1. Objectives

  • Reading data sheets and extracting required parameters.

  • Understanding the significance of some major input and output electrical specifications.

  • Understanding the limitations of driving various loads.

  • Develop experiments to measure and verify some of these specs.

2. Materials Required

  • IC – 7404

  • Potentiometers / Variable resistors

  • Wires

  • Wire stripper

  • Prototyping board with power and ground connections

  • IC Tester

  • Multi-meters

3. Background

driver load

  • If the output of some gate A is connected to the input of another gate B, gate A is said to be the driver gate, while gate B is said to be the load gate. Equivalently, we say:

    • A drives B, or

    • B loads A

multiple loads

  • A single driver gate may have more than one load gate.

different loads

  • The load does not necessarily have to consist of similar gate(s). The load may be a combination of a variety of other devices, e.g. a purely resistive load, a servo motor, etc.

3.1. Requirements for Proper Load Driving

Load input voltage and current

  • To drive a given load, the driver must be capable of providing the load with the proper values of:

    1. Input voltage (high and low), and

    2. Current (in and out)

3.2. Voltage Requirements

voltage high low

  • Using electric signals to represent logic values, a logic 0 value is allocated a range of low voltages while a logic 1 value is allocated a range of high voltages:

voltage output input

  • If both the driver and load are IC gates, the driver output voltage Vhigh and Vlow values and the load input voltage Vhigh and Vlow values are different and are designated as VOH and VOL for outputs and as VIH and VIL for inputs.

v oh v ih

  • Worst case driver high output voltage (VOH) should be within the range of acceptable input logic 1 voltages, i.e.

    VOH ≥ VIH

    Typically, VOH = VIH + NM1, where NM1 is the maximum tolerable noise voltage that can be subtracted from the driver high output voltage with the load gate still functioning properly.

v ol v il

  • Worst case driver low output voltage (VOL) should be within the range of acceptable input logic 0 voltages, i.e.

    VOL ≤ VIL.

    Typically, VIL = VOL + NM0, where NM0 is the maximum tolerable noise voltage that can be added to the driver low output voltage with the load gate still functioning properly.

3.3. Current Requirements

i oh

  • When the driver output is high (Vout ≥ VOH), the maximum current that can be delivered by the driver is known as IOH.

    If the current drawn from the driver exceeds IOH, the output voltage ay drop below the minimum acceptable logic 1 voltage of VOH.

    Thus, for proper operation:

    IOH ≥ ILoad

i ol

  • When the driver output is low (Vout ≤ VOL), the maximum current that can be sunk into the driver from the load is designated as IOL.

    If more current than IOL is sunk into the driver, the output voltage may exceed the maximum acceptable logic 0 voltage of VOL.

    Thus, for proper operation:

    IOL ≥ ILoad

Exercise

Check out the SN7404 [1] or the SN5404 [2] IC that you will use in this lab, read its corresponding data sheet, and answer the following:

  1. What is the difference between the 7400 and the 5400 series?

  2. Define the following electrical parameters in your own words, and extract their values from the data sheet of the specific IC type you will be using:

Parameter Definition Value

VIL

VIH

VOL

VOH

IIL

IIH

IOS

4. Tasks

In this experiment, we characterize some major electrical characteristics of IC input and output pins. The IC used in the experiment is a hex inverter 54/7404.

The tasks are broken down into two parts:

  1. Determination of the current drive capability of the output of an IC driver, i.e. measuring IOH, and IOL.

  2. Determination of some major voltage specifications of an IC:

    • Output specifications (VOH and VOL)

    • Input specifications (VIH and VIL)

4.1. Current Drive Capability (IOH and IOL)

In this part, we characterize the drive capability (IOH and IOL) of output pin drivers of the SN7404 IC.

Strategy

  1. Plot the output voltage of one of the hex inverters versus the output load current in two cases:

    1. Vout = Logic 1 = High Voltage, i.e. Vout ≥ VOH

    2. Vout = Logic 0 = Low Voltage, i.e. Vout ≤ VOL

  2. Together we will develop, in a step-by-step manner, the circuit needed to test case 1 (a).

  3. Independently, you will develop the circuit needed to test case 1 (b).

4.1.1. Plotting Vout vs. Iout with Vout = High (Vout ≥ VOH)

inverter v in low

  • First, for an inverter of the SN7404 IC, make sure that Vout = High, i.e. force Vin = Low.

  • This way, when connecting Vout to the load, the current will be sourced out of the driver.

inverter voltage current

  • We need to draw different values of load current from the driver and see how this affects the output voltage value.

inverter open circuit

  • For the highest impedance [3] load (open circuit), there will be no load (no current drawn, i.e. Iout = 0), and Vout assumes its highest value.

inverter short circuit

  • The lowest impedance load (short circuit) yields the largest load current, and Vout assumes its lowest value (Vout = 0).

    In this case, Vout = 0 in spite of the fact that the inverter input Vin = 0.

    Thus,

    • Vout = highest value when Iout = 0 (no load condition, i.e. open circuit)

    • Vout = lowest value = 0 when Iout = maximum (short circuit)

Conclusions
v out vs i load

  • As the load current Iout increases, Vout goes down.

