ISSUE with replacement of Linear Technology LF398N8 with national semiconductor LF398AN
ic run at + and - 11.7 volts
cap 1.0uf
pin 7 to gnd
The ics were purchased to enable support for a pcb in the future
LF398 N8 is in a weighscale pcb to be in future replaced with nat semi LF398an An issue was found that weighscale pcb had only 1.31 volts on control logic pin 8 which is under stated typical threshold voltage of 1.4v but the original Linear Technology ics all worked in all 6 pcbs (these are 15 year old pcbs )sampling and holding correctly On fitting the National Semiconductor LF398an all the boards malfunction when pin 8 is given logic trigger sample signal in this case 1.31v ,Pin 5 would climb to supply approx 10v. I compared the linear tech ic's with the national ones and the Linear tech ones performed ok with sample happening at 1.26v with no abnormal operation at any other logic trigger voltage, However all the ones from national exhibit this strange operation at approx 1.31 to 1.34 volts on pin 8 with pin 5 climbing to approx 10v with or without input signal do you have any reason why this happens? I will be increasing the logic trigger signal to 1.6 volts as this makes the national ones work ok
I would never of noticed this issue if the original boards had a higher trigger signal The only thing that worries me is the strange 1.31 to 1.34v issue I am looking forward to your prompt response
Update
contacted farnel who i purchased the ic's from They said the data sheets seem compatible and they should work which is in fact true if you have the 1.4 typical trigger input but I want to know why they behave different at the 1.31 to 1.34 voltage I have tried to contact Texas direct and was guided to this forum as they are not interested in this as obviously i do not spend enough money with them I would be interested if anyone else can recreate this problem to back me up. The only difference i can find in the data sheets is that the internal resistor at pin 6 is 150 ohms in a linear ic and 300 ohms in a texas one
