New tricks for the Pico voltmeter
The voltmeter built at the end of the last issue introduced basic metrology. It suffered from low accuracy because the input current of the Raspberry Pi Pico caused high voltage losses across the protection resistor. In this follow-up instalment, we’ll use an operational amplifier to solve this problem. We’ll also add a mode for measuring resistances.
Just as in last month’s issue, the goal of this tutorial isn’t to create the perfect tabletop multimeter. Instead, we want to demonstrate additional metrological concepts and inspire you to perform more experiments on your own.
The current must flow
A resistor in series with the ADC input turned out to be a great way to make the multimeter more resilient. Connecting 5V to the ADC input would normally destroy the Raspberry Pi Pico, but our system survived even with the reduced value of the protection resistor. This, however, isn’t ideal. Practical multimeters regularly face hundreds or even thousands of volts in spurious inputs. Undoing this resistor value reduction would be beneficial for the longevity of the product, but we need to solve the problem of the ADC input current.
To recap, our problem is caused by the current flowing into the ADC. It’s
