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CD4070B: CD4070BE - Need replacement that doesn't drift with temperature

Plain Wacky
Prodigy 10 points
Part Number: CD4070B


Tool/software:

I am using a CD4070BE Exclusive Or gate in a self-oscillating configuration (see circuit below) and I noticed that the frequency drifts when the device is at 0-degrees C and about 38-degrees. I know this isn't an isolated chip issue or drift caused by passive components (resistors/caps) used for self-excitation/oscillation.

Can anyone recommend a replacement? 

Some of the components have been blocked out for proprietary reasons. 

  • 0
    Guru 318050 points

    What is the supply voltage?

    This device is intended for digital signals. In your application, the frequency depends on the exact input switching thresholds and propagation delays, which are not very well defined. If you want to control the frequency better, use any comparator with an explicit reference voltage.

  • 0 in reply to Clemens Ladisch
    Prodigy 10 points

    Thanks for contributing to my thread. 
    The supply voltage is 5v. 
    This circuit is on a PCB and not suitable for a redesign.
    I am currently using a thermistor (blacked out) connected to inputs 2 and 6 of the comparator to compensate for the drift. It works "OK", but if I had a temp stable exclusive OR chip, I wouldn't have to rely on the thermistor to compensate. If I had to redesign this, I'd use a stable frequency generator. 
    I know it's this chip, cause when I touch it (quickly) with a soldering iron, the frequency drifts...same is if I spray it with a freeze. 

  • 0 in reply to Plain Wacky
    Guru 318050 points

    The CD4070B pinout is different from the 74xx86 pinout, so there is no replacement. 

  • 0 in reply to Clemens Ladisch
    Prodigy 10 points

    I know I can't use that series, for the reason you just mentioned. it perplexes me why T1 made them with a different pinout...one would think they would have used the same pinout for easy replacement. 

    I was hoping there might be an automotive or military version of the CD4070, (or even a different manufacture) that would be more temperature stable.


  • 0 in reply to Plain Wacky
    Guru 318050 points

    The CD4000 series was originally designed by RCA, and in 1968, they did not know how important the 74 series would become.

    For digital logic inputs, temperature stability never has been a design concern; in fact, the VIL/VIH levels were chosen to allow large variations.

    It might be possible to create an SMD-on-DIP daughterboard with two XOR gates and two comparators.

  • 0 in reply to Clemens Ladisch
    Prodigy 10 points

    I don't have the knowledge to follow what you are suggesting and if it would be possible to easily do on an existing PCB. 

  • 0 in reply to Plain Wacky
    TI__Mastermind 40325 points

    Hi,

    You mention that this is already on a PCB that can't be redesigned, so unfortunately there isn't a P2P replacement we can give you. 

    What Clemens is suggesting is to make a daughter board that can slot into the existing PCB. However, again this depends on how the PCB is designed. 

    SMD - surface mount device
    DIP - dual-inline packaging

    I did a quick google search for a visualization. 

  • 0 in reply to Albert Xu1
    Prodigy 10 points

    Thanks...that is what I thought, but wanted to make sure. 
    Unfortunately, there are 250-units and this would be unpractical. 

    As I mentioned, i am currently using a Thermistor to compensate for the drift. It works, with a modest rework, but it would be a lot easier to replace the chip.

    What about the CD4030? Does anyone know if it will oscillate given my circuit? I was thinking of ordering a few and see if it drifts when heated. 

  • 0 in reply to Plain Wacky
    TI__Mastermind 40325 points

    Hi,

    Unfortunately as Clemens mentioned, the devices aren't really specced for this. I do not know if it will oscillate in your usecase. Your best bet will be to order some and check physically. 

  • 0 in reply to Albert Xu1
    Prodigy 10 points

    I agree. 
    I created this thread hoping I could find a replacement, unfortunately, it doesn't look like there is one. 
    Thanks everyone for helping out!