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Level shifter for 2MHz signal. (5V to 12V)

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moeedmughal

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Hi all,

I have a 2MHz/5V signal. I want to shift its level at 12V then transmit it over a wire (70cm long) to another board. On the other end i would like to shift level of 12V signal back to 5V. I tried CD4504B. But the signal get distorted ..... and duty cycle changed to 60% from 50%.

Kindly suggest me an alternate level shifter for this specific application.


Many thanks.

Moeed.
 

Hi KlausST,

Thank you for your suggestion. For RS-485 i need two wires to transmit each signal between the boards. Interconnect cable is already 12 way and has 12 way molex connectors on both ends. Space on both boards is limited.

I can not change the cable/connector. Under these circumstances it would be great if you can propose a solution.

Reagrds
Moeed.
 

Hi,

Why do you think things are better with higher voltage?

Old RS232 runs with up to +/-12V (maybe 100kBaud, single ended)
RS485 with +/- 5V (some megaBaud, differential)
modern LVDS with 0.3V (some gigaBaud, differential)

--> My advice: Don´t use 12V., it´s no improvement. Use RS485 differential signalling instead. Drivers available.


Klaus

- - - Updated - - -

Hi,

12 way cable .. this is more than enough.

Klaus
 

I mean to say that i already have 12 way cable and no wire is spare and can not be replaced by cable with more interconnections. Hence, if i use RS485, more wires are required to transmit differential signals which is not possible at the moment. Therefore, solution should be single ended, unfortunately.

Regards,
Moeed
 

That sounds like bad planning!

Your best solution is probably to use a Schmitt trigger input gate at the receiving end and possibly a series RC network to ground to supress ringing. I would not advise raising the level to 12V, all that does is increase the drive current you need (= probably more waveform distortion) and increase cross coupling to adjacent wires.

Brian.
 

Hi,

I mean to say that i already have 12 way cable and no wire is spare
I assume ... one single wire is available to transmit the signal.

I wonder why the cable is "fix" but both involved PCBs may be modified.

And I wonder what signals are at the other 11 cables? Maybe one can modify /improve something.

Is it a constant clock?, or what is it?
Or is it data transfer?

It sounds as if the cable is unshielded, not twisted, not terminated....
What about EMI (EMC, too)? The overtones of the 2MHz signal (not distorted ... means sharp edges ... means high frequency components) combined with the 70cm cable may give a good transmitting antenna.

Generally this is not a good idea.
If there is no way around the "single wire" solution, then I´d rather lower the signal voltage than increase it.
And I´d reduce the bandwidth....(close to 2MHz) then the signal becomes more "sine shaped", wich means less EMI. Then the suggested schmitt-trigger solution.

Klaus
 

Thanks Brain to reply. Indeed it is a poorly planned setup. RC circuit on output of Schmitt trigger ? As per my understanding, series RC circuit will add delay. I am not able to see throughput of schmitt triggers on data sheets.

Further details about series RC circuit would highly be appreciated. Can you suggest a schmitt trigger IC with data rate more then 2MHz.

Regards,
Moeed
 

Hi,

Can you suggest a schmitt trigger IC with data rate more then 2MHz.

Any modern logic schmitt trigger will do. Datasheet is your friend. (Either there is a frequency specification or timing specification)

Klaus
 

Thank you to reply Klaus. Rest of the wires are supplies, ground and TTL signals. The signals i want to transmit are data and permanent clock. Cable is unshielded. Signals at source are 5V. Available supplies are 5V & +12V with common ground. i can up level the signal to 12V and then down level back to 5V.

I feel Schmitt trigger is a suitable option in this scenario.

I do not understand this bit "If there is no way around the "single wire" solution, then I´d rather lower the signal voltage than increase it.
And I´d reduce the bandwidth....(close to 2MHz) then the signal becomes more "sine shaped", wich means less EMI."


I can lower the signal by passing it through a voltage divider. What you mean by reduce the bandwidth ? I can not tweak data rate and clock.

Regards,
Moeed

- - - Updated - - -

I can find timing details like transition delay and propagation delay on datasheet, but not bandwidth. How can i use them to check the bandwidth of schmitt trigger ?

Moeed.
 

Hi,

several times we recommended not to use 12V signals. But you are free to do what you want.

I´d reduce the voltage level to decrease EMI. The same time I´d make the signal line low ohmic.
But the problem with single ended signals is, that the signal current needs a return path. In your case GND. The current causes a voltage drop on the GND line. But with high frequencies the GND line is high impedance and thus the voltage drop is high. Not to compare with a volage drop caused by DC current.
The voltage drop reduces signal integrity.
****
The signals i want to transmit are data and permanent clock
You want to change more than one signal? How many?

****
Bandwidth:
A square wave signal has overtones. A lot of overtones. And the frequency (in theory) is unlimited. Therefore a 2MHz signal may send out overtones above the 100MHz region.
This is called EMI. The radiated energy may harm electronic devices nearby. You are responsible to comply with EMI standards of your country.

The solution is to suppress the overtones at the transmitter side with low pass filters. But the signal is no square wave anymore. The more you suppress the overtones the signal becomes shaped like a sine.
Usually it´s no problem to use a schmitt trigger (or comparator) at the receiver side to generate a square wave from the sine.

****
You talk about delay. If this is an issue you have to tell us the specifications.
You talk about duty cycle. If this is an issue you have to tell us the specifications.

Klaus
 
Further details about series RC circuit would highly be appreciated. Can you suggest a schmitt trigger IC with data rate more then 2MHz.

As you are using a 5V supply, I would suggest a 74HC14 would be suitable.

By series RC network I meant a resistor and capacitor in series then wired from the signal to ground at the receiving end, for example from a 74HC14 input pin to it's ground pin. You may have to experiment to find the optimum values but the idea is the capacitor stops DC current flowing but it 'invisible' to the signal then the resistor acts as a terminator to the cable impedance. If the impedance is matched, any ringing or overshoot in the signal will be minimized. Good starting values might be 1nF and 220 Ohms but the impedance of such a cable and connectors will be almost impossible to calculate so trying different resistor values while watching the waveform on an oscilloscope would be my way to do it.

Brian.
 
Along with your 70cm of forward cable, you also probably
have a similarly poor ground return.

A generic power MOSFET driver could certainly slap that
load around, and this might help with duty cycle distortion
(which in some logic families might come from the drive
strength asymmetry of the outputs). But I'd first recommend
looking at whether you have adequate drive for the load,
like by paralleling the gates of a hex / octal buffer (or just
trying one of the stronger types like octal bus transceivers,
24mA vs normal 4mA type outputs). Also look at the far end
for signs of transmission line effects (like a "porch" halfway
up, which makes logic decision very unrepeatable). The
nature of the signal degradation ought to indicate what the
driver needs to be. Asymmetric dV/dt on rising & falling edges
might just want more drive. Ugliness like porches and ringing
wants termination (but this might end up reducing amplitude
to the point that standard logic fails).

At 2MHz you can find comparators that could be used as a
front end, accommodating the transmission line termination's
attenuation.
 
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