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

Most newer systems use a single power supply to improve efficiency and to simplify power-supply design. The OPAx837 can be used with single-supply power (ground for the negative supply) with no change in performance from split supply, as long as the input and output pins are biased within the linear operating region of the device. The outputs nominally swing rail-to-rail with approximately a 100-mV headroom required for linear operation. The inputs can typically swing 0.2 V below the negative rail (typically ground) and to within 1.2 V of the positive supply. For DC-coupled single-supply operation, the input swing is below the available output swing range for noninverting gains greater than 1.30 V/V. Typically, the 1.2-V input headroom required to the positive supply only limits output swing range for a unity-gain buffer.

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 69 depicts changing from a ±2.5-V split supply to a 5-V single-supply. The load is shown as mid-supply referenced but can be grounded as well.

Figure 69. Single-Supply Concept

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

Figure 70 shows a typical noninverting amplifier circuit. With 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 V/V is required and R_F = 2 kΩ, select R_G = 2 kΩ to set the gain, and R₁ = 1 kΩ for bias current cancellation which reduces the output DC error to I_OS × R_F. The value for C is dependent on the reference, and TI recommends a value of at least 0.1 µF to limit noise. The frequency response flatness is impacted by the AC impedance, including the reference and capacitor added to the R_G element.

Figure 70. Noninverting Single-Supply Operation With Reference

Figure 71 shows 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 V/V is required and R_F = 2 kΩ, selecting R_G’ = R_G” = 4 kΩ gives an quivalent parallel sum of 2 kΩ, sets the gain to 2, and references the input to mid-supply (2.5 V). R₁ is set to 1 kΩ for bias current cancellation. The resistor divider costs less than the 2.5-V reference in Figure 70 but increases the current from the 5-V supply. Any noise or variation on the 5-V supply now also comes into the circuit as an input through the biasing path.

Figure 71. Noninverting Single-Supply Operation With Resistor Mid-Supply Biasing

Figure 72 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 V/V is required and R_F = 2 kΩ, select R_G = 1 kΩ to set the gain and R₁ = 667 Ω 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 72. Inverting Single-Supply Operation With Reference

Figure 73 shows 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 value of R₁ and R₂ equal to the parallel value of R_F and R_G. C must be added to limit coupling of noise into the positive input. For example, if gain of –2 V/V is required and R_F = 2 kΩ, select R_G = 1 kΩ to set the gain. R₁ = R₂ = 2 × 667 Ω = 1.33 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 72 but increases the current from the 5-V supply. Any noise or variation in the 5-V supply also comes into the circuit through this bias setup but be band-limited by the pole formed with R₁ || R₂ and C.

Figure 73. Inverting Single-Supply Operation With Resistor Midsupply Biasing

These examples are only a few of the ways to implement a single-supply design. Many other designs exist that can often be simpler if AC-coupled inputs are allowed. A good compilation of options can be found in the Single-Supply Op Amp Design Techniques application report.