Create A High Input Impedance, Rail-To-Rail Measurement System
Two very desirable features for a precision measurement system based on an analog-to-digital converter (ADC) are high input impedance and a wide input range, ideally including or extending slightly beyond the power-supply rails. The circuit described here does just that. Its very high input impedance is complemented by an input range that extends 300 mV beyond the supply rails.
The example circuit uses a thermocouple and a resistance temperature detector (RTD) connected to an LTC2449 high-performance delta-sigma ADC (Fig. 1). Thermocouple outputs produce very small changes (tens of microvolts per degree C), and the output will be negative if the thermocouple junction is colder than the "cold junction" connection from the thermocouple to the copper traces on the pc board.
The RTD is measured by comparing the voltage across the RTD to the voltage across the reference resistor, RREF. This provides a very precise resistance comparison, and it doesn't require a precise current source. Grounding the sensors as shown is a good first line of defense for reducing noise pickup. However, the ADC must accommodate input signals that are very close to, or slightly outside, the supply rails. The LTC2449 handles these signals very well.
The analog inputs to the ADC are routed to the device's MUXOUT pins, and an external buffer isolates these signals from the switched-capacitor ADC inputs. The external buffer, an LTC6241 precision CMOS dual op amp, provides a high impedance through the multiplexer and back to the analog inputs. This has a distinct advantage over integrated buffers because the analog inputs are truly railto-rail—and even slightly beyond—with appropriate buffer supply voltages.
The LTC6241 has a rail-to-rail output stage and an input common-mode range from the negative supply to 1.5 V lower than the positive supply. Because no rail-to-rail amplifier can actually pull its outputs to the rails, an LT3472 boost/inverting regulator is used to create a 1.25/7-V supply for the op amp from the 5-V LTC2449 supply (Fig. 2). The regulator can provide enough current for several amplifiers and other circuitry that really needs to swing to the rails.
In addition, the LT3472's 1.1-MHz switching frequency is close to the middle of the LTC2449 digital filter stopband. The center of the stopband is 900 kHz when using the internal conversion clock and is independent of the selected speed mode. The amplifier's 0.01- µF capacitive load and compensation network provide the ADC with a "charge reservoir" to average the ADC's sampling current while the 2.5-k feedback resistor maintains dc accuracy.