cause of AND gate breaking
cause of AND gate breaking
We have an ultrasound medical iamging device which has a motor attached to it. The motor is a galil controller. The galil controller issues triggers via a 3V high off signal which is converted to 0V for the trigger. The signal which recieves the trigger keeps breaking. It could also be that a test execuationer for the board hooked up the boards wrong too though.
The reason the doppler imgign modlues failed is that trigger input (pin 2) from the galil controller to IC U54 (AND gate) was grounded. Upon removing the AND gate, the short circuit went away, showing it was broken. A schematic is as follows
The input from the galil motion control is labeled as ‘pulse trigger’. The Doppler select is a signal which is high for ultrasound imaging and then low for Doppler imaging, it is a 5V signal when on which has little ripple or noise. Im not even sure in what state the chips are breakign in.
The chips that is breaking is IC U54 which and has a description as follows. Part number: FAIRCHILD 74AC08SC (IC Gate and Quad 2-Input AND gate 14-SOIC). The 74AC designated high speed CMOS. The pinout for the device is as follows74AC08SC (IC Gate and Quad 2-Input AND gate 14-SOIC).
The galil trigger signal that is going into the pin 2 is shown below. It is a normally off at a 3V high signal that turns on for a trigger pulse of 0V for 600ns at 0V. There is likely some overshoot on the negative pulse although the image below will also show RF induced noise too. The following image as taken with the probe touching pin 2 and the probe in an erroneous state (it completes one image sequence movement before erroring out). There is probably inducatance is the approximat,ly 10ft of cable which the galil signal passes through.
From the 74AC08SC datasheet, the minimum voltage the pin can withstand is -0.5V. It is likely we are below this value at the lowest voltages. Another likely event is that if someone plugs the image pulse into this connector, it will also break the gate by exceeding the VCC+0.5V rating. The supply voltage (VCC) was a flat, contast 5V and is unlikely to be the issue.
An example of another IC in the console that handles a galil trigger is the SN74LS123N in a different module, this is a IC- Dual Monostable Multivibrator. I have no record of this chip ever breaking in our consoles and it handles the same signal as the pulse that breaks. I cant seem to find hte low voltages it can withstand in the datasheet.
Anyone have any ideas, does the write up seem logical?
What about ways to fix it. A capactior across the output with a time constant between the ripple of overshoot and that of the main pulse. How about a fast acting diode at around 0V? A replacment for AND gate that is more robust. I believe the galil pulse is fixed.