Design Goals

Design Description

This circuit combines an inverting and non-inverting amplifier to make a reference voltage adjustable from the negative of the input voltage up to the input voltage. Gain can be added to increase the maximum negative reference level.

Design Notes

1. Observe the common-mode and output swing limitations of the op amp.
2. Mismatch in R₁ and R₂ results in a gain error. Selecting R₂ > R₁ increases the maximum negative voltage, and selecting R₂ < R₁ decreases the maximum negative voltage. In either case, the maximum positive voltage is always equal to the input voltage. This relationship is inverted if a negative input reference voltage is used.
3. Select the potentiometer based on the desired resolution of the reference. Generally, the potentiometers can be set accurately to within one-eighth of a turn. For a 10-turn pot this means alpha (∝) may be off by as much as 1.25%.

Design Steps

Alpha represents the potentiometer setting relative to ground. This is the fraction of the input voltage that will be applied to the non-inverting terminal of the op amp and amplified by the non-inverting gain.

The transfer function of this circuit follows:

1. If R₂ = R₁ = 20kΩ, then the equation for V_o simplifies as the following shows:
   ${\mathrm{V}}_{\mathrm{o}}=(2\mathrm{\alpha }-1)\times {\mathrm{V}}_{\mathrm{i}}$
2. If V_i = 10V and ∝ = 0.75, the value of V_o can be determined.
   ${\mathrm{V}}_{\mathrm{o}}=(2\times 0.75-1)\times 10=\text{5}\mathrm{V}$

Design Simulations

DC Simulation Results

Design References

Texas Instruments, SBOMAU2 TINA-TI™ circuit simulation, file download

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