From 63fc30d79ce1b0d65acf713e23b0ec136d153fbb Mon Sep 17 00:00:00 2001
From: Michael Hohenstein <michael@hohenste.in>
Date: Tue, 6 Feb 2024 22:53:25 +0100
Subject: [PATCH] [hard] script to calculate resistor values based on defined
 input ranges

---
 hardware/calc_resistors.py | 50 ++++++++++++++++++++++++++++++++++++++
 1 file changed, 50 insertions(+)
 create mode 100755 hardware/calc_resistors.py

diff --git a/hardware/calc_resistors.py b/hardware/calc_resistors.py
new file mode 100755
index 0000000..c27be1e
--- /dev/null
+++ b/hardware/calc_resistors.py
@@ -0,0 +1,50 @@
+#!/usr/bin/env python3
+
+"""
+# This script is used to calculate the resistor ratios for the hardware layout of the Parallax oscilloscope
+# In the beginning all the input ranges are defined for the analog inputs in the list $In_Ranges$
+# These have to be mapped to a voltage between 0V and $U_{max}$.
+# The script calculates the resistor and capacitor ratio for the attenuator.
+
+# Afterwards it compares all the input ranges with the largest one in the list and calculates
+# the gains for the op-amp and the reference voltage for all the input ranges.
+
+"""
+
+
+# ########## Initial value definition ##########
+
+# define the input voltage ranges
+# this scripts expects the ranges to be symmetrical e.g. 15V means the range goes from -15V to +15V
+In_Ranges = [15, 5, 2, 0.5]  # all values in Volts
+
+# define the input range of the ADC. The minimum ADC input voltage is expected to be Ground
+U_max = 3  # 3 V, better than 3,3 V because for 3 V the resistor values are closer to real resistors and there is some margin before the clamping diodes kick in
+
+# define the reference voltage for the ADC to get the middle in between the ADC range
+U_aref = 3.3  # 3,3 V
+
+
+# print the input rages as the first information
+print(f"Input ranges: {In_Ranges}")
+
+# calculate the resistor and capacitor ratio for the attenuator
+Attenuator_Resistor_ratio = (2 * max(In_Ranges)) / U_max - 1  # R_1 / R_2
+print(f"Resistor ratio: R_1 / R_2 = {Attenuator_Resistor_ratio}")
+
+# the amplifier has to map a smaller voltage range to the range of the maximum input voltage since the attenuator stays the same for each input range
+amp_gains = [max(In_Ranges) / i for i in In_Ranges]
+print(f"Values for the amplifier gains: {amp_gains}")
+
+# the offset voltage needs to be calculated for each individual input range
+# if only one would be used then this will interfere with the Op-Amp also amplifying the baseline
+offsets = [i / Attenuator_Resistor_ratio for i in In_Ranges]
+print(f"Offset Voltages: {offsets}")
+
+# correct the offsets to be in the middle:
+corrected_offsets = [(U_aref / (2 * amp_gains[i])) * (1 / Attenuator_Resistor_ratio + 1) for i in range(len(In_Ranges))]
+print(f"Corrected offset: {corrected_offsets}")
+
+# now calculate the resistor ratios for the voltage follower providing the offset voltage
+resistor_ratios = [U_aref / i - 1 for i in corrected_offsets]
+print(f"Resistor ratios for the offset voltages: R_a / R_b = {resistor_ratios}")