Measuring 18 2-Wire RTDs with the LTC2983 Design Note 1035 Tom Domanski Introduction A single LTC ®2983 temperature measurement device can support up to 18 2-wire RTD probes, as shown in Figure 1. Each RTD measurement involves simultaneous sensing of two voltages developed across RSENSE and the RTD probe RTDx due to the current IS. Each voltage is sensed differentially, and given the LTC2983’s high common mode rejection ratio, the number of RTDs in the stack does not adversely affect the individual measurements. The choice of the RTD probe depends on the system accuracy and sensitivity requirements. For example, given that 2-wire probes are used, the PT-1000 may prove more robust in the presence of wiring’s parasitic resistance. Once the RTDs are selected, IS and RSENSE should be chosen so that voltage at the top of the resistor stack (V at the CH1 input) does not exceed the input common mode limit of the LTC2983 over the operating temperature range of the system. This requirement is expressed as: N ⎛ ⎞ VDD − 0.3 ≥ ⎜⎜RSENSE + ∑RTDi ⎟⎟Is , N = 1,2…18 ⎝ ⎠ i=1 Consider the system shown in Figure 1 and assume the following constraints: 5V supply rail, all RTD probes are PT-100, and the maximum expected temperature measurement is at 150°C. Table 1 shows the channel assignment word for each one of the PT-100 probes. Consult the “Channel Assignment Memory Map” in the LTC2983 data sheet. Note that in this example CH3 senses the RTD1 probe, CH4 senses RTD2, etc. L, LT, LTC, LTM, Linear Technology and the Linear logo are registered trademarks of Linear Technology Corporation. All other trademarks are the property of their respective owners. Table 1. CH2 Through CH20 RTD Channel Assignment Word FUNCTION BIT FIELD VALUE DESCRIPTION Sensor Type 31:27 01100 PT-100 Sense Resistor Channel Pointer 26:22 00010 CH2 Sensor Configuration 21:18 0001 2-Wire Excitation Current 17:14 1000 1mA RTD Curve 13:12 01 American Curve Address 11:6 000000 NA Length 5:0 000000 NA Custom RTD Data Pointer The sense resistor, connected to CH2, is configured as shown in Table 2. Table 2. Sense Resistor Channel Assignment Word FUNCTION BIT FIELD Sensor Type Sense Resistor Value 10/15/1035 VALUE DESCRIPTION 31:27 11101 Sense Resistor (29) Integer 26:10 000000 1111101000 1kΩ Fraction 9:0 0000000000 1k RTD1 C2 RTD2 C3 RTD3 C4 RTD4 C5 RTD5 C6 RTD6 C7 RTD7 C8 RTD8 C9 RTD9 C10 RTD10 C11 RTD11 C12 RTD12 C13 RTD13 C14 RTD14 C15 RTD15 C16 RTD16 C17 RTD17 C18 RTD18 C19 C20 CH1 IS CH1 CH1 LTC2983 CH1 CH1 CH1 CH1 5V VDD Q1 Q2 RTD Stack Settling Time Once the excitation current source is enabled, it takes a finite amount of time for the R and C chain to settle. That is, the settling time, tS. tS is dependent on the number and value of the individual resistors (RSENSE and RTDs) and capacitors at each input node. The upper bound on tS can be estimated by lumping the total RC, but that yields an overly pessimistic result. Another method to obtain tS is to simply simulate a circuit as shown in Figure 2: 0.1µF CH1 CH1 VREFP 1µF VREF_BYP LDO 1µF C RTDN C The results of simulation are shown in Figure 3. Here all capacitors are chosen to be 100nF, and RSENSE is 1k. Each line represents settling time tS to within 0.1% of the final value of the voltage across last RTD in the stack. For each graph, all RTDs are of the same type. 103 10µF CH1 RTDN-1 Figure 2. Delay Line Model of the RTD Stack Q3 VREFOUT 1k C IS C 10µF 10µF CH1 RTD1 RSENSE PT-1000 PT-100 PT-10 102 CH1 t S (ms) RSENSE C1 CH1 CH1 CH1 CH1 CH1 100 RESET INTERRUPT 10–1 CS SDI 0 2 4 6 8 10 12 14 16 NUMBER OF RTDs IN THE STACK 18 SDO Figure 3. Simulated Settling Time of the RTD Stack SCK The LTC2983, by default, inserts a delay time tDELAY = 1ms between enabling the excitation source and the beginning of the ADC conversion. This, however, is insufficient for any more than two PT-100 probes in the RTD stack (see Figure 3). CH1 CH1 CH1 COM 101 GND Figure 1. LTC2983 with 18 RTD Sensors The tDELAY may be increased by setting the value in the MUX configuration register, 0x0FF. By default the register is cleared. Each LSB added to the register value represents 100µs added to default tDELAY. Consult the “Supplemental Information” section in the data sheet for more detail on the MUX delay. For example, writing 0x10 into 0x0FF results in: tDELAY = 1ms + 0x10 • 100µs = 2.6ms Note that the maximum value of programmable delay is 26.5ms, which is sufficient for settling of at most six PT-1000 devices, given the C = 100nF. See Figures 3 and 4. The tDELAY is inserted prior to each individual ADC cycle. Each RTD measurement consists of two ADC cycles. Therefore the total conversion time of the stack of RTDs is approximately: t TOTAL = (2tDELAY + t CONV ) N Where tDELAY is programmable by the user, tCONV is given in the “Complete System Electrical Characteristics” table in the data sheet, typically 164ms including the default MUX delay, and N is the number of RTDs to be measured. tTOTAL is summarized in Figure 4. Conclusion The LTC2983 can interface to as many as 18 2-wire RTD probes, but be sure to take into account the settling delay incurred by RC systems. The issue may be exacerbated by the number and type of RTD probes used. The delay issues can be examined using the model and simulation presented here. INITIATE CONVERSION COMMAND tDELAY 1ST CYCLE tDELAY 2ND CYCLE (N-1) RTD1 CONVERSION TIME tTOTAL Figure 4. Total Conversion Time of the RTD Stack Data Sheet Download www.linear.com/LTC2983 Linear Technology Corporation For applications help, call (408) 432-1900 dn1035f LT 1015 • PRINTED IN THE USA 1630 McCarthy Blvd., Milpitas, CA 95035-7417 (408) 432-1900 ● FAX: (408) 434-0507 ● www.linear.com LINEAR TECHNOLOGY CORPORATION 2015