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There are various reasons why you might need to split a voltage rail in your design. Perhaps the design contains a sensor or an IC that requires a bipolar supply, or you need to make the best use of the dynamic range of a bipolar-input analog-to-digital converter (ADC). Another reason to split a voltage rail is if you need a mid-rail bias voltage in an otherwise single-supply rail design.
The term “rail splitter” describes the creation of a new 0-V reference point for a circuit, usually the midpoint of the supply voltage (VDD), divided by 2, of a single-supply-rail VDD. The total available voltage remains the same, except that you can view it as being distributed as a bipolar supply ±VDD/2 above and below the new 0-V reference, which is called a “virtual ground”.
A rail splitter that creates a new virtual ground must be able to source or sink load current, and it must be stable with a capacitive decoupling load on its output. One way to generate a virtual ground is to use an operational amplifier (op amp) configured as a unity-gain buffer.
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Op amp in rail splitting
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The input voltage to the op-amp buffer comes from a resistor-divider that is set to half of the supply voltage (VDD/2). The divider is decoupled with C3 to stabilize it against noise or ripple on VDD (Figure 1). Adding R3 limits any current flowing into the noninverting pin of the op amp. This current can flow as VDD ramps up or down and as the C1, C2 and C3 capacitors are charging or discharging.
Figure 1 Here is how a voltage rail splitter uses an op amp. Source: Texas Instruments
The output of the op amp is therefore Vsplit = VDD/2. The output of the buffer should be capacitively decoupled, as is usually the case with supply rails. However, most op amps will become unstable with even a few tens of picofarads of output load capacitance, requiring additional techniques to make them stable.
So, choose an op amp that is inherently stable with unlimited output load capacitance. For this design case study, we use OPA994, which automatically detects the load capacitance on its output and optimizes its internal compensation to allow for large output capacitors. It also has the ability to maintain the VDD/2 output while sourcing or sinking tens of milliampere loads, as shown in the datasheet curves for different values of VDD.
A safe value for the maximum source or sink current is ±30 mA, given the worst-case effects of temperature and supply-voltage variation, and whether the device is sourcing or sinking current. It’s possible to increase this ±30 mA current by using the datasheet curves that correspond to the application.
We simulated the OPA994 rail splitter in TINA-TI simulation software to check its output frequency response stability. The Bode plot in Figure 2 shows a stable response, with a phase margin of 66.7 degrees at a crossover frequency of 16.65 kHz. To achieve that response, the OPA994 automatically reduced its bandwidth from 18 MHz to 16.65 kHz.
Figure 2 Bode plot simulation of the OPA994 shows output to its inverting input. Source: Texas Instruments
We ran a time-domain simulation of Figure 1 where we applied load transients to the output (Figure 3). This involved switching a 120 Ω resistive load connected between Vsplit and the original 0 V. We applied the load at t = 2 ms and removed it at t = 6 ms. A second 120 Ω resistive load connected between VDD and Vsplit switched in at t = 4 ms and was removed at t = 8 ms. The 120 Ω load is a 2.5 V, 120 Ω ≈ 21 mA transient load current.
Figure 3 Simulation shows how load transients are applied to the output. Source: Texas Instruments
The purpose of the simulation was to look at the voltage deviation of Vsplit and to check for stability, indicated by a well-damped response. In most applications, the load transient will be much smaller, so this simulation is showing a worst case. As you can see, the response is well damped. The deviation of the output is 9 mV for a 21-mA load step (source or sink) and the recovery time is 0.37 ms.
Reference op amp
In an actual application, the load being powered can often be an op-amp signal chain. In the TINA-TI simulation shown in Figure 4, OPA171 op amp is referenced to the Vsplit virtual ground output of the OPA994. The OPA171 op amp is configured as an inverting amplifier with a gain of –100. The voltage input (VIN) to the OPA171 is a ±20 mV peak sine wave, which is also referenced to Vsplit.
Figure 4 The above diagram shows OPA994 split rail and OPA171 inverting amplifier simulation. Source: Texas Instruments
The simulation output of Figure 5 shows that the OPA994 (ILOAD) supplies only a small amount of current (±20 μA) and that the disturbance to the Vsplit rail is negligible. The quiescent current of the OPA171 is sourced from VDD to 0 V, and the only current sourced or sunk by the OPA994 split rail comes from the Vsplit-referenced input and output signals.
Figure 5 The simulation output shows that OPA994 supplies only a small amount of current and the disturbance to the Vsplit rail is negligible. Source: Texas Instruments
The OPA171 output is ±2 V, and there is an additional output-voltage component because of the input offset voltage of the op amp multiplied by its noise gain (that is, a gain of 101). The common-mode rejection ratio of OPA171 attenuates disturbances or offsets on Vsplit.
We tested the circuit shown in Figure 1 along with a switched load of 120 Ω from Vsplit to the original 0 V to test the load transient response. Figure 6 shows the AC-coupled Vsplit output and gives a 4-mV peak-to-peak deviation that mirrors the simulation result.
Figure 6 Test result shows a 120-Ω load switched in and out. Source: Texas Instruments
We then tested the circuit shown in Figure 4 using a ±20 mV, 1 kHz sine-wave input. The test result in Figure 7 shows a ±2 V output (with respect to the Vsplit virtual ground) plus an offset, given the op amp’s input offset voltage.
Figure 7 The above diagram shows OPA171 output (G = –100) with a ±20 mV input. Source: Texas Instruments
As with all op amps, there is a shift in the Vsplit output offset voltage with load current. Figure 1 shows this simulation, while Figure 8 shows the results over a load range of ±40 mA and a 14-mV deviation across that load range.
Figure 8 Vsplit variation is shown with load current. Source: Texas Instruments
The 3-mV offset voltage at 0 mA is attributable to the offset voltage of the op amp, plus the offset voltage from the input bias current in the R1, R2 and R3 resistors. Adding a resistor in the feedback path of the op amp equal to the parallel combination of the divider resistors (R1//R2) plus R3 (6 kΩ) nulls the offset voltage caused by the bias current.
Overcoming rail split challenges
Splitting a voltage rail can seem easy, but capacitance on the output quickly adds complexity. The OPA994 has a supply range of 2.7 to 32 V and can split common voltage rails including 3.3 V, 5 V, 12 V and 15 V. The resistor-divider is external, so it’s also possible to create an unequal split. The device is stable with a large output capacitance and responds in a well-damped way, with good phase margin to load transients.
Dan Tooth is an analog field application engineer at Texas Instruments.
Tyler Holmvik is an analog field application engineer at Texas Instruments.
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