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Analogue inputs (low-level)
Thermocouples and mV sources
The recommended barrier for thermocouples and mV sources is the MTL7760ac. This 2-channel non-polarised barrier retains the ‘earthfree’ nature of the signal and, providing the receiver’s input ‘floats’, rejects common-mode ac and dc interference up to at least 7V and is unaffected by earth faults on the primary element.
AC sensors, photocells, microphones and turbine flowmeters
The MTL7760ac is the recommended choice for these devices. While many of these are designated ‘simple apparatus’ and thus do not need certification, note that some ac sensors may be subject to a significant level of inductance and will therefore need to be designed and certified for hazardous-area locations.
Slidewire displacement transducers
The simplest choice is the MTL7760ac. This barrier supplies power and brings back a unipolar signal.
For 3-wire RTDs, a single MTL7755ac barrier is the most economical choice. This is suitable for use with a floating bridge – the two leads from the bridge arms are protected by the barrier with the third (supply) lead being earthed through the barrier. The barrier has a low end-toend resistance of only 19Ω/channel to minimise span changes and its channels track within 0.15Ω (between –20°C and +60°C) to minimize zero shift with temperature.
If the bridge circuit is already earthed, the third barrier channel provided by an MTL7756ac is needed. For extreme accuracy, 3 channels and an earth-free bridge can be used, a configuration that cancels out the small errors due to barrier leakage.
Channels 1 and 2 (those between terminals 1 & 2 and 3 & 4 respectively) track to within 0.15Ω (between –20°C and 60°C).
4-wire constant-current circuits do not need matched barrier resistances and can be protected by two MTL7761ac barriers. If the increase in loop resistance is too great, use two MTL7755ac barriers instead.