8.4.2 Single-Supply Operation (2.7 V to 5.4 V)

Often, newer systems use a single power supply to improve efficiency and reduce the cost of the power supply. The OPA2834 is designed for use with single-supply power operation and can be used with single-supply power with no change in performance from split supply, as long as the input and output are biased within the linear operation of the device.

To change the circuit from split-supply to single-supply, level shift all voltages by half the difference between the power-supply rails. For example, Figure 51 shows changing from a ±2.5-V split supply to a 5-V single supply.

Figure 51. Single-Supply Concept

A practical circuit has an amplifier or some other circuit providing the bias voltage for the input, and the output of this amplifier stage provides the bias for the next stage.

Figure 52 shows a typical noninverting amplifier circuit. With a 5-V single-supply, a mid-supply reference generator is needed to bias the negative side through R_G. To cancel the voltage offset that is otherwise caused by the input bias currents, R₁ is selected to be equal to R_F in parallel with R_G. For example, if a gain of 2 is required and R_F = 3.6 kΩ, select R_G = 3.6 kΩ to set the gain, and R₁ = 1.8 kΩ for bias current cancellation. The value for C is dependent on the reference, and TI recommends a value of at least 0.1 µF to limit noise.

Figure 52. Noninverting Single-Supply Operation With Reference

Figure 53 illustrates a similar noninverting single-supply scenario with the reference generator replaced by the Thevenin equivalent using resistors and the positive supply. R_G’ and R_G” form a resistor divider from the 5-V supply and are used to bias the negative side with the parallel sum equal to the equivalent R_G to set the gain. To cancel the voltage offset that is otherwise caused by the input bias currents, R₁ is selected to be equal to R_F in parallel with R_G’ in parallel with R_G” (R₁= R_F || R_G’ || R_G”). For example, if a gain of 2 is required and R_F = 3.6 kΩ, selecting R_G’ = R_G” = 7.2 kΩ gives an equivalent parallel sum of 3.6 kΩ, sets the gain to 2, and references the input to mid supply (2.5 V). R₁ is set to 1.8 kΩ for bias current cancellation. The resistor divider costs less than the 2.5-V reference in Figure 53 but can increase the current from the 5-V supply.

Figure 53. Noninverting Single-Supply Operation With Resistors

Figure 54 shows a typical inverting-amplifier circuit. With a 5-V single-supply, a mid-supply reference generator is needed to bias the positive side through R₁. To cancel the voltage offset that is otherwise caused by the input bias currents, R₁ is selected to be equal to R_F in parallel with R_G. For example, if a gain of –2 is required and R_F = 3.6 kΩ, select R_G = 1.8 kΩ to set the gain and R₁ = 1.2 kΩ for bias current cancellation. The value for C is dependent on the reference, but TI recommends a value of at least 0.1 µF to limit noise into the op amp.

Figure 54. Inverting Single-Supply Operation With Reference

Figure 55 illustrates a similar inverting single-supply scenario with the reference generator replaced by the Thevenin equivalent using resistors and the positive supply. R₁ and R₂ form a resistor divider from the 5-V supply and are used to bias the positive side. To cancel the voltage offset that is otherwise caused by the input bias currents, set the parallel sum of R₁ and R₂ equal to the parallel sum of R_F and R_G. C must be added to limit the coupling of noise into the positive input. For example, if a gain of –2 is required and R_F = 3.6 kΩ, select R_G = 1.8 kΩ to set the gain. R₁ = R₂ = 2.4 kΩ for the mid-supply voltage bias and for op-amp input-bias current cancellation. A good value for C is 0.1 µF. The resistor divider costs less than the 2.5-V reference in Figure 55 but can increase the current from the 5-V supply.

Figure 55. Inverting Single-Supply Operation With Resistors