10.1.1.1 V_A and V_IO

Any ADC architecture is sensitive to spikes on the analog voltage, V_A, digital input/output voltage, V_IO, and ground pins. These spikes may originate from switching power supplies, digital logic, high power devices, and other sources. To diminish these spikes, the LMP900xx’s V_A and V_IO pins should be clean and well bypassed. A 0.1 µF ceramic bypass capacitor and a 1 µF tantalum capacitor should be used to bypass the LMP900xx supplies, with the 0.1 µF capacitor placed as close to the LMP900xx as possible.

Since the LMP900xx has both external V_A and V_IO pins, the user has two options on how to connect these pins. The first option is to tie V_A and V_IO together and power them with the same power supply. This is the most cost effective way of powering the LMP900xx but is also the least ideal because noise from V_IO can couple into V_A and negatively affect performance. The second option involves powering V_A and V_IO with separate power supplies. These supply voltages can have the same amplitude or they can be different.

10.1.1.2 V_REF

Operation with V_REF below V_A is also possible with slightly diminished performance. As V_REF is reduced, the range of acceptable analog input voltages is also reduced. Reducing the value of V_REF also reduces the size of the LSB. When the LSB size goes below the noise floor of the LMP900xx, the noise will span an increasing number of codes and performance will degrade. For optimal performance, V_REF should be the same as V_A and sourced with a clean source that is bypassed with a ceramic capacitor value of 0.1 µF and a tantalum capacitor of 10 µF.

LMP900xx also allows ratiometric connection for noise immunity reasons. A ratiometric connection is when the ADC’s V_REFP and V_REFN are used to excite the input device’s (i.e. a bridge sensor) voltage references. This type of connection severely attenuates any V_REF ripple seen the ADC output, and is thus strongly recommended.

10.2.1.1 Design Requirements

• VA = 3 V
• VIO = 3 V
• 3-Wire RTD using 2 current sources

10.2.1.2 Detailed Design Procedure

Figure 72 shows the first topology for a 3-wire resistive temperature detector (RTD) application. Topology #1 uses two excitation current sources, IB1 and IB2, to create a differential voltage across VIN0 and VIN1. As a result of using both IB1 and IB2, only one channel (VIN0-VIN1) needs to be measured. As shown in Equation 14, the equation for this channel is IB1 x (RTD – RCOMP) assuming that RLINE1 = RLINE2.

Equation 14.

VIN Equation for Topology #1

The PT-100 changes linearly from 100 Ω at 0°C to 146.07 Ω at 120°C. If desired, choose a suitable compensating resistor (RCOMP) so that VIN can be virtually 0 V at any desirable temperature. For example, if RCOMP = 100 Ω, then at 0°C, VIN = 0 V and thus a higher gain can be used.

The advantage of this circuit is its ratiometric configuration, where VREF = (IB1 + IB2) x (RREF). Equation 15 shows that a ratiometric configuration eliminates IB1 and IB2 from the output equation, thus increasing the overall performance.

Equation 15.

ADC_DOUT Showing IB1 & IB2 Elimination

10.2.1.3 Application Curve

Figure 73. PT-100 RTD Resistance from –200°C to 850°C

10.2.2.1 Design Requirements

• VA = 3V
• VIO = 3V
• 3-Wire RTD using 1 current source

10.2.2.2 Detailed Design Procedure

Figure 75 shows the second topology for a 3-Wire RTD application. Topology #2 shows the same connection as topology #1, but without IB2. Although this topology eliminates a current source, it requires two channel measurements as shown in Equation 16.

Equation 16.

VIN Equation for Topology #2

10.2.2.3 Application Curve

Figure 76. PT-100 RTD Resistance from –200°C to 850°C

10.2.3.1 Design Requirements

• VA = 5V
• VIO = 2.7V
• Thermocouple with Cold Junction Compensation

10.2.3.2 Detailed Design Procedure

The LMP900xx is also ideal for thermocouple temperature applications. Thermocouples have several advantages that make them popular in many industrial and medical applications. Compare to RTDs, thermistors, and IC sensors, thermocouples are the most rugged, least expensive, and can operate over the largest temperature range.

A thermocouple is a sensor whose junction generates a differential voltage, VIN, that is relative to the temperature difference (T_hot – T_cold). T_hot is also known as the measuring junction or “hot” junction, which is placed at the measured environment. T_cold is also known as the reference or “cold” junction, which is placed at the measuring system environment.

Because a thermocouple can only measure a temperature difference, it does not have the ability to measure absolute temperature. To determine the absolute temperature of the measured environment (T_hot), a technique known as cold junction compensation (CJC) must be used.

In a CJC technique, the “cold” junction temperature, T_cold, is sensed by using an IC temperature sensor, such as the LM94022. The temperature sensor should be placed within close proximity of the reference junction and should have an isothermal connection to the board to minimize any potential temperature gradients.

Once T_cold is obtained, use a standard thermocouple look-up-table to find its equivalent voltage. Next, measure the differential thermocouple voltage and add the equivalent cold junction voltage. Lastly, convert the resulting voltage to temperature using a standard thermocouple look-up-table.

For example, assume T_cold = 20°C. The equivalent voltage from a type K thermocouple look-up-table is 0.798 mV. Next, add the measured differential thermocouple voltage to the T_cold equivalent voltage. For example, if the thermocouple voltage is 4.096 mV, the total would be 0.798 mV + 4.096 mV = 4.894 mV. Referring to the type K thermocouple table gives a temperature of 119.37°C for 4.894 mV.

10.2.3.3 Application Curve

Figure 78. Thermocouple Output as Function of Temperature