How to Test a Nand Gate
NAND gates are a common logic element used in many electronic circuits. The most common configuration is a two-input NAND gate, although configurations with more inputs are common. A NAND gate can either be a discrete circuit, usually mounted in a small dual inline package (DIP), or incorporated into a larger integrated circuit. A microprocessor chip contains millions of NAND gates or their equivalents. A NAND gate operates by outputting a logical high unless both inputs are logical high, in which case its output is a logical low.
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Things You'll Need
- NAND Gate and Associated Electrical Specification
- 3 Voltage Sources Capable of 0 to 5 volt operation
- Voltmeter
- Signal Generator (optional)
- Dual channel storage oscilloscope and probes (optional)
Instructions
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Testing NAND Gate Function
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1
Read the electrical specification for the NAND gate and identify the following levels: VOH, VOL, VIH and VIL. These are the output and input voltages for logical high and low signals. Also note the required supply voltages (Vdd and Vgnd).
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2
Set the first programmable voltage supply to Vdd. Connect the positive lead of this supply to the Vdd pin of the NAND gate, and connect the negative lead to the Vss or GROUND pin of the NAND gate.
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3
Set the second programmable voltage supply to VIL. Connect the ground lead of this supply to the ground lead of the first supply.
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4
Set the third programmable voltage supply to VIH. Connect the ground lead of this supply to the ground lead of the first supply.
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5
Connect the positive lead of the VIL power supply to both inputs of the NAND gate. Use the voltmeter to measure the voltage between the NAND gate output and ground. It should be higher than the VOH specification.
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6
Connect the positive lead of the VIH power supply to both inputs of the NAND gate. Use the voltmeter to measure the voltage between the NAND gate output and ground. It should be lower than the VOL specification.
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7
Connect the positive lead of the VIH power supply to one input of the NAND gate and the positive lead of the VIL power supply to the other input. The voltage at the NAND gate output should be less than VOH. Reverse the inputs and verify the output remains below VOH.
Testing NAND Gate Propagation Delay
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8
Connect the Vdd supply power and ground to the NAND gate.
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9
Connect the positive lead of VIL supply to one input of the NAND gate. Connect the ground to the ground from the Vdd supply.
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10
Program the signal generator to deliver a single low to high transition. Connect the signal generator to the other input of the NAND gate.
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11
Connect one channel of the oscilloscope to the signal generator, and the second channel to the NAND gate output. Program the oscilloscope for edge trigger and capture..
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12
Initiate a single transition from the signal generator. The oscilloscope will capture both the input and output to the NAND gate. The time difference between the transition in these signals is the propagation delay of the NAND gate.
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13
Repeat the previous step with a high to low transition.
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14
Exchange the connections on the inputs, and capture the propagation delays on the other input.
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1
Tips & Warnings
Never apply a voltage to an input without Vdd being supplied to the NAND gate. Circuit damage may result.
NAND gates almost never have propagation delay issues, and the second part of the procedure is often skipped.
Depending on the manufacturer and process technology, the signal names in this article may differ from those used in the specification.
If you are not concerned with input voltage sensitivity, one power supply can be used for the NAND Gate Function Test. Use the Vdd supply positive lead to generate VIH, and the ground lead to generate VIL.
Temperature can affect all voltage and timing measurements of a NAND gate. Be sure to test at the operating temperature. Do not exceed the specified temperature range.
