I am using a voltmeter to test the output voltage of the SN74LS08N AND gate. The inputs are simply jumper wires that I can conveniently plug into a voltage source (5V), GND, or otherwise leave it hanging. The output of the gate I hook it up to the voltmeter, whose COM port is hooked to my circuit's GND. When I plug both input jumper wires into GND, my voltmeter reads 0V as expected. When I plug both input jumper wires into 5V, my voltmeter reads 5V as expected. However, if I leave the jumper wires hanging, the voltmeter hovers around 2.5V. Why is this the case? I am suspecting this has to do with noise interference from the environment.
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\$\begingroup\$ If your voltage-measuring device was an oscilloscope instead of a voltmeter, you wouldn't have to ask. Your output is technically a logic high at +2.5V, but a weak high. I suspect you'd find some 50/60 Hz signal superimposed. A voltmeter can't easily display such a fast-changing result. It might be interesting to see what your voltmeter says when you switch it to read AC volts. \$\endgroup\$– glen_geekSep 2, 2022 at 13:11
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3 Answers
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The datasheet shows the internal circuit:
In theory, open inputs should behave as if high. But high-impedance inputs can easily pick up noise, and the circuit was not designed to work with open inputs.
A DC value of 2.5 V does not make sense; maybe the output is oscillating.
Please note that the base current of the two high output transistors must be able to flow out of the output, so a voltmeter alone will not show the behaviour as in a TTL circuit. (With a small load, the high output voltage usually is near 3.4 V.)
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TTL has some rules:
Open inputs are logic high. This is by design, as driving an input high just reverse-biases the input diode.
Outputs have “nice” voltages only when pulled via 20k or less to VCC.
A logic low is between 0 and 0.8V while a logic high is anything above 2.0V.
Assuming the output you’re measuring is not oscillating - and it shouldn’t be - you’re seeing a valid HIGH level. With a pull-up resistor it would go up a bit higher and perhaps look “more convincing”, but as far as TTL goes, 2.0V and up is a valid high and that’s that.
answered Sep 2, 2022 at 13:31
Kuba hasn't forgotten MonicaKuba hasn't forgotten Monica
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Invalid input voltages cause invalid output voltages.
Measure with multimeter what the input voltages are. Compare the results with datasheet range of valid logic 0 and logic 1 voltage ranges.
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\$\begingroup\$ TTL inputs are diodes. Driving them high is equivalent to leaving them open (high impedance), but the external voltage source acts as an electrostatic shield and decreases potential noise pickup. Such noise pickup is not normally a problem unless the layout is very poor - and back in the day it usually was. With a ground plane, TTL inputs left open are perfectly acceptable and connecting them has no benefit IIRC. \$\endgroup\$ Sep 2, 2022 at 13:34