1. Field of the Invention
This invention relates generally to measurement and data acquisition systems and, more particularly, to programmable gain instrumentation amplifier (PGIA) design.
2. Description of the Related Art
Scientists and engineers often use measurement systems to perform a variety of functions, including measurement of a physical phenomena or unit under test (UUT), test and analysis of physical phenomena, process monitoring and control, control of mechanical or electrical machinery, data logging, laboratory research, and analytical chemistry, to name a few examples.
A typical measurement system comprises a computer system with a measurement device or measurement hardware. The measurement device may be a computer-based instrument, a data acquisition device or board, a programmable logic device (PLD), an actuator, or other type of device for acquiring or generating data. The measurement device may be a card or board plugged into one of the I/O slots of the computer system, or a card or board plugged into a chassis, or an external device. For example, in a common measurement system configuration, the measurement hardware is coupled to the computer system through a PCI bus, PXI (PCI extensions for Instrumentation) bus, a GPIB (General-Purpose Interface Bus), a VXI (VME extensions for Instrumentation) bus, a serial port, parallel port, or Ethernet port of the computer system. Optionally, the measurement system includes signal conditioning devices which receive the field signals and condition the signals to be acquired.
A measurement system may typically include transducers, sensors, or other detecting means for providing “field” electrical signals representing a process, physical phenomena, equipment being monitored or measured, etc. The field signals are provided to the measurement hardware. In addition, a measurement system may also typically include actuators for generating output signals for stimulating a unit under test.
Measurement systems, which may also be generally referred to as data acquisition systems, may include the process of converting a physical phenomenon (such as temperature or pressure) into an electrical signal and measuring the signal in order to extract information. PC-based measurement and data acquisition (DAQ) systems and plug-in boards are used in a wide range of applications in the laboratory, in the field, and on the manufacturing plant floor.
In a measurement or data acquisition process, analog signals may be received by a digitizer, which may reside in a DAQ device or instrumentation device. The analog signals may be received from a sensor, converted to digital data (possibly after being conditioned) by an Analog-to-Digital Converter (ADC), and transmitted to a computer system for storage and/or analysis. When a measurement system generates an analog output signal, the computer system may generate digital signals that are provided to one or more digital to analog converters (DACs) in the DAQ device. The DACs convert the digital signal to an analog output signal that is used, e.g., to stimulate a UUT.
The input stage of many measurement devices (e.g., a DAQ device) typically consists of one or more instrumentation amplifiers. Generally, analog signals that are received at a DAQ device are first routed from a particular channel via a multiplexer. The signals may then enter an instrumentation amplifier, typically a programmable gain instrumentation amplifier (PGIA). A distinctive feature of an instrumentation amplifier is that it provides very high input impedance and gain. A PGIA typically applies a specified amount of gain to an input signal, which amplifies or attenuates the signal and ensures proper A/D conversion. The PGIA may also convert differential input signals applied to the DAQ board to a single-ended output so that the ADC can correctly digitize the data.
As the resolution and speed of ADCs increase, the accuracy, noise, and speed requirements on PGIAs may similarly increase. Some common PGIA designs may not be able to meet the additional requirements. One conventional design is a three op-amp PGIA design. This design may suffer from a decrease in bandwidth as the gain is increased. Its accuracy and linearity may also suffer at high gains due to the reduction of negative feedback necessary to effect higher gain. Also, the rejection of common-mode signals may be limited by resistor matching.
Other PGIA designs typically suffer from output offset errors and drifts due to matching errors between multiple DC current sources and mirrors. These current sources and mirrors may also contribute a significant amount of output-referred noise. Also, in most PGIA designs, as more components are added to the input stage, the settling time of the overall amplifier may suffer significantly. Each component may introduce additional delays due to parasitics, some of which exhibit dielectric absorption effects, and therefore may change the overall time constants and step response of the amplifier. Moreover, device tolerances, mismatches, and any asymmetrical topology in the PGIA may result in a relatively low common mode rejection ration (CMRR).
Furthermore, PGIAs may be susceptible to overload-induced errors, such as saturation recovery time or excessive current draw. Insufficiently protected PGIAs may pass along overload conditions to the next stage of circuitry, typically the ADC, which may not be designed to handle such overload conditions.