You can provide temperature compensation in weigh-scale applications by simultaneously measuring both the temperature of the bridge and the primary output of the bridge transducer. Traditionally, an integrated multiplexer connects multiple input variables to a single sigma-delta ADC. Each time the multiplexer switches the input, the ADC must flush the digital filter of all data pertaining to the previous channel. Before the new data becomes valid, the system must account for the settling time and latency, reducing the maximum throughput rate. For example, with an ADC containing a second-order sigma-delta modulator and a third-order digital filter, the output-settling time for a step input is three times the period of the data rate. Switching from the primary to the secondary channel can reduce the primary channel's throughput by a factor of six when you need to monitor primary and secondary variables together. In many cases, you can monitor the secondary variable only intermittently and thus minimize the reduction in throughput. Figure 1 shows a solution to the throughput problem that uses the two independent channels of an AD7719 dual sigma-delta ADC.

The ADCs convert in parallel, so you can simultaneously measure both the bridge output and the bridge temperature. The output data from both measurements is available in parallel, thereby removing the latency associated with multiplexed data-acquisition systems. The main channel monitors the bridge transducer, and the secondary channel monitors the bridge temperature. The bridge transducer develops a differential output voltage between the Out(+) and Out(-) terminals. A bridge sensitivity of 3 mV/V produces a full-scale output of 15 mV when a 5V excitation source powers the bridge. The ADC's reference voltage can assume any value between and including the supply voltages, so you can use the bridge-excitation voltage to provide the reference to the ADC. A resistive divider, however, allows you to use the full dynamic range of the input. This implementation is fully ratiometric, so variations in the excitation voltage do not introduce errors in the system.

The resistor values of 20 and 12 kV in Figure 1 yield a 1.875V reference voltage for the AD7719, with a 5V excitation voltage. The main-channel (programmable) gain is 128, resulting in a full-scale input span of the ADC equal to the full output span of the transducer. A low-side switch disables the transducer to save power in standby mode. The AD7719 features factory calibration, and its signal chain uses a chopping scheme to reduce gain and offset drifts, eliminating the need for field calibration. A key requirement in weighscale applications is the ability to reject line-frequency components (50 and 60 Hz). You can achieve simultaneous 50- and 60-Hz rejection by programming the AD7719 for an output data rate of 19.8 Hz. With a gain of 128, the ADC achieves 13-bit resolution at this data rate. You can increase the resolution by reducing the update rate or by providing additional digital filtering in the controller.

The secondary channel of the AD7719 monitors the bridge temperature with the aid of a thermistor. An on-chip current source excites the thermistor and generates the reference voltage for the AD7719. As a result, excitation signals do not affect performance, and the configuration is fully ratiometric. The circuit uses a four-wire force/sense configuration to reduce the effects of lead resistance. Lead resistance of the drive wires shifts the common-mode voltage but does not degrade the performance of the circuit. Lead resistance of the sense wires is immaterial because of the high impedance of the AD7719's analog inputs. The reference-setting resistor, R_REF, must have a low temperature coefficient. The AD7719 achieves 16-bit resolution in the secondary channel, using a 19.8-Hz update rate. The thermistor determines the operating range of the circuit. The maximum voltage on the auxiliary input is REFIN 2 or 2V. With a Betatherm 1K7A1 thermistor (www.betatherm.com) and 200-µA excitation current, the operating range is -26 to +70°C.