Why do two different ICs have wildly different high level output voltages?

Question

I have two ICs on a breadboard (74ls374 D type flipflop and 74ls04 hex inverter) and I have an arduino measuring the voltage on the respective output pins of those ICs. The outputs are both at a logic high, but the flip flop output voltage sits around 3.3V, while the inverter is at 4.5V. There is no load resistor or anything like that.

Why might this be happening?

EDIT: The reason I am using the arduino ADC is because I am a skint college student who cannot afford a multimeter at the moment. Arduino is the best I have

Answer 1

The logic family specification defines min./max. levels for H and L output states.

As long as both ICs satisfy the specification (which I assume is the case; i.e. in this case voltage is above min. H output Level 2.7V) there is nothing to wonder about.

The output voltages don't have to be exactly the same; they just have to be above the minimum level.

See e.g. \$V_{OH(MIN)}\$ "minimum guaranteed voltage at an output terminal" here.

So after it is clear that they don't have to be the same: here are some reasons for them not being the same:

• different temperatures (maybe even different elevated temperatures sometime in the past; e.g. by overload)
• output circuits inside ICs differ for some reason (e.g. because they have different max. fan out)
• ICs are made by different manufacturers
• ICs are made in different process technologies
• ICs are made at different plants
• ICs are made at different times
• IC dies come from different wafers
• IC dies come from different parts of the same wafer
• ...

The answer is similar to the answer for "Why may \$\beta\$ vary so much for two samples of a BC547C transistor?"

Answer 2

You give no enough information - you must check if they both are within specifications in terms of +5 V power supply at least.

Next, if you look into the datasheets of these devices -

• 74LS374
• 74LS04

You will find that typical Voh voltage is exactly around 3.3 V, which is valid TTL signal level.

Regarding this big difference - look at 74HCT04, depending on the source of your component you hay have remarked HCT part, which outputs CMOS level. I guess exact internal circuit may differ from manufacturer to the manufacturer, thus the difference in output voltage. Generally not possible to say as you do not provide the manufacturer name, and pictures of the ICs.

It would also be really good if you use multimeter to confirm your Arduino readings. You actually do NOT need to buy multimeter to use it once, borrow it from the lab for the half of an hour.

Answer 3

I have two ICs on a breadboard (74ls374 D type flipflop and 74ls04 hex inverter)...outputs are both at a logic high, but the flip flop output voltage sits around 3.3V, while the inverter is at 4.5V. There is no load resistor or anything like that.

Is ANYTHING connected to the outputs? The TTL high output level, if there is a TTL input being driven, may rise above the (nominal) level, which is about 3.5V. An SN74LS04 input is a circa 25k ohm pullup resistor, and a 7404 input is a 1k ohm pullup resistor plus one diode drop, from +5V.

Odd though it may seem, an unloaded output, when high, is lower voltage than a loaded output (if the 'load' is a TTL input). The minimum and typical values in the data sheet are for PULLDOWN loads, and do not represent an average value for the common situation of an output pin that drives another logic gate of the 74nn or 74LSnn families. There's nothing in the output circuit that holds the output pin voltage down when the logic level is HIGH.

The 74LS logic family is a good example for students, because it has so many strange behaviors. Understanding its input/output foibles is a kind of rite of passage