/* Summing and difference amplifiers */
kill(all)$
display2d:false$

/* Inverting weighted sum */
vo_sum : -Rf*(v1/R1+v2/R2+v3/R3)$
print("weighted-summer output =",vo_sum)$
numeric_sum : ev(vo_sum,Rf=20e3,R1=10e3,R2=20e3,R3=40e3,
                 v1=.30,v2=-.20,v3=.80,numer)$
print("worked weighted sum (V) =",float(numeric_sum))$

/* General difference amplifier */
vp : v2*R4/(R3+R4)$
vo_diff : factor((1+R2/R1)*vp-(R2/R1)*v1)$
print("general difference-amplifier output =",vo_diff)$
matched : ratsimp(subst([R2=k*R1,R4=k*R3],vo_diff))$
print("matched-ratio output =",matched)$

/* Common-mode and differential gains from v1=vcm-vd/2, v2=vcm+vd/2 */
vdecomp : expand(subst([v1=vcm-vd/2,v2=vcm+vd/2],vo_diff))$
Acm : ratsimp(diff(vdecomp,vcm))$
Ad : ratsimp(diff(vdecomp,vd))$
print("common-mode gain =",Acm)$
print("differential gain =",Ad)$

/* 0.1% high-side ratio error for nominal k=10 */
vals : [R1=10e3,R2=100e3,R3=10e3,R4=100.1e3]$
Acm_n : ev(Acm,vals,numer)$
Ad_n : ev(Ad,vals,numer)$
CMRRdB : 20*log(abs(Ad_n/Acm_n))/log(10)$
print("Acm =",float(Acm_n)," Ad =",float(Ad_n)," CMRR(dB) =",float(CMRRdB))$