Zero-crossover amplifiers use a unique topology which eliminates the error induced by the crossover region that standard rail-to-rail amplifiers have. TI’s zero-crossover topology assures high linearity across the entire common-mode voltage range and lowest distortion for precision and general applications. This application brief explains the differences between standard rail-to-rail input and zero-crossover amplifiers.
A traditional rail-to-rail input CMOS architecture contains two differential pairs. Figure 1 highlights two differential pairs; one PMOS transistor pair (blue) and one NMOS transistor pair (red). PMOS transistors can operate in common-mode input voltages from VSS to (VDD-1.8 V) and NMOS transistors can operate in common-mode input voltages from (VDD-1.8 V) to VDD. The two input transistor pairs have independent and uncorrelated input offset voltages, temperature coefficients, and noise.
During the transition from the PMOS pair to the NMOS pair, and vice versa, there is a crossover region at approximately 1.8 V below the positive rail where both inputs are conducting (see Figure 2). Within this region, the DC input offset voltage can change. This is a source of distortion known as input crossover distortion. This offset error can be simulated using the TINA-TI SPICE tool.
Figure 3 shows the simulated results of applying a
[–2.4 V,
2.4 V] DC sweep to a traditional rail-to-rail CMOS input, buffer-configured op amp.
The graph shows the input offset voltage abruptly shifts when the common-mode
voltage is within the crossover region. If this error source is beyond the error
budget, a zero-crossover amplifier is required.
Zero-crossover topology uses a internal voltage charge pump to achieve linear operation with input voltages up to the rail with a single input transistor pair (PMOS or NMOS). This use of a single transistor pair allows true rail-to-rail operation without distortion over the entire input common-mode range since there is no crossover region. Zero-crossover amplifiers such as the OPA388 include an internal voltage charge pump. The charge pump boosts the input stage voltage approximately 1.8 V above VDD. This is enough to overcome the non-linearity that occurs when the transistor enters triode operation at VDS < 1.8 V. Figure 4 shows a simplified representation of the charge pump topology used in zero-crossover amplifiers.
Figure 3 also shows the simulated results of applying a [–2.4 V, 2.4 V] DC sweep on a buffer-configured OPA388. The input offset voltage trace in the graph shows no abrupt shift with input common-mode change because there is no crossover region. Figure 5 contrasts the measured performance between a complementary rail-to-rail input and zero-crossover amplifier. Note the large variance in offset voltage across the input common-mode voltage.
A zero-crossover and a standard rail-to-rail CMOS amplifier were used in identical, unity-gain buffer configurations. These amplifiers were both fed a pure sine wave with an amplitude of 2 V (4 VPP). The outputs of these circuits were captured and the FFT was computed. Figure 6 shows the output voltage spectrum for the OPA388 (red) and a typical CMOS rail-to-rail amplifier (black). The output of the zero-crossover amplifier contains few spurs and harmonics compared to the typical rail-to-rail CMOS amplifier. This is the effect of eliminating the crossover region with zero-crossover topology.
Traditional rail-to-rail input CMOS op amps use two parallel differential input transistor pairs. When the common-mode is in the transition region (deadband), there is an abrupt shift in the input offset voltage which results in output voltage error and distortion. Zero-crossover op amps vastly reduce any changes in input offset voltage across the entire input common-mode range.
Table 1 lists some of TI’s zero-crossover amplifiers. For a full list, see the operational amplifiers parametric search tool.
Device | Optimized Parameters |
---|---|
OPA328 | Vos(max): 25 μV, GBW: 40 MHz, CMRR: 120 dB, IB(max): 1 pA, 2.2 V < VS < 5.5 V, Noise: 9.8 nV / √Hz |
OPA323 | Vos(max): 1.25 mV, CMRR: 114 dB, GBW: 20 MHz, IB(max): 20 pA, Noise: 5.5 nV / √Hz, Slew Rate: 33 μV, 1.7 V < VS < 5.5 V |
OPA388 | Zero-drift, Vos(max): 5 μV,
dvos/dt(max): 0.05 μV / °C, CMRR: 138 dB, GBW: 10 MHz, Noise: 7 nV / √Hz |
OPA320 | Vos(max): 150 μV, CMRR: 114
dB, IB(max): 0.9 pA, GBW: 20 MHz, 1.8 V < VS < 5.5 V, Noise: 7 nV / √Hz |
OPA325 | Vos(max): 150 μV, CMRR: 114
dB, IB(max): 10 pA, GBW: 10 MHz, 2.2 V < VS < 5.5 V, Noise: 9 nV / √Hz |
OPA365 | Vos(max): 200 μV, CMRR: 120
dB, GBW: 50 MHz, Noise: 4.5 nV / √Hz, Slew rate: 25 V / μs, 1.8 V < VS < 5.5 V |
OPA322 | Vos(max): 2 mV, CMRR: 100 dB,
GBW: 20 MHz, Noise: 8.5 nV / √Hz, Slew Rate: 10 V / µs, 1.8 V < VS < 5.5 V |
OPA363,OPA364 | Vos(max): 2.5 mV, CMRR: 90
dB, GBW: 7 MHz, Noise: 17 nV / √Hz, IB(typ): 1 pA, 1.8 V < VS < 5.5 V |
OPA369 | Vos(max): 750 µV, CMRR: 114
dB, GBW: 12 kHz, IB(typ): 10 pA, 1.8 V < VS < 5.5 V |
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