Thermocouple amplifier AD8494 temperature measurement

Built-in an on-chip temperature sensor, generally used for cold junction compensation, ground the thermocouple input, the device can be used as an independent Celsius thermometer. In this configuration, the amplifier generates an output voltage of 5 mV/°C between the output pin of the on-chip instrumentation amplifier and (usually connected) the reference pin. This method has a shortcoming, when measuring a narrow range of temperature, the system resolution is not good. Consider this situation: A 10-bit ADC powered by a single 5 V power supply has a resolution of 4.88 mV/LSB. This means that the system shown in Figure 1 has a resolution of approximately 1°C/LSB. If the target temperature range is narrow, such as 20°C, the output change amplitude is 100 mV, and only 1/50 of the available dynamic range of the ADC is utilized.

Figure 1. Simple thermometer

The circuit shown in Figure 2 can solve this problem. As with the above configuration, the amplifier produces an output voltage of 5 mV/°C between the output pin of the instrumentation amplifier and the reference pin. However, now the reference pin is driven by the AD8538 operational amplifier (configured as a unity gain follower), so a voltage of 5 mV/°C appears across R1. The current flowing through R1 will also flow through R2, thereby generating a temperature-dependent voltage across the series combination, the magnitude of which is (R1 + R2)/R1 multiplied by the voltage across R1. Using the values shown in the figure, it can be concluded that the output voltage changes by 20 × 5 mV/°C = 100 mV/°C, so a 20°C temperature change will produce a 2 V output voltage change. The resolution of the new system is 0.05°C/LSB, which is 20 times higher than the original circuit. AD8538 buffers this resistor network and drives the reference pin with low impedance to maintain good common-mode rejection performance and gain accuracy.

Figure 2. High resolution

It is necessary to ensure that the sensitivity of the system matches the required temperature range. For example, if the output voltage is 2.5 V at 25°C and the output voltage varies from 0.5 V to 4.5 V, the system can measure the temperature in the range of 5°C to 45°C.

The circuit shown in Figure 3 can provide higher sensitivity and a customizable temperature range. R3 and R4 form a resistor divider that is used to simulate the required thermocouple voltage to adjust the amplifier so that the output voltage is set to 0 at the required temperature. If the VDD noise is high, a precision reference voltage source and voltage divider circuit can be used to provide quieter and more offset adjustment. As shown in the figure, the output voltage of this circuit at 25°C is about 0.05 V, the sensitivity is 100 mV/°C (0.05°C/LSB resolution), and the measurement range is about 25°C to 75°C.

The initial offset error of the AD8494 is ±1°C to ±3°C, and the user must perform offset calibration to improve accuracy.

Figure 3. Higher resolution temperature measurement with offset adjustment function