, and C2 and go from there.

If you really want the symbolic expression:

[num,den]=numden(C/(s*eye(2)-A)*B+D)

and then proceed from there.