Uses matplotlib to plot linearity error in softpot output with pulldown resistor

softpot_pulldown_error_graph.py

# coding=utf-8
#!ipython --pylab

from pylab import *

def format_resistor_value(value):
    value = int(value) * 1000
    for unit in (u"Ω", u"kΩ", u"MΩ", u"GΩ"):
        value = value / 1000
        if value < 1000:
            break
    return "%d %s" % (value, unit)


fig = figure()
fig.suptitle("SoftPot with pulldown: error relative to linear response")

plot1 = fig.add_subplot(2,1,1)
plot2 = fig.add_subplot(2,1,2)

plot1.set_xlabel("Position (x)") 
plot1.set_ylabel("Vout/Vref") 

plot2.set_xlabel("Position (x)") 
plot2.set_ylabel("Error (%)") 

x = np.linspace(0, 1, 100, endpoint=True)
x[0] = x[1] / 100       # prevent divide by zero

for Ro_val in (1000, 10000, 100000, 1000000):
    Ro = np.linspace(Ro_val,Ro_val, len(x), endpoint=True)
    R1 = 10000 * x
    R2 = 10000 * (1-x)

    Rp = 1/(1/R1 + 1/Ro)
    Vout = Rp / (Rp+R2)
    Videal = R1/(R1+R2)
    error = 100*abs(Vout - Videal)/Videal

    plot1.plot(x, Vout, label="Pulldown Ro=%s" % format_resistor_value(Ro_val))
    plot2.plot(x, error)
    print "Ro = %6s  max error = %4.1f%%" % (format_resistor_value(Ro_val),
                                                max(error))

plot1.legend(loc="upper left", prop={"size":12})