Tips about PCB design: Part 3 - Static and dynamic PCB effects
In static printed circuit board designs, leakage resistance is the dominant effect. Contamination of the PCB surface by ﬂux residues, deposited salts, and other debris can create leakage paths between circuit nodes. Even on well-cleaned boards, it is not unusual to ﬁnd 10 nA or more of leakage to nearby nodes from 15 V supply rails.
Nanoamperes of leakage current into the wrong nodes often cause volts of error at a circuit’s output; for example, 10 nA into a 10 MΩ resistance causes a 0.1 V error. Unfortunately, the standard op amp pinout places the VS supply pin next to the input, which is often hoped to be at high impedance.
To help identify nodes sensitive to the effects of leakage currents, ask the simple question: If a spurious current of a few nanoamperes or more were injected into this node, would it matter?
Figure C.26: A high-impedance differential input ADC also allows high transmission accuracy between source and load.
If the circuit is already built, it is possible to localize moisture sensitivity to a suspect node with a classic test. While observing circuit operation, blow on potential trouble spots through a simple soda straw. The straw focuses the breath’s moisture, which, with the board’s salt content in susceptible portions of the design, disrupts circuit operation upon contact.
There are several means of eliminating simple surface leakage problems. Thorough washing of circuit boards to remove residues helps considerably. A simple procedure includes vigorously brushing the boards with isopropyl alcohol, followed by thorough washing with deionized water and an 85ºC bakeout for a few hours.
Be careful when selecting board-washing solvents, though. When cleaned with certain solvents, some water-soluble ﬂuxes create salt deposits, exacerbating the leakage problem.
Unfortunately, if a circuit displays sensitivity to leakage, even the most rigorous cleaning can offer only a temporary solution. Problems soon return upon handling or exposure to foul atmospheres and high humidity. Some additional means must be sought to stabilize circuit behavior, such as conformal surface coating.
Fortunately, there is an answer to this problem, namely guarding, which offers a fairly reliable and permanent solution to the problem of surface leakage. Well-designed guards can eliminate leakage problems, even for circuits exposed to harsh industrial environments. Two schematics illustrate the basic guarding principle, as applied to typical inverting and noninverting op amp circuits.
Figure C.27 below illustrates an inverting mode guard application. In this case, the op amp reference input is grounded, so the guard is a grounded ring surrounding all leads to the inverting input, as noted by the dotted line.
Figure C.27: Inverting mode guard encloses all op amp inverting input connections within a grounded guard ring.
Basic guarding principles are simple: Completely surround sensitive nodes with conductors that can readily sink stray currents, and maintain the guard conductors at the exact potential of the sensitive node (as otherwise the guard will serve as a leakage source rather than a leakage sink).
For example, to keep leakage into a node below 1 pA (assuming 1000 MΩleakage resistance) the guard and guarded node must be within 1 mV. Generally, the low offset of a modern op amp is sufﬁcient to meet this criterion.
There are important caveats to be noted with implementing a true high-quality guard. For traditional through-hole PCB connections, to be most effective the guard pattern should appear on both sides of the circuit board. It should also be connected along its length by several vias.
Finally, when either justiﬁed or required by the system design parameters, do make an effort to include guards in the PCB design process from the outset—there is little likelihood that a proper guard can be added as an afterthought.
Figure C.28 below illustrates the case for a noninverting guard. In this instance the op amp reference input is directly driven by the source, which complicates matters considerably. Again, the guard ring completely surrounds all of the input nodal connections. In this instance however, the guard is driven from the low impedance feedback divider connected to the inverting input.
Figure C.28: Noninverting mode guard encloses all op amp noninverting input connections within a low impedance, driven guard ring.
Usually the guard-to-divider junction will be a direct connection, but in some cases a unity gain buffer might be used at X to drive a cable shield or to maintain the lowest possible impedance at the guard ring.
In lieu of the buffer, another useful step is to use an additional, directly grounded screen ring, Y, which surrounds the inner guard and the feedback nodes as shown. This step costs nothing except some added layout time and will greatly help buffer leakage effects into the higher-impedance inner guard ring.
Of course what hasn’t been addressed to this point is just how the op amp itself is connected into these guarded islands without compromising performance. The traditional method using a TO-99 metal can package device was to employ double-sided PCB guard rings, with both op amp inputs terminated within the guarded ring.
Many high-impedance sensors use the above described method. The next section illustrates how more modern IC packages can be mounted to PCB patterns and take advantage of guarding and low leakage operation.