Part Number: CD74HC4067
Other Parts Discussed in Thread: LM4030, TPS2491, LMR16030, SN65HVD1781, TMUX1108
Hi.
I'm designing an IoT industrial data collector for machinery.
Here is my current schematic about the analog portion:
Regarding analog inputs, it has 7x 4-20mA current inputs (10uA resolution) and 7x voltage inputs up to 100V (25mV resolution). For voltage inputs there are 7 voltage dividers, the mid of dividers go to 7 inputs of the 16:1 Mux, for curent inputs there are 7x 100R sampling resistors that converts current to voltage, then there are 10K series resistors and then go to the Mux input.
The voltage reference is intended to be a LM4030 of 4096mV, so the ADC supply and conversion voltage reference is 4096mV, as it is a 12-bit ADC with 4096 possible results, in theory 1 LSB = 1mV.
At the current input the first component is the TVS diode SMAJ36CA, according to the datasheet of it of Bourns Inc, for a 8/20us pulse, the maximum clamp voltage is 75V, so I will consider that this can occur at some moment, like in a induced transient, mainly when the input is not in use. I will consider the worst case possible. If this occur, the voltage at the 100R resistor will be around 75*(100/(100+17.7)) = 63.7V, this would be the voltage at the left side of the 10K resistor, the right side of the 10K resistor is clampled in around 4.5V, 4096mV + ~0.5V of the internal ESD diode of the Mux that goes from the input to VCC pin. So at the 10K resistor there is around 63.7-4.5 ~= 60V. 60V / 10K = 6mA. Then if I considerer that this situation can occur on all 7 current inputs at the same time, the worst case, there is 6x7 = 42mA, this clamp current will pass through the the voltage reference LM4030, correct? The fact is that also there is around 10mA constant current flowing through it coming from the 5V supply and the series 100R resistor (R52). The total would be 52mA, which exceds the maximum of 50mA of transient withstand shown the datasheet of LM4030. Its datasheet shows: "Maximum Shunt Current (<1s) = 50mA"
To explain more, on the current inputs, the sampling resistor should be protected against a constant connection of a DC source from 20V to 36V, that why of the 36V TVS, if we consider that it is undestrutible it clamps at 75V maximum for 8/20us pulse, and the circuit connected to it should withstand such transient.
(1) The problem is that the TBU devices that protect over-current on the current inputs, they can take a "maximum of 1us" to open the path, and my concern is about that 1us possible overload, so one thing I would like to ask is if the LM4030 can withstand transients greater than 50mA for periods of few microseconds.
(2) As I said, for the ADC each 1mV is 1 LSB in theory, but on its datasheet shows that on its analog pin there is a +-1uA leakage (min/max), typ is not specified. 1uA * 10K = 10mV = 10 LSB error can take place only due to this series resistor, so at least at the moment I don't plan to increase the value of the 10K resistor (if keep this circuit) to enhance the protection of the LM4030. I'm seeing the value of this 10K resistor as a trade-off, if I increase it I gain in protection, but loss in accuracy.
(3) The Mux of the schematic, CD74HC4067SM96, when an input is connected to the IO COM pin, I did not find in its datasheet what is the leakage current. Passing all these 14 signals through it, is it possible to know what would be the maximum error inserted for current and voltage inputs? I mean for voltage and current input types.
(4) If CD74HC4067SM96 have considerable leakage for the current that is flowing between the input and IO COM, what would be another 16:1 Mux with better specs? (smaller leakage)
(5) I also thought about placing in parallel with the 100R resistors, the sampling resistors of the current inputs, a TVS diode of 3.3V or 5V with low leakage.
(6) I was also thinking to use quad OpAmp ICs to buffer some signals. For example, place a OpAmp after the 10K resistor of the current input, then I would be able to increase its resistance without problem.
(7) I think I was able to explain a little about the analog application of this device, I would like to know if you could suggest another different hardware solution using TI parts.
Regards,
Jeferson.
