Handheld meters, data loggers, and automotive and monitoring systems typically require a multiplexed system with the low-cost combination of high accuracy and high system resolution. A system that can handle this diversity requires a multiplexer, a gain cell, and an ADC. A feasible approach has a 10-channel PGA (programmable-gain amplifier) teaming up with a medium-speed, 12-bit SAR (successive-approximation-register) ADC (Figure 1). The single-supply, 10-channel PGA has a rail-to-rail I/O with a gain adjustment of 1 to 200V/V. The PGA’s low-noise performance of 12 nV/ at 10 kHz is appropriate for a 12-bit system.

The analog interface between these two devices includes an operational amplifier in a buffer configuration and an RC circuit. The 12-bit, capacitor-based SAR ADC has an inherent sample/hold function and requires the RC circuit, which facilitates the charging action of the ADC’s input structure. The calculated value of the PGA noise, referred to the output, is equal to the PGA’s noise density at 10 kHz (12 nV/) times the square root of the PGA’s closed-loop bandwidth times the square root of π/2. The multiple of accounts for the noise in the frequency region beyond the PGA’s bandwidth. You then multiply this number by the gain of the PGA. The following equation uses a PGA gain of 16V/V: PGA_RMS-NOISE=12 nV/××16V/V=304 µV rms. The ADC noise of 431 µV rms from this converter is well below 1 LSB or 1.22 mV in this 5V system. The noise from the buffer amplifier, which is 39 µV rms, contributes little or no noise to this system.

The combined noise of the PGA, op amp, and ADC is 529 µV rms, which is still less than 1 LSB of the 12-bit converter. You calculate this value using a root-sum-square equation or the following equation: Noise referred to output= The equivalent 12-bit accuracy of this system when the PGA is in a gain of 16V/V is 0.432 LSB: Noise referred to output×2^N/FSR (full-scale range), where N is 12 and FSR is 5V/V. If you look at this system across the PGA’s gain range of 1 to 200V/V, you find that the PGA dominates the noise-contribution portion in this circuit. Once the PGA gain exceeds approximately 125V/V, this system no longer matches the 12-bit-accuracy criterion. However, the system’s referred-to-input LSB voltage size becomes smaller (Figure 2). The trade-off for a smaller LSB is a decrease in the system’s effective number of bits.

The system in Figure 1 provides an adequate gain range for the PGA when 12-bit accuracy is required and an equally adequate gain range when good system resolution is required.