Background
As supply voltages fall while signal quality and integrity improve, basic analog components used in applications such as high-speed, DS, or SAR converter systems are strained by amplifiers that cannot achieve high rail-to-rail input performance. Simple rail-to-rail op amps require transistor designs that truly span the supply rails without introducing distortion. In many application circuits, this requirement is non-negotiable.
Historical Approaches
In the 1970s, single-supply op amp design trends began with a single differential input stage that covered part of the common-mode input range. Later designs used two differential input stages (complementary input stages) that share rail-to-rail input operation across the amplifier common-mode range, albeit with some distortion. None of these solutions fully met the stringent system requirements needed to cover the amplifier's entire common-mode input range.
Charge Pump Input Stage
IC designers adopted techniques from other devices to address this issue. A common solution is to use a charge pump to drive one differential input stage above the positive supply rail (see Figure 1). Designers set the switching frequency above the amplifier bandwidth and keep switching noise below the amplifier's theoretical noise floor.
Figure 1 Input topology of OPA635 and OPA333 single-supply amplifiers
Effect on CMRR and THD
A single differential input stage with a charge pump can increase the amplifier's common-mode rejection ratio (CMRR) by about 20 to 30 dB. In buffering configurations, this improvement is beneficial. The amplifier's total harmonic distortion (THD) can also be reduced by nearly 10x. Therefore, using an amplifier with a charge-pumped input stage to drive high-performance SAR or DS converters can improve overall system performance.
System THD Calculation
For example, the total harmonic distortion of an ADC driven by a buffered op amp equals the root-sum-square of the ADC and op amp distortion contributions. In this configuration, the system THD is:
THD_OPA-% is the op amp THD specification in percentage units from the datasheet.
Using these formulas, if a complementary-input op amp specifies 0.004% THD (VIN = 4 Vp-p) and a 16-bit SAR ADC specifies -99 dB THD, the system THD is -88 dB. Alternatively, if the op amp input stage includes a charge pump and specifies 0.0004% THD (VIN = 4 Vp-p), the system THD becomes -98 dB.
Conclusion
Single-supply amplifiers continue to evolve alongside high-performance converters through innovative circuit topologies, such as charge-pumped input stages. Although the charge pump is a practical solution, designers continue to pursue lower system supply voltages and higher signal integrity. Demand for skilled analog circuit designers remains strong.
