Electronic Design

Create a quiet −5 V supply for 14-bit ADCs

Many high-performance data-acquisition systems reap multiple benefits when using ±5-V supplies rather than a single 5-V supply. These benefits include the ability to handle larger signal magnitudes than is possible with a single 5-V supply. This increases a system’s dynamic range and helps improve the signal-tonoise ratio. Operating on ±5 V also increases headroom, which is important for signal conditioning.

Compared to operating on 5 V, conditioning circuitry operating on ±5 V has twice the headroom, allowing it to easily handle ±2.5-V signals without clipping. In addition, the greater headroom avoids the limitations of rail-to-rail operation and widens the selection of high-performance op amps and analog-to-digital converters (ADCs).

A switching or charge-pump power supply is an efficient way to create a −5-V supply from a single 5-V supply. However, these circuits aren’t generally recommended for use with ADCs, which typically have inadequate power supply rejection ratio (PSRR) that decreases with increasing frequency.

The circuit described here combines two features to enable the ADC to achieve 14-bit performance (Fig. 1). The first feature is the low-noise Cuk-configured switching regulator (U2). This configuration has the advantage of a small triangular switching-current waveform through the secondary inductor. This current waveform is continuous, producing much less harmonic content than is created by a typical positive-to-negative voltage converter with its rectangular switching current waveform. The second feature is U1’s very high PSRR (Fig. 2). Figure 2 shows that when operating on ±5 V, the negative and positive PSRR are typically 80 dB and 90 dB, respectively, up to 200 kHz for a 100-mV ripple voltage. Combined with the proper layout, the U1’s high PSRR allows it to convert signals without signal degradation while using Figure 1’s Cuk switching regulator.

Excellent results were derived with the fast Fourier transforms (Figs. 3a and 3b). Figure 3a is an FFT of U1 operating on ±5 V from a lab supply and converting a full-scale 91-kHz sinewave at 800 ksamples/s. The noise floor is approximately 114 dB below full scale; the second harmonic’s amplitude is approximately 90 dB below full scale; and the SINAD is 80.5 dB. Figure 3b shows the FFT of U1 operating on a 5-V lab supply and −5 V from the U2-based Cuk circuit. The noise floor and the second harmonic’s amplitude remain the same relative to full scale, and the SINAD remains the same at 80.5 dB.

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