DESIGN
T&M: SIGNAL CONVERSION
This article presents a low power analog-to-digital converter (ADC) solution for high precision measurement applications. A typical application in electrical engineering is the recording of physical quantities by sensors and forwarding of these quantities to a microcon- troller for further processing. ADCs are needed to convert the analog sensor output signals into digital signals. In high precision applications, either SAR-ADCs or sigma-delta ADCs are used. With low power applications, every mW that can be saved counts. How to Realise Power-Saving Analog- to-Digital Conversion for Highly Accurate Measurements
By Thomas Brand, Field Applications Engineer, Analog Devices
Figure 1. A schematic diagram showing a simplified bridge measurement circuit combined with an in- amp and a SAR-ADC.
so that it can be more easily managed by the ADC. A PGA analog front end (AFE) is required to connect a small voltage with a 10mV output voltage. For example,
Signal conversion with Sigma-delta ADCs Sigma-delta ADCs offer a few advantages over SAR-ADCs. For one, they often have higher resolutions. In addition, they often come integrated with programmable gain amplifiers (PGAs) and general-purpose inputs/outputs (GPIOs). Thus, sigma-delta ADCs are well suited for DC and low frequency high precision signal conditioning and measurement applications. However, due to the high, fixed oversampling rate, a sigma-delta ADC often has a higher power consumption, which translates to a lower lifetime in the case of battery-operated applications. If the input voltage is small - that is, in the millivolt range - it first has to be amplified
to connect small voltages from a bridge circuit to a sigma-delta ADC with a 2.5V input range, the PGA has to have a gain of 250. This, however, leads to additional noise at the ADC input because the noise voltage is also amplified. The effective resolution of a 24-bit sigma-delta ADC is thereby drastically reduced down to 12-bit. However, in some circumstances, there is no requirement to use all codes in the ADC and at some point, further amplification no longer provides a dynamic range improvement. Another disadvantage of sigma-delta ADCs is the usually higher costs resulting from their internal complexity. Benefits of combining a SAR-ADC with an in-amp
20 ELECTRONICSPECIFIER.COM
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