  • Vout must have acceptable logic 1 voltage for proper inverter operation, i.e. Vout ≥ VOH.

  • Thus, the load current Iout must not exceed the value that brings Vout down to VOH (i.e. at Vout = VOH) → ILoad = IOH.

Test Circuit
inverter r fixed

  • The above circuit may inadvertently cause the variable load resistor to be 0 (RL = 0). Thus, the driver inverter output is short circuited to ground causing huge currents that may damage the IC.

  • To avoid such damaging large current, a fixed resistor, Rfixed, is connected in series with the variable resistor.

Estimating a Reasonable Value for Rfixed

  • Maximum output current when Vout = High (Vout ≥ VOH) is IOH.

  • The IOS current is damaging if sustained for more than a very short duration (about 1 second).

  • Let’s select the maximum allowed output current Iout(max) to be such that:

    IOH < Iout(max) < IOS

  • Assume that Iout(max) = IOS/2. Then,

    Rfixed = Vout(max) / Iout(max) = 2 Vout(no load) / IOS

Steps

  1. Vary the variable resistor from its maximum value to its lowest value in steps, each time recording Vout and the corresponding Iout.

  2. Plot Vout vs. Iout. Determine the value of Iout corresponding to the spec. value of VOH.

  3. Compare your suggested IOH value with the spec. value. Which value is lower? Why?

Questions

  1. Is the value you selected for IOH different from that specified by the manufacturer?

  2. If yes, what are some of the factors that the manufacturer might have taken into account in defining his specified values of IOH?

4.1.2. Plotting Vout vs. Iout with Vout = Low (Vout ≤ VOL)

Follow the same steps in the previous section. Use the following hints as a guide.

  1. Make sure the inverter output is low.

  2. Make sure that Iout direction is into the driver (inverter) output, not out of it.

  3. Make sure that your circuit allows varying Iout versus output voltage Vout.

  4. Make sure that your circuit protects the inverter output from passing excessive currents, i.e. the max current should be limited to an acceptable value (> IOL).

Steps

  1. Vary the variable resistor from its maximum value to its lowest value in steps, each time recording Vout and the corresponding Iout.

  2. Plot Vout vs. Iout. Determine the value of Iout corresponding to the spec value of VOL.

  3. Compare your suggested IOL value with the spec. value? Which value is lower? Why?

Questions

  1. Is the value you selected for IOL different from that specified by the manufacturer?

  2. If yes, what are some of the factors that the manufacturer might have taken into account in defining his specified values of IOL?

  3. What is meant by the output drive? What are the parameters that define the driving-ability of an IC output pin?

4.2. Major Voltage Specifications (VOH, VOL, VIL, and VIH)

In this part, we determine VOH, VOL, VIL and VIH for one of the hex inverters available in the SN7404 IC.

Strategy
v out vs v in

Determination of these voltage quantities will be done through plotting the voltage transfer characteristics of the inverter.

4.2.1. The Test Circuit

In this section, we are going to incrementally build the test circuit, justifying each revision as we go.

Revision 1
inverter v test circuit r1

  • Variable voltage source

  • Vin = VSRC

  • Vin is varied by varying the input voltage source from 0 to VCC.

  • For each Vin value, measure and record the corresponding Vout.

Merits: Directly connecting the voltage source to the IC input is not recommended as the absolute maximum rating of the input may accidentally be exceeded causing device permanent damage.

Revision 2

inverter-v-test-circuit-r2

  • A potentiometer acts as a voltage divider where both Rup and Rdown vary as we vary the rotating position of the third tapping terminal such that:

    Rup + Rdown = RPot

    where RPot = Total potentiometer resistance.

  • If the input impedance of the inverter is much higher than RPot, then:

    Vin = Vfixed / (1 + Rup / Rdown)

  • Vin is varied by varying the potentiometer knob.

  • For each Vin value, measure and record the corresponding Vout.

Merits: If Rup ≈ 0, direct connection of the voltage source to the IC input may cause an input voltage overshoot and is not recommended.

Revision 3
inverter v test circuit r3

  • The lowest Vin in this case is when Rdown = RPot(min) ≈ 0, leading to: Vin(min) ≈ 0.

  • The highest Vin in this case is when Rdown = RPot and is given by:

    Vin(max) = Vfixed / (1 + Rfixed / RPot)

  • For Vin(max) ≈ Vfixed, the value of Rfixed should be selected such that:

    Rfixed << RPot → Rfixed ≤ 0.1 RPot

Steps

  1. Vary the potentiometer rotary switch knob, and read Vin and the corresponding Vout values.

  2. Plot Vout vs. Vin.

Questions

  1. Are the values you selected for VOH, VOL, VIL, and VIH different from those specified by the manufacturer?

  2. If yes, what are some of the factors that the manufacturer might have taken into account in defining the specified values of VOH, VOL, VIL, and VIH?

5. Grading Sheet

Task Points

Plot and discuss Vout vs. Iout with Vout = High

25

Plot and discuss Vout vs. Iout with Vout = Low

25

Plot Vout vs. Vin

25

Lab notebook and discussion

25
1. SN7404, SN74LS04, or SN74S04.
2. SN5404, SN54LS04, or SN54S04.
3. resistance