Achieve High SNR with the PGA855, Fully Differential Programmable-Gain Amplifier
Abstract
The PGA855 is a precision, wide-bandwidth programmable gain instrumentation amplifier with fully differential outputs optimized to drive high-performance analog-to-digital converters (ADCs) with fully differential inputs. The PGA855 is equipped with eight binary gain settings, from an attenuating gain of 0.125V/V to a maximum of 16V/V, using three digital gain-selection pins. The low-noise current-feedback front-end architecture offers excellent gain flatness, even at high frequencies, making the PGA855 an excellent high-impedance sensor readout device. Integrated protection circuitry on the input pins handles overvoltages up to ±40V beyond the power supply voltages. The PGA855 offers an excellent combination of AC performance and DC accuracy, making the PGA855 a versatile choice for a variety of sensors.
This application note shows the AC performance of the PGA855 driving the ADS127L11 and ADS127L21 delta-sigma ADCs. The focus of this document is on intrinsic noise, signal-to-noise ratio (SNR) and effective resolution performance. In particular, this application note provides guidelines of the analog filter selection and provides the performance of the PGA855 and ADS127Lx1 with different data rates and digital filter settings.
The document illustrates a step-by-step intrinsic noise analysis of the acquisition system, provides a calculator tool to estimate the system performance with different ADC digital filter and data rate settings, and shows the bench measurements of the PGA855 driving the ADS127Lx1.
Trademarks
Microsoft® and Excel® are registered trademarks of Microsoft.
All trademarks are the property of their respective owners.
1 PGA855 and ADS127L21, 24-Bit, Delta-Sigma ADC Driver Circuit
Analog signal acquisition in industrial systems is a challenging problem. Sensor output signals often present full-scale signals in the millivolt range, requiring resolutions in the microvolt range or even nanovolt range. The system designer's challenge is optimizing the sensor amplifier front-end to achieve the best signal to noise performance. Hence, understanding and minimizing these sources of noise is essential.
The instrumentation amplifier (INA) and the programmable gain instrumentation amplifier (PGA) are essential in industrial acquisition systems. These components offer high accuracy, low noise signal-conditioning and level shifting, and versatile gain programmability. INAs and PGAs offer a high-impedance front-end and have evolved into excellent ADC drivers in the back-end, offering a complete integrated signal acquisition design. Applications include industrial analog input modules measuring a wide variety of bridge, pressure, and temperature sensors; data acquisition cards; surgical equipment; vibration analysis, and power metering/battery testing systems.
Figure 7-13 shows a circuit example for the PGA855 driving the ADS127Lx1, a fully-differential input, high resolution, wide-bandwidth, delta-sigma ADC.
Figure 1-1 PGA855 and ADS127L21, 24-Bit, Delta-Sigma ADC CircuitThe PGA accepts single-ended or fully-differential input signals while driving the differential ADC inputs. Pin-controlled gains scale the input signal to the ADC input range. The super-beta input transistors offer a low input bias current, providing a very low input current noise density of 0.3pA/√Hz, making the PGA855 a versatile choice for many sensor types.
The PGA855 offers independent input and output power supplies. In this example, ±15V input power supplies are used for the PGA input section, allowing a wide voltage input range. The output stage is powered with the ADC 5V power supply. The 5V output stage supply operation prevents overloading the ADC inputs during PGA overdrive conditions. The VCM output pin of the ADC drives the PGA855's VOCM pin setting the common mode voltage of the PGA outputs.
The goal of the circuit on Figure 7-13 is to provide a high level of SNR and total harmonic distortion (THD) performance for a given circuit bandwidth requirement. The effective bandwidth of the circuit is affected by the PGA855 analog front end bandwidth as well as the ADC digital filter.
