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OPEN CHANNEL PROTECTION FOR MULTIPLEXED RTD CIRCUITS

IP.com Disclosure Number: IPCOM000226825D
Publication Date: 2013-Apr-22
Document File: 4 page(s) / 196K

Publishing Venue

The IP.com Prior Art Database

Abstract

In control systems, two-wire, three wire or four wire resistance temperature detectors (RTDs) are used for temperature measurements. Typically, a large number of RTD channels are multiplexed using an analog multiplexer. The purpose of this invention is to protect the circuit associated with measuring RTD resistance from failure due to increased stress under open channel conditions. The open RTD channel protection circuit proposed consists of a comparator and resistors RA, RB, and a fixed/dynamic reference voltage generator. Multiple RTDs can be present in the measurement system and each RTD gets connected to the measurement circuit and the constant current source (CCS), one at a time. The CCS is designed to prevent its output from hitting rail in case of open load. With the comparator in place, CCS output is monitored continuously and compared against the reference set by fixed/dynamic reference generator. If due to an open RTD condition or any other reason, the output of CCS increases above the reference, the comparator output will be pulled low to a safe value as will be decided by RA and RB. The invention offers improved circuit reliability and uninterrupted operation without the accuracy being compromised in the event of an open RTD as a low cost protection solution, as no additional input voltage protection circuit is required for AD conversion section.

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OPEN CHANNEL PROTECTION FOR MULTIPLEXED RTD CIRCUITS

BACKGROUND

In control systems, two-wire, three wire or four wire resistance temperature detectors (RTDs) are used for temperature measurements. Typically, a large number of RTD channels are multiplexed using an analog multiplexer. The RTDs are connected to a constant current source (CCS) for excitation and to the other measurement circuitry, one at a time using such a multiplexer. In order to know the RTD resistance, two parameters are measured: RTD excitation current, for each individual RTD and voltage across the RTD after the wire drop is compensated, in case of three and four wire RTDs. In the event of any open channel due to no connection of transducer to the particular channel, Open RTD, or break in wires carrying the excitation current, the ccs providing RTD excitation tries to maintain the constant current and in the process its increased output voltage saturates the succeeding gain stages that hit the rail voltage.

In high accuracy applications, a precision op-amp is configured to work as CCS. The CCS itself can be designed not to hit the rail in case of open load. However, the other op-amp based amplifier gain stages in the measurement circuit should not be used near rail voltages. Also, the ADC or a VCO used as a digitizer could be damaged due to excessive voltage at its input pins, a risk which might require additional protection circuitry in the A to D conversion section. In RTD measurement applications, especially where there is more than one RTD multiplexed to a single constant current source and measurement circuit, an open RTD can cause stress to the measurement circuits and results in costs for additional protection devices. The proposed inventionoffers a solution to prevent damage to any of the circuit blocks while ensuring un-interrupted operation without affecting the accuracy of the subsequently scanned channels.

BRIEF DESCRIPTION OF DRAWINGS

Open channel protection for multiplexed rtd circuits is disclosed with reference to the following diagram:

Figure 1 shows the open RTD channel protection circuit.

DETAILED DESCRIPTION

The purpose of this invention is to protect the circuit associated with measuring RTD resistance from failure due to increased stress under open channel conditions. The open RTD channel protection circuit proposed consists of a comparator and resistors RA, RB, and a fixed/dynamic reference voltage generator, as shown in Fig. 1 within the dotted area. Multiple RTDs can be present in the measurement system and each RTD gets connected to the measurement circuit and the constant current source (CCS), one at a time.

FIG. 1: Open RTD channel protection circuit (shown within the dotted area)

For example, in an 8 RTD multiplexing scheme as shown in Fig. 1, analog multiplexers (U1-U3) share common address lines and switch the RTDs. The excitation current flows through the Exc net into the RTD and returns back to the CCS circuit through the Ret net. The actua...