3 Key Care Abouts when Implementing a Fault Protected Multiplexer into an Analog I/O Module

Now that it has been shown the fault protection features on TI’s Fault Protected Switches and Multiplexers can help prevent system damage during multiple types of fault conditions it is now time to focus the attention on the impact the device has on the signal chain during normal – non-fault condition- operation. This topic is discussed in detail in TI’s application note, Discrete ADC Input Expansion Using Precision Multiplexers which greatly mimics the analog input module scheme of either Op Amps or ADC’s following the multiplexer/switch. While the parts discussed in that app note are labeled precision – TI’s Fault Protected Switches and Multiplexers are also precision. Following is a brief synopsis of the key care abouts in these systems as well as a look into some fault protected specific parameters.

Ideally a multiplexer or switch will have no effect on the signal chain when the switch is closed and it acts a perfect open circuit when the switch is open. This is not the case as in reality there are multiple parameters that affect switch operation that deviate from the ideal case.

The first deviation is that of the switch channel itself – ideally it is assumed to a be perfect conductor. In reality the switch, when closed, acts as a RC filter with the resistance being the channel resistance and the C being the parasitic capacitance to ground. The load can then add to the output capacitance and change the channel parameters as well. If the RC were constant the only real worry would be attenuation of higher frequencies – however the channel resistance (Ron) will vary with input voltage, so for a variable voltage signal the RC time delay to the output of the switch/mux will be different depending on what input voltage was applied – this can cause distortion to the system. This is mitigated by using multiplexers and switches with a flat on resistance and a flat on capacitance response. Where flatness means how much the values vary due to input signal levels. A system that has a variable R and C can create distortion by creating different propagation delays for different input voltages. The flatter these responses are the more constant of a propagation delay will be observed which in turn results in very little to no added distortion. Using a device with a low on resistance response also helps alleviate any signal attenuation across the switch/multiplexer. For a quick example the TMUX7462F at supplies of +/-15V has a typical on resistance flatness of 0.6Ω with a worst case of 1Ω and has an on capacitance of 16pF. Assuming 0pF of additional capacitance from the system – the prop delay differences can be typically around 9.6ps to 16ps. These are very small differences and when compared to the input signal generally these differences will be negligible showing that precision multiplexers like TI’s Fault Protected Switches and Multiplexers. However large output capacitances can greatly increase these delays and therefore distortion.

Besides the channel impact there are other errors that can result in issues for the system. Multiplexers and Switches will have leakage current associated with them – these are currents that ideally don’t exist. Off leakage is current that is being sourced/sunk by a pin when it is unselected or opened – this is specifically leakage during normal conditions with no faults and VDD is active. On leakage is leakage current through an on channel – essentially this causes the current going into the switch to not equal the current coming out of the switch – this can cause offsets and errors at the output of the multiplexer. These leakage currents are worst at high temperatures and to mitigate the effects devices with low leakage should be utilized. The TMUX7308F for example at +/-15 V supplies has typical off leakages of 100pA and worst case off leakage peaking at +/-8nA for source pins and +/- 15nA for the drain pin. While typical on leakages of 300pA with the on leakage worst case peaking at +/-25nA. With low currents and low on resistances offset and voltage errors can be kept low.

On the topic of leakage currents there are three other values – the leakage under powered off conditions, leakage when supply pins are floating, leakage during fault condition. These are all called leakage currents under fault – the TMUX7462F data sheet is shown in Figure 3-1 to see how these specs are reported.

Figure 3-1 TMUX7462F Fault Condition Leakage Table