3.5.1.1 Current Sense Amplifier

To measure the Peltier current, it is necessary to amplify the value from the current shunt to a value that is more usable by the MCU. Without amplification, the voltage induced on the current shunt is simply Ohm’s law:

V_{S H U N T} = I_{P L A T E} * R_{S H U N T}

If the system had a current target of 10A and a shunt resistor of 10 mΩ, this would induce a nominal voltage of 100 mV. For our comparators, we used the internal 8-bit Digital-to-Analog Converters (DACs) with the internal Fixed Voltage Reference (FVR) at 2.048V to set the allowed current conditions. The 2V reference was used instead of the 1V reference due to requirements of the internal Temperature Indicator Module that was also used. By knowing the resolution and voltage reference used for the DAC, the resolution per bit can be found as:

\frac{V_{R E F}}{2^{n}} = \frac{2.048 V}{2^{8}} = 8 \frac{m V}{b i t}

Solving for measurement range, it becomes clear that only a small fraction of the available range of the DAC is utilized:

\frac{V_{N O M}}{V_{R A N G E}} = \frac{100 m V}{2.048 V} ≌ 4.88 %

To improve signal range utilization, the op amp inside of the PIC16F17146 was set up with a gain of 8x using the internal resistor ladder in the configuration shown below, in Figure 3-5.

Figure 3-5. Current Sense Amplifier with the Op Amp Peripheral

This immediately increases the nominal signal range utilization by the gain of 8x to:

\frac{800 m V}{2.048 V} ≌ 39.06 %

Which provides a lot more measurement headroom than before.