negative to positive level shifting
negative to positive level shifting
(OP)
Ok here we go, for very complicated reasons I don't want to explain, here is my setup: I have a 68hc12 that needs to talk to a digital resistor chip over SPI.
Microcontroller: Gnd: 0V Vcc: 5V
dig resistor : Gnd: -5V Vcc: 0V
So as you can see I need to level shift into the negative voltage range. I was thinking about using pulldown resistors, however I would also need to put a resistor in series from the MCU to the dig resistor. Does anybody know of some level shifting chips I might be able to use in order to make this happen? Thank u very much.
Microcontroller: Gnd: 0V Vcc: 5V
dig resistor : Gnd: -5V Vcc: 0V
So as you can see I need to level shift into the negative voltage range. I was thinking about using pulldown resistors, however I would also need to put a resistor in series from the MCU to the dig resistor. Does anybody know of some level shifting chips I might be able to use in order to make this happen? Thank u very much.





RE: negative to positive level shifting
Keith Cress
kcress - http://www.flaminsystems.com
RE: negative to positive level shifting
http://www.avagotech.com/
RE: negative to positive level shifting
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Do I want Transistor, Darlington, Transistor with VCC, or Transistor with Base?
I'm not sure but I would think I would just want transistor.
Thanks!
RE: negative to positive level shifting
The key thing with optocouplers is to look at the TRANSFER RATIO. That is of key importance. You will never get more than that thru the transistor.
In your case you want to pay attention to the switching delays too. Rise time and fall times as all optos have them. You don't want your SPI frequency to exceed about 1/3 the time alloted: 1/3 rise 1/3, at level, 1/3 drop.
Keith Cress
kcress - http://www.flaminsystems.com
RE: negative to positive level shifting
RE: negative to positive level shifting
htt
RE: negative to positive level shifting
Drive the emitter through a resistor from the 0 to +5v signal source, and connect a pullup resistor of similar or slightly higher value from the collector down to -5v.
This circuit will be non inverting, it just level shifts, so a further refinement is to capacitively couple direct from one level (signal input) to the other level (signal output). The capacitor makes for very fast edges without introducing significant transmission delays, and the transistor, maybe even a slow transistor, handles the dc shifting function, so it works right down to continuous dc,
This circuit is fast, cheap and simple.
RE: negative to positive level shifting