Circuit 04: Variable Input from 0.625 V to 1.25 V

In this circuit, we want to display a voltage range between 0.625 - 1.25 V.
The battery voltage is 12 V.

Signal Input

The voltage divider with R1 and R_POT1 limits the maximum voltage at SIG to

$$\mathrm{V\_SIG\_MAX}=\frac{\mathrm{R\_POT1}}{\mathrm{R1+R\_POT1}}*\mathrm{12\; V}$$

We want to limit the input voltage at SIG to 1.25 V.

$$\mathrm{V\_SIG\_MAX}=\mathrm{1.25\; V}$$

A trim potentiometer with a value of 1K Ω is easily available.

With these choices, the value for R1 becomes

$$\mathrm{R1}=\mathrm{R\_POT1}*\frac{\mathrm{<mi>12\; V\; -\; V\_SIG\_MAX</mi>}}{\mathrm{V\_SIG\_MAX}}$$

$$\mathrm{R1}=\mathrm{1000\; \Omega }*\frac{\mathrm{<mi>12\; V\; -\; 1.25\; V</mi>}}{\mathrm{1.25\; V}}$$

$$\mathrm{R1}=\mathrm{8600\; \Omega }$$

An 8.6K Ω resistor is not easy to find, but with an 8.2K Ω resistor we get

$$\mathrm{V\_SIG\_MAX}=\frac{\mathrm{1000\; \Omega }}{\mathrm{1000\; \Omega +8200\; \Omega }}*\mathrm{12\; V}=\mathrm{1.3\; V}$$

Lower Threshold

If we set the voltage at RLO to 0.625 V, the comparators will not start to turn on the outputs until the voltage at SIG reaches 0.625 V.

This can be accomplished by adding a voltage divider with the values

• R2 = 3.3 K Ω
• R3 = 3.3 K Ω

and connecting the midpoint to RLO.

The upper end of the voltage divider is connected to REFOUT which is 1.25 V.

The lower end of the voltage divider is connected to ground.