Technically speaking Test equipment loading effects
Back to basics! Yes, sometimes it is important to take the time to digress to the basics that we were taught in technical school. Maybe some us missed class that day, or spring fever had a grip on our thoughts, or a number of other diversions might have captured our attention! Anyway, for whatever reason, some of the basic points sometimes seem so far away and very cloudy!
Have you ever been led on a wild goose chase by misleading readings from your test instruments? Practically all of us who have worked in this business for any length of time have been led astray from time to time by misleading or misinterpreted instrument readings. Let’s look at some of the loading effects of test instruments and the underlying causes.
The multimeter All multimeters are not created equal. Generally speaking, you get what you pay for in test equipment. You can save money up front but pay for it in time and frustration down the road. Let’s compare some of the old analog multimeters and see what those sensitivity specifications really mean.
*The VOM–The old VOM (volt-ohm-milliammeter) has long been an essential piece of test equipment. Sensitivity of the old VOM multimeters was specified as so many ohms-per-volt. Let’s examine the meaning of this in terms of measurements on a practical circuit.
The circuit shown in Figure 1 top, left is a simple dc circuit. Because the two series-connected resistors are both of equal resistance (1M), the 12V supply voltage should divide equally between the two resistors, producing a voltage drop of 6V across each resistor. So, we pull out our old trusty multimeter with a sensitivity specification of 20,000 ohms/volt to measure the voltage drop across the resistor R[sub2]. Because the expected voltage reading is 6V, the multimeter scale is switched to the 10V range. (Actually, the old professor at technical school said to start with the highest voltage range and work downward–does anyone do that?)
So, with the multimeter switched to the 10V dc scale, the probes are touched across resistor R[sub2]. What? The voltage reading is only 1.71V. (See Figure 2 on page 8.) Obviously, the reading indicates a malfunction in the circuit. Suspecting the supply voltage to be low, we place the multimeter probes across the supply voltage source. The voltage here reads the full 12V, as it should. The measurements taken thus far would tend to indicate that resistor R[sub1] has increased in value. (Resistors do it all the time!)
Because the voltage reading across R[sub2] was 1.71V, the reading across R[sub1] should be 12 minus 1.71, or 10.29V. (You do remember what the professor said about Kirchoff’s law, don’t you?) So, expecting a reading of 10.29V across R[sub1], the multimeter is switched up to the 50V scale, and the probes are connected across R[sub1]. (See Figure 3 on page 8.) What? Only 4V? How can that be? According to what the professor said about Kirchoff’s law, the sum of the voltage drops across the circuit should equal the supply voltage. But 4 plus 1.71 only adds up to 5.71V–nowhere close to the 12V supply voltage. What happened to the other volts?
To further confuse the issue, the multimeter connected across R[sub1] was switched from the 50V scale down to the 10V scale. (See Figure 4 on page 8.) Now the voltage reading becomes 1.71V. What’s going on? Don’t throw out the multimeter just yet! Let’s explore the situation to find out what is really happening here.
Remember the ohms/volt sensitivity specification? That is the basis for solving this mystery. Back in Figure 2 where the first voltage measurement was made, the multimeter was switched to the 10V scale. The dc sensitivity of this particular multimeter is 20,000 ohms/volt. Thus, on the 10V scale the instrument presents a shunt impedance of 20,000 X 10 = 200,000 ohms. Now, if we substitute a 200,000-ohms resistor for the multimeter, we can better understand what is happening to our circuit. (See Figure 5 on page 8.) Here the circuit is redrawn with a 200K resistor representing the multimeter shunt impedance. The two resistors in parallel can be reduced to a single 166.667K resistor as shown in Figure 6 on page 8. The voltage drop across the 166.667K resistor should be equal to:
(Rsub2/(Rsub1+Rsub2)) * EsubS = (0.16667/(1+0.166667)) * 12 = 0.14286 * 12 = 1.71V
Thus, the reason for measuring 1.71V across R[sub2] is the shunting effect of the input impedance of the multimeter. A multimeter with lower sensitivity (lower ohms/volt) would produce even worse readings. See Figure 7 top, right where a “bargain” 2,000 ohms/volt meter is used to measure the voltage across R[sub2]. With the meter switched to the 10V range, the impedance of the multimeter is 20,000 ohms. The meter reading is only 231mV. This is the actual voltage across the resistor when the meter is connected, but when the meter is removed, the voltage across R[sub2] goes back up to 6V. If the original multimeter with 20,000 ohms/volt sensitivity were switched to the 2.5V range and connected across R[sub2], the meter would read 545mV as shown in Figure 8 center, right.
From this we can reason that to determine whether the multimeter is causing significant loading of the circuit under test, simply increase the range setting on the multimeter. If the voltmeter reading increases substantially, then the meter is causing serious loading of the circuit. To minimize loading of the circuit under test, the multimeter should be switched to the highest useable range. If the multimeter input impedance is 10 times greater than the resistance of the circuit being tested, then the error caused by loading will be extremely small and generally acceptable for all but the most critical applications. *Digital multimeters–Digital multimeters are popular, even among the “old salts.” The input impedance of these instruments is generally around 10M and doesn’t vary with the range setting. Figure 9 on page 70 shows such an instrument connected to the same circuit we have been discussing. Here, the voltage measurement is 5.71V. Because the voltage should be 6V, the error is less than 5%. For most servicing applications, this is well within the acceptable limits.
Other instruments Multimeters are not the only instruments that cause loading errors. Almost any instrument connected to high-impedance circuits can produce loading errors. Let’s look at the RF voltmeter that is used to measure RF voltages on tuned circuits.
*RF voltmeter–In Figure 10A on page 76 is an RF resonant circuit. The resonant frequency of the tuned circuit is about 160MHz. (See the graph in Figure 11 on page 76.) The RF voltmeter has an impedance consisting of 2pF across a resistance of 100K. If the RF voltmeter is connected to point 2 at Figure 10B, the probe loading is represented by the red capacitor and resistor. With the RF voltmeter connected to the circuit, the resonant frequency changes from 160MHz to 145MHz as shown by the graph in Figure 12 on page 78. To determine just how much loading error your RF voltmeter is causing to the circuit under test, you can use the setup in Figure 13 on page 78. Here, two RF voltmeters are used. One is connected to a point early in the amplifier chain, and the other is connected to a point at the end of the amplifier chain. First, connect RF voltmeter No. 2 to the circuit. Then, while observing the reading on voltmeter No. 2, touch the probe of RF voltmeter No. 1 to the circuit. If the reading on voltmeter No. 2 changes little, then the loading of voltmeter No. 1 is insignificant. Make sure that the stages between the two voltmeters are not limiter stages! If limiter stages separate the voltmeters, then the reading on voltmeter No. 2 may not change much even if voltmeter No. 1 is causing serious loading on the circuit.
Summary The point of all this is to increase your awareness of instrument loading effects. Don’t let the loading errors set you on a wild goose chase! In critical, high-impedance circuits, use instruments that cause minimal disturbance to the circuit under test. Sometimes compensations for minor RF voltmeter detuning or loading can be made by temporarily retuning the circuit to which the meter is connected to “peak” the meter reading. Then, once the meter is removed, the tuning inductor or capacitor is returned to the original setting.
It is just as important (or more so) to know the limitations of your equipment as it is to know its capabilities. The more you know about your test equipment, the less likely you are to go wild-goose chasing!’Til next time…stay tuned!
