Easy Drive ADC Simplifies Measurement of High Impedance Sensors

With high accuracy and strong noise immunity, delta-sigma ADCs are ideal for direct measurement of many types of sensors. However, the input sampling current can overwhelm high source impedance or low bandwidth, micropower signal conditioning circuits. The LTC2484 delta-sigma converter family solves this problem by balancing input currents, thereby simplifying or eliminating the need for signal conditioning circuits. A common application for delta-sigma ADCs is thermistor measurements.

With high accuracy and strong noise immunity, delta-sigma ADCs are ideal for direct measurement of many types of sensors. However, the input sampling current can overwhelm high source impedance or low bandwidth, micropower signal conditioning circuits. The LTC2484 delta-sigma converter family solves this problem by balancing input currents, thereby simplifying or eliminating the need for signal conditioning circuits. A common application for delta-sigma ADCs is thermistor measurements. Figure 1 shows how the LTC2484 is connected when directly measuring thermistors up to 100kΩ. The data I/Os are connected via a standard SPI interface, and the sampling current per input is approximately:

Easy Drive ADC Simplifies Measurement of High Impedance Sensors

in

Easy Drive ADC Simplifies Measurement of High Impedance Sensors

Or about 1.67µA when VREF is 5V and both inputs are grounded.

Easy Drive ADC Simplifies Measurement of High Impedance Sensors

4-WIRE SPI INTERFACE: 4-wire SPI interface

Figure 2 shows how to balance the thermistor to minimize ADC input current. If the reference resistors R1 and R4 are exactly equal, then the input current is zero and no error occurs. If the tolerance of the reference resistor is 1%, the maximum error in the measured resistance is 1.6Ω due to slight drift in the common-mode voltage, which is much less than the 1% error of the reference resistor itself. This solution requires no amplifiers, making it ideal for micropower applications.

Easy Drive ADC Simplifies Measurement of High Impedance Sensors

It may be necessary to ground one end of the sensor to reduce noise picked up, or to simplify wiring if the sensor is at the far end. If this circuit were used without buffering, the changing common-mode voltage would cause a 3.5kΩ full-scale error in the measured resistance.

Figure 3 shows how to connect a very low power, very small bandwidth op amp to the LTC2484. For a 1.5µA supply current amplifier, the LT1494 has very good DC performance specifications, with a maximum offset voltage of 150µV and an open loop gain of 100,000, but its 2kHz bandwidth makes the device unsuitable for driving conventional delta-sigma ADCs. Adding a 1kΩ, 0.1µF filter solves this problem by providing a charge bank that supplies the LTC2484’s instantaneous sampling current, while the 1kΩ resistor isolates the capacitive load from the LT1494. Don’t try to do this with a normal delta-sigma ADC, because in the circuit shown in Figure 3, an ADC with similar performance specifications to the LTC2484 family would have a 1.4mV offset and 0.69mV full-scale error for sampling current. The equalized input current of the LTC2484 allows these errors to be easily removed by placing an identical filter on the INC side.

Easy Drive ADC Simplifies Measurement of High Impedance Sensors

The Links:   SKD160-12 L1652