Re: Wide voltage logic input design

On Mon, 3 Apr 2006 22:01:36 +0000 (UTC), the renowned Uwe Bonnes
<bon@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx> wrote:

John <spam@xxxxxxxx> wrote:

"Nobody Here" <nobby@xxxxxxxxxxxxxxx> wrote in message
news:44314496$0$9259$ed2619ec@xxxxxxxxxxxxxxxxxxxxxxxxxxxxx
John <spam@xxxxxxxx> wrote:
I need to provide an interface to a logic device (3.3V) from an input
logic
signal which can be anything from 5V to 30V. I've come up with a few
ideas
to do this with the least board area but I'd be interested to hear anyone
else's views.

It rather depends. For example, where does the input come from - is it
some other clean-edged signal or is it a sensor that might have slow
or noisy transitions? How noisy is the signal in the first place? Do
you need to control or vary the threshold voltages? Does it need
hysteresis? Are there any ESD-type issues with the input? What switching
frequency do you need to cope with? Is it actually a logic input or
is it a close-contact type with a pullup or down or an open collector
type output? Opto-isolation?

Depending on the answer to these you could get by with something as simple
as a transistor with a couple of base resistors to a comparater with
a threshold voltage controlled by a DAC output. Primarily if you need
a well defined input switching characteristic at low frequencies you'll
probably end up with a comparator of some sort.

--
Nobby Anderson

Thanks for the reply. I really should have provided more information.

The input is a pulsed waveform and will not be noisy. I don't need to
control the threshold or have hysteresis but there is a need for ESD
protection though. The frequency is low (5KHz).

A npn transistor with a base series resistor (perhaps 10k) and and equal
base-emitter and an 1 k pull-up at the collector to VCC should to the job.

A (normally reverse-biased) diode is required across the E-B junction.


Best regards,
Spehro Pefhany
--
"it's the network..." "The Journey is the reward"
speff@xxxxxxxxxxxx Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog Info for designers: http://www.speff.com
.

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