What is the power rating or wattage of the integrated resistor in SN65LVDT34D?
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We are operating in a closed system in which the digital ground for the differential driver and the differential receiver are the same.
The SN65LVDT34D datasheet has an equivalent input schematic diagram as in the datasheet page 3. If a TTL level signal were applied to input A and B such that the Vod (Va-Vd) max were violated (>1V), the integrated resistor (mathematically) appears to have sufficient margin for power dissipation. In other words, we have reliability issue with a situation in which the LVDT receiver is driven by a RS-422 level differential signal and appears to work (and has worked for a period of time, years even) without issue and the integrated resistor is still within tolerance of 110 ohm.
Question 1- The 200K series resistor in the attenuation network on both the A and B inputs presents a high input impedance and a low base current to the NPNs. Although I’m not sure how “low” low really is on this base current. What part of the input circuit should we have expected the circuit to fail?
Question 2- We plan to correct this, but the design is a bit space constrained. Does TI have a RS-422 differential receiver with an integrated termination resistor? Also see question 4.
Question 3- The datasheet indicates (in the recommended operating conditions) that without the integrated resistor the max differential input voltage is 3V, up from the 0.8V with the resistor (444mW power dissipation). What limits the voltage differential for the integrated resistor design versus without.
Question 4 - The datasheet (revised Nov 2004) states this LVDS part is pin-compatible with the AM26LS32 which is a RS422 differential receiver. Was this a typo to include a 422 receiver in the list of pin-compatible parts or was there another intent to include a Rs422 differential receiver in a LVDS receiver datasheet?
We don't have RS-422 receivers with integrated termination. Looking at the respective datasheets, this part is pin-compatible with the AM26LS32, as all the pins are named the same and have the same function between devices. However, this does not mean it is a drop-in replacement in regards to operating conditions.
Since this a very old device, unfortunately I do not have answers to your other questions as they would require looking into the actual design of these devices. Your design may have been working fine up until now but we can only guarantee correct operation of the device if it is operated within the recommended operation conditions specified in the datasheet. There is an application report detailing the use of RS-422 level signals with an LVDS receiver (no integrated termination) that you can reference though: https://www.ti.com/lit/an/slla031a/slla031a.pdf