Datasheet

Application Hints (Continued)

This would cause a 100 times degradation from the

LPC662’s actual performance. However, if a guard ring is

held within 5 mV of the inputs, then even a resistance of

ohms would cause only 0.05 pA of leakage current, or

perhaps a minor (2:1) degradation of the amplifier’s

performance. See

Figure 5a

Figure 5b

Figure 5c

for typical

connections of guard rings for standard op-amp

configurations. If both inputs are active and at high

impedance, the guard can be tied to ground and still provide

some protection; see

Figure 5d

The designer should be aware that when it is inappropriate

to lay out a PC board for the sake of just a few circuits, there

is another technique which is even better than a guard ring

on a PC board: Don’t insert the amplifier’s input pin into the

DS010548-19

FIGURE 4. Example of Guard Ring in P.C. Board Layout, using the LPC660

DS010548-20

(a) Inverting Amplifier

DS010548-22

DS010548-21

(b) Non-Inverting Amplifier

DS010548-23

(d) Howland Current Pump

FIGURE 5. Guard Ring Connections

LPC662