Page 71 - Troubleshooting Analog Circuits

58 5. Preventing Material and Assembly Problems

terminal or pin of your circuit under test. If you don’t control the supply voltage
precisely, you might start failing parts that are actually meeting their specs.
For example, let’s say we want to test the load regulation of an LM323, a 5-V
regulator, when Vi” is held at exactly +8.00 V, and the load changes from 5 mA to
3.00 A (Fig 5.4). In this circuit, there are four pairs of Kelvin connections at work.
The first pair is located at the power supply’s output. This programmable supply’s
remote-sense terminals permit it to maintain an accurate 8.00-V output right up to the
pin of the DUT. This is commonly called “remote sense,” when you are in the power-
supply business, but actually it represents a Kelvin connection. This is important
because if the 8.00-V supply dropped to 7.9 or 7.8, that would be an unfair test.
The second Kelvin connection in Figure 5.4 is located at the output of the DUT. In
order for you to observe the changes in V,,, as you apply various loads, the Kelvin
contacts provide Force leads for the 3 A of output current. They also provide Sense
leads, so you can observe the DUT’s output with a high-impedance voltmeter. Note
that there are two Sense and two Force connections to the ground pin of the DUT.
You don’t really need allfour contacts-you can tie both Force leads together and
also both Sense leads together. You can do that because there is no significant current
flowing in the Sense lead; and in the Force lead, we don’t care how much current
flows, nor do we care exactly what the voltage drop is.
The op amp in Figure 5.4 forces the DUT’s output current through the Darlington
transistor and then through a 0.1 R precision resistor. The only way to use a 0.1 -R
resistor with any reasonable accuracy and repeatability is to use the 4-wire (Kelvin)
connections as shown. The op amp can force the upper Sense lead to be precisely
300 mV above the lower Sense level, even if the lower end of the resistor does rise
above ground due to various IR drops in wires or connectors.
There are several places in this circuit that we could call ground, but the only
ground we can connect to the 300-R resistor and get good results is the Sense lead at
the bottom of the 0.142 resistor. If you connect the bottom of the 300-R resistor to
any other “ground,” shifts in the IR drops would cause relatively large and unpre-
dictable and unacceptable shifts in the value of the 3-A current-in other words,
Inaccuracy and Trouble. So, when you’re running large currents through circuits,
think about the effects of IR drops in various connectors and cables. If you think IR
drops will cause trouble, maybe Kelvin connections can get you out of it.
Figure 5.4’s fourth Kelvin connection is hidden inside the LM323 5-V regulator,
which has separate Force and Sense connections to the output terminal. A fifth Kelvin
connection is also concealed inside the current-limit circuitry of the regulator. Here,
the device senses the load current with a 4-wire, Kelvin-connected resistor and sends
that voltage to the current-limit sense amplifier.
The use of Kelvin sockets is not confined to large power transistors or high-current
circuits. Consider a voltage reference with 2 mA of quiescent current. If you’re trying
to observe a 1 -ppm stable reference and the ground connection changes by 5 mR
(which most socket manufacturers do not consider disastrous), the lO-p,V shift that
results from this change in ground impedance could confound your measurements. If
you want to avoid trouble in precision measurements, avoid sockets or at least avoid
sockets that do not have Kelvin contacts. Lord Kelvin-William Thomson before he
was appointed a baron-did indeed leave us with a bag full of useful tools.

Avoid Cold-Soldered Joints

I have a few comments to add about solder; most of the time we take it for granted.
You’ll normally use ordinary rosin-core tin-lead solder. If you avoid jiggling the