LM308A

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4

INPUT GUARDING

Special care must be taken in the assembly of printed

circuit boards to take full advantage of the low input currents

of the LM308A amplifier. Boards must be thoroughly

cleaned with alcohol and blown dry with compressed air.

After cleaning, the boards should be coated with epoxy or

silicone rubber to prevent contamination.

Even with properly cleaned and coated boards, leakage

currents may cause trouble at +125°C, particularly since the

input pins are adjacent to pins that are at supply potentials.

This leakage can be significantly reduced by using guarding

to lower the voltage difference between the inputs and

adjacent metal runs. The guard, which is a conductive ring

surrounding the inputs, is connected to a low−impedance

point that is at approximately the same voltage as the inputs.

Leakage currents from high voltage pins are then absorbed

by the guard.

(1) Power Bandwidth: 250 kHz

Small Signal Bandwidth:

3.5 MHz

Slew Rate: 10 V/

µs

(2) C5 =

R1

(3) In addition to increasing speed,

the LM101A raises high and low

frequency gain, increases output

drive capability and eliminates

thermal feedback.

RS

C5 (2)

Input

R1

R4

150pF

1

6

2

3

1.0M

Output

LM101A(3)

or equiv

Compen B

1 M

R2

LM308A

0.002

µF

0.002

µF

300pF

150k

Figure 7. Fast (1) Summing Amplifier with

Low Input Current

(1) Teflon, Polyethylene or Polycarbonate

Dielectric Capacitor

Output

30pF

Q1

Q2

Input

Sample

1.0M

1.0

µF (1)

Figure 8. Sample and Hold

Inverting Amplifier

Follower

Noninverting Amplifier

(1) Used to compensate for large source resistances.

Note:

R1 R2

R1 +R2

must be an impedance.

C1

R1

R2

Input

Output

R3 (1)

C1

Input

Output

R3 (1)

C1

R2

Input

Output

R1

R3 (1)

Figure 9. Connection of Input Guards