Digital-to-analog converters (DACs) are widely used in various applications, often paired with amplifiers to condition the output signal. The amplifier can enhance current drive capability, convert differential signals to single-ended outputs, isolate downstream signal paths, or provide complementary bipolar voltage outputs. Figure 1 illustrates a typical signal chain for single-supply operation, which includes a reference, a DAC, and a buffer. To achieve high dynamic range and excellent signal-to-noise ratio, DACs are usually designed to operate at full swing, with the reference voltage (VREF) set equal to the supply voltage (VDD). This ensures maximum utilization of digital codes. When operating on a single supply, both the DAC and the output buffer are typically connected to the same power line, requiring rail-to-rail input and output amplifiers.
Figure 1: Typical signal chain for single-supply operation
The input stage of a conventional non-rail-to-rail amplifier uses a p-type or n-type differential pair. P-type amplifiers allow the input common-mode voltage to approach the lower supply rail, making them suitable for ground detection applications. In contrast, n-type amplifiers support input voltages from just above the low supply rail up to the high supply rail, making them ideal for high-side current sensing. Rail-to-rail input amplifiers combine both n-type and p-type input stages to extend the common-mode voltage range across both supply rails.
These amplifiers feature two parallel differential pairs—n-type and p-type—that activate based on the input common-mode voltage. The p-type pair turns on when the input approaches the low rail, while the n-type pair activates at higher input levels. This design allows the amplifier to handle a wide input range and swing close to both supply rails. However, this dual-pair configuration introduces a known issue called "crossover distortion," where different offset voltages between the pairs create a step-like response during transitions. This distortion is inherent in all rail-to-rail amplifiers with such topologies. Figure 2 illustrates this phenomenon, showing a crossover region at around 3.4 V under a +5V and ground supply.
Figure 2: Relationship between input offset voltage and common-mode voltage in a rail-to-rail amplifier
Figure 3 displays the integral nonlinearity (INL) error of a 16-bit DAC system using a typical rail-to-rail buffer. INL measures the deviation of the actual transfer function from the ideal one, expressed in LSB. The DAC scans from code 200 to 2¹â¶-200, excluding approximately 15 mV due to the limited output swing of the rail-to-rail amplifier. Crossover distortion appears at around 45,000 digital codes, corresponding to 3.4 V. This distortion degrades INL performance, causing nonlinearities that cannot be corrected by calibration. For a 16-bit system, the crossover nonlinearity can reach up to 4–5 LSB.
Figure 3: Integral Nonlinearity (INL) for a 16-Bit DAC with a Typical Rail-to-Rail Input Buffer
To address this issue, zero-crossover distortion amplifiers are an effective solution. These amplifiers use an internal charge pump to boost the supply voltage, enabling rail-to-rail input swing without relying on complementary differential pairs. As a result, they eliminate crossover distortion. The ADA4500-2 from Analog Devices is a prime example. Figure 4 shows its stable offset voltage across the entire input range, proving its reliability.
Figure 4: Offset voltage vs. input common-mode voltage in a zero-crossover distortion amplifier
Using such amplifiers significantly improves INL performance. Figure 5 demonstrates a 16-bit DAC system with the ADA4500-2, achieving INL better than ±1 LSB. Another alternative is to use a reference voltage lower than the supply voltage, avoiding the crossover region within the DAC’s digital code range. While this method reduces the output range, it can be useful if the signal level is sufficient. If external amplification is needed, additional circuitry may be required.
Alternatively, increasing the amplifier's supply voltage allows the use of non-rail-to-rail amplifiers, providing more headroom for the input stage. However, this approach sacrifices power efficiency. In summary, selecting the right amplifier as a DAC buffer is crucial. You can reduce the reference voltage to expand the output range, increase the supply voltage for better headroom, or opt for zero-crossover distortion amplifiers to maintain precision and performance.
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