# Electronics project notes/Device voltage and impedance

(Redirected from Impedance-bridge)

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# Digital logic voltage levels

This article/section is a stub — some half-sorted notes, not necessarily checked, not necessarily correct. Feel free to ignore, or tell me about it.

An image search like this may be the best answer or at least good indicator of the variation out there.

I prefer diagrams like this one: http://www.jsykora.info/2014/05/logic-voltage-levels/

...because these summarize and compare many more, and some related conventions, like the voltages in LVDS communication[1].

The ones you'll probably run into most (in home DIY context):

### 5V TTL

5V TTL: (Vcc is ideally between 4.75V and 5.25V)

• VOL: 0.4V
• VIL: 0.8V
• VIH: 2V
• VOH: 2.4V (can be slightly higher in some logic families(verify))
• VCC: 5V

A more intuitive view on that is:

on inputs
low is 0V to 0.8V
high is 2V to 5V
on outputs
low is 0V to 0.4V
high is 2.4V to 5V

The difference between input and output is mainly there so that you can tie such components together, and noise voltages lower than ~0.4V introduced between them won't bother anything.

### CMOS

CMOS defines levels as a percentage of VCC, which can itself be 5, 3.3, 2.5, 1.8, 1.5, 1.2V. (there have also been higher-voltage CMOS systems but this is fairly rare now(verify))

• VOL: 10%
• VIL: 30%
• VIH: 50%
• VOH: 70%

Where you may want to controlling a 5V Led string from a 3.3V device,

you can control 5V TTL but not 5V CMOS, because 5V TLL is high above ~2.4V and 5V CMOS is high above 70%*Vcc = ~3.5V (all the highness happens above 3.3V so it'l never go high). For completeness:

3.3V CMOS means (OL=0.33V, IL=1V, IH=1.65V, OH=2.3V) and matches 5V TTL well enough
5V CMOS means (OL=0.5V, IL=1.5V, IH=2.5V, OH=3.5V)
5V TTL means (OL=0.4V, IL=0.8V, IH=2V, OH=2.4V)

### LVTTL

LVTTL: 0 to 3.3V. All threshold levels are identical to the above, and only the VOH to VCC region is smaller.

Note that the above means that 3.3 and 5V doesn't always imply the same thresholds unless you know the logic family.

### Serial

Simple serial

• RS232
space between +3..+15V, mark between -3V..-15V
mostly replaced by 5V or 3V level, same protocol -- but you still see this bipolar, higher-voltage variation (mostly on non-PC devices?)

• "TTL serial" is RS-232-style in communication,
0 and whatever the IC's Vcc is, usually 5V or 3.3V
common on modern boards and ICs that do serial

Seeing a DE-9 connector, you probably want to use a multimeter to check that it is oldschool RS232, and not connect it directly to the latter. Doing so may work, but will also everntually burn out the 5V side.

• RS485
• RS422

See Electronics notes/IO and wired communication#Common_serial_port_variants for more functional-level details.

### Other notes

Note:

• VCC and Gnd tend to be correlated with BJT transistors and TLL logic
(Vcc seems to have originally meant "the voltage common to all BJT collector pins", which is often just the positive voltage supply)
• VDD and VSS tend to refer to FET style ICs and boards ('source', 'drain')
• V+ and V- are more generic, seen e.g. on board power connectors
• ...but people use them fuzzily, and they may mean little more than "the higher and lower voltage that the power supply puts out"
• Also, while V-, Vss, and Gnd in many cases are
what you'd call 0V and/or
used as a voltage reference and/or
the lowest voltage around in the circuit
...but there are even more footnotes to that one.
• I'm assuming 2-level logic here
there are more-level logic out there, but most are specialized/niche [2]
and that three-state logic that you get from tristating is understood as something distinct

# Semi-sorted

On a circuit board schematics, and possibly its silkscreen, you see labels like:

• VCC - positive supply, BJT
• VEE - negative supply, BJT (may be Gnd)
• VDD - positive supply, FET
• VSS - negative supply, FET (may be Gnd)

In more general use you might use

V+ and V-
VS for supply voltage
Schematics often mention a rail's voltage, e.g. +12V

...though people have their habits