An Overview of the LTC2983 Precision Temperature-to-Bits Converter by Analog Devices

One of the most commonly measured physical phenomena is temperature. Sensors capable of creating electronic signals as a function of temperature are widely used to accurately digitize temperature in many applications. One such sensor is the RTD.

An RTD is a device that changes resistance as a function of temperature. RTDs can be configured as two, three, or four-wire connection and have several standards, i.e., temperature coefficients as well as nominal resistance values at 0 degrees C.

An RTD's resistance is determined by applying an excitation current through an RTD connected in series with a known sense resistor. This type of measurement is ratiometric and does not depend on the absolute value of the ADC reference or excitation current.

An RTD's resistance variation with temperature can be very small. For example, a PT100 RTD varies approximately 0.03 ohms per tenth of a degree C. can be compromised by external input protection and filter circuits, lead resistance, and parasitic thermocouple effects. A device targeting RTD measurements must accommodate two, three, and four-wire configurations, requires high input impedance, includes fault reporting, and excitation current sources.

The sense resistor can be expensive. Its voltage output filtering circuits and parasitic lead resistance are difficult to interface to the reference input of an ADC. The LTC2983 solves these design challenges by combining three low noise, continuously self-calibrated 24-bit delta sigma ADCs with an excitation current DAC lie with zero buffer amplifiers with high input impedance and hundreds of switches to automatically perform the necessary measurements.

Two ADCs simultaneously measure the voltage drop across the sense resistor and RTD, eliminating the conventional problems of measuring Rsense with the ADC reference input. The LTC2983 calculates the RTD resistance, performs and reports fault detection, and outputs that results in degrees C or F and the RTD resistance in ohms.

In order to evaluate the device performance, the LTC2983 and dedicated RTD demonstration circuits are used with their corresponding demonstration software, an oil bath, and an RTD calibrator. The LTC2983 demo board interfaces to the Linduino or DC590 USB board and connects to the dedicated RTD board. The demonstration system is useful in evaluating the RTD performance and features of the LTC2983.

It includes a simulated RTD, which is a 100-ohm resistor, with parasitic thermocouple for demonstrating the LTC2983's current source excitation reversal. It also includes a PT100 RTD with a jumper to connect it as a three or four-wire RTD and a 10k pot for demonstrating RTD ranges and fault reporting. All sensors take advantage of the Rsense sharing capabilities of the LTC2983.

An accurate method to measure temperature with a real sensor is a temperature-controlled oil bath. In this case, a four-wire PT100 RTD is inserted in an oil bath set to 100 degrees C. As you can see, the demonstration system is collecting the LTC2983 output, which is precisely reading 100 degrees C. The corresponding Rsense resistor is tied between channel one and two and its value is stored in the LTC2983.

The 2k ohm sense resistor is shared between all three sensors on the demonstration board. The LTC2983 contains many fault-reporting mechanisms. By adjusting the variable resistor RTD simulator, it is possible to create soft faults, sensor temperature above or below of the RTD range and hard faults, RTD short or open, Rsense short or open.

Soft faults report the calculated temperature while hard faults reports -999 degrees, indicating an invalid reading. The dedicated RTD board includes a fixed 100-ohm resistor used as an RTD at 0 degrees C with a parasitic thermocouple inserted in the connection between the RTD and the LTC2983. One physical phenomenon affecting the accuracy of RTD measurements are parasitic thermocouples resulting from the wire connections to the RTD.

These effects lead to temperature errors and drift. This demonstration circuit along with a heat gun simulates the real-world effects of parasitic thermocouples. In order to cancel these effects, the LTC2983 can perform automatic current excitation rotation. When the rotation is turned on, parasitic thermocouple errors are continuously removed from the result.

This circuit was run over a two-day period where the first day the rotation was off, the blue curve, and the second day, it was turned on, red curve. Effects due to heating and cooling of the room are canceled by auto rotation. All switches and current sources enabling auto rotation are built into the LTC2983 and can be easily enabled and disabled in software.

In order to measure the full RTD temperature range with sub degree C precision, a temperature calibrator is required. In this case, the calibrator temperature is manually walked up a PT100 type RTD in 100 degrees C increments. The result is well within 1/20th of a degree C accuracy. This can be done for each RTD type. A calibrator can also be used to automatically, under software control, sweep across all temperatures and RTD types.

Shown here is the LTC2983 performance using a PT100 type RTD with errors well within 1/20th of a degrees C. In addition to digitizing RTDs, the LTC2983 can also measure the temperature of thermocouples and thermistors. The LTC2983 20 inputs can be software configured to digitize any of these types of sensors. This allows one hardware design to be shared across many sensor types.

The universal demonstration board includes a terminal block for directly interfacing to RTDs, thermocouples, and thermistors. In addition to standard RTDs, the LTC2983 can also digitize custom table-driven RTDs. The LTC2983 is a highly-accurate simple-to-use temperature measurement system. It directly interfaces to RTDs, applies the excitation current, simultaneously measures the RTD and sense resistor and reports the results in degrees C or F.

It does not require any external references, buffers, or level shift circuitry and can be universally configured to digitize PT10, PT50, PT100, PT200, PT500, PT1000, and NI120 type RTDs as well as thermocouples, diodes, and thermistors. Refer to other videos in this series for more information on the unique capabilities of the LTC2983 relative to thermocouples and thermistors.

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