This design offers good DC performance and high accuracy in DC measurements, making it ideal for applications that require precise voltage readings.
This reference design provides a comprehensive overview of the theory, design, and testing of a high-performance signal chain optimised for DC measurements. While the primary application for this design is digital multimeters (DMMs), it is also highly suitable for other applications such as data acquisition (DAQ) and condition monitoring systems. This reference design is ideal for digital multimeters (DMMs), data acquisition systems (DAQ), and condition monitoring applications, where precision and reliability in DC measurements are crucial. The easy gain selection and wide input range further enhance its adaptability for various high-accuracy measurement tasks.
The design TIDA-010945 by Texas Instruments (TI) delivers exceptional DC accuracy by integrating a high-performance 24-bit analog-to-digital converter (ADC), the ADS127L21, and a low-noise programmable gain amplifier (PGA), the PGA855. These components are specifically chosen for their superior performance characteristics, including a typical nonlinearity of ±2.4ppm and minimal drift over temperature. The high level of integration in these devices simplifies the overall signal chain design, making it easier to achieve the desired performance. The integration of the ADS127L21 ADC and PGA855 amplifier simplifies the signal chain, enhancing both reliability and ease of implementation.
The design includes a programmable gain stage that supports a broad input range, making it particularly suitable for applications like digital multimeters (DMMs) that need to handle various input ranges. The design incorporates key components that contribute to its superior performance. The ADS127L21 is a 24-bit delta-sigma (ΔΣ) ADC that features a programmable digital filter and supports data rates up to 512kSPS with a wideband filter or up to 1365kSPS with a low-latency filter. Its high resolution and configurable filter settings enable an excellent signal-to-noise ratio (SNR) of 110dB when using the Sinc4 filter, all while eliminating the need for high-speed ADC input drivers and reference buffers. The PGA855 amplifier is a high-bandwidth, programmable gain instrumentation amplifier that offers fully differential outputs and eight binary gain settings, ranging from an attenuating gain of 0.125V/V to a maximum of 16V/V. These gain settings are managed through three digital gain selection pins, providing flexibility in signal conditioning.
The REF70 series, known for its high-precision voltage references, delivers industry-leading low noise (0.23ppmp-p), minimal temperature drift (2ppm/°C), and high accuracy (±0.025%). These references also offer excellent power supply rejection ratio (PSRR), low dropout voltage, and outstanding load and line regulation, ensuring consistent performance under varying conditions. In terms of performance specifications, the design achieves a total unadjusted error (TUE) of ±6ppm and integral nonlinearity (INL) of ±2.4ppm at 25°C after calibration, with maximum estimated values of TUE ±32ppm and INL ±12.2ppm. The design also demonstrates low offset drift (350nV/°C) and gain drift (1.5ppm/°C), ensuring stable performance across a wide temperature range.
Texas Instruments (TI)has tested this reference design. It comes with a bill of materials (BOM), schematics, assembly drawing, printed circuit board (PCB) layout, and more. The company’s website has additional data about the reference design. To read more about this reference design, click here.
