LM359M datasheet(9/24 Pages) NSC

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Application Hints (Continued)

these parameters are significant in wide band amplifier de-

sign. The input stage current is approximately 3 times

SET(IN) and by using this relationship the following first order

approximations for these AC parameters are:

where C

comp is the total capacitance from the compensation

pin (pin 3 or pin 13) to ground, A

VOL is the low frequency

open loop voltage gain in V/V and an ambient temperature of

25˚C is assumed (KT/q = 26 mV and

typ = 150). ISET(IN)

also controls the DC input bias current by the expression:

which is important for DC biasing considerations.

The total device supply current (for both amplifiers) is also a

direct function of the set currents and can be approximated

by:

supply ≅ 27xISET(OUT) +11xISET(IN)

with each set current programmed by individual resistors.

PROGRAMMING WITH A SINGLE RESISTOR

Operating current programming may also be accomplished

using only one resistor by letting I

SET(IN) equal ISET(OUT). The

programming current is now referred to as I

SET and it is cre-

ated by connecting a resistor from pin 1 to pin 8 (

Figure 8).

This configuration does not affect any of the internal set cur-

rent dependent parameters differently than previously dis-

cussed except the total supply current which is now equal to:

supply ≅ 37xISET

Care must be taken when using resistors to program the set

current to prevent significantly increasing the supply voltage

above the value used to determine the set current. This

would cause an increase in total supply current due to the re-

sulting increase in set current and the maximum device

power dissipation could be exceeded. The set resistor val-

ue(s) should be adjusted for the new supply voltage.

One method to avoid this is to use an adjustable current

source which has voltage compliance to generate the set

current as shown in

Figure 9.

This circuit allows I

SET to remain constant over the entire

supply voltage range of the LM359 which also improves

power supply ripple rejection as illustrated in the Typical Per-

formance Characteristics. It should be noted, however, that

the current through the LM334 as shown will change linearly

with temperature but this can be compensated for (see

LM334 data sheet).

Pin 1 must never be shorted to ground or pin 8 never shorted

to V

+ without limiting the current to 2 mA or less to prevent

catastrophic device failure.

CONSIDERATIONS FOR HIGH FREQUENCY

OPERATION

The LM359 is intended for use in relatively high frequency

applications and many factors external to the amplifier itself

must be considered. Minimization of stray capacitances and

their effect on circuit operation are the primary requirements.

The following list contains some general guidelines to help

accomplish this end:

Keep the leads of all external components as short as

possible.

Place components conducting signal current from the

output of an amplifier away from that amplifier’s

non-inverting input.

Use reasonably low value resistances for gain setting

and biasing.

Use of a ground plane is helpful in providing a shielding

effect between the inputs and from input to output. Avoid

using vector boards.

Use a single-point ground and single-point supply distri-

bution to minimize crosstalk. Always connect the two

grounds (one from each amplifier) together.

Avoid use of long wires (> 2") but if necessary, use

shielded wire.

Bypass the supply close to the device with a low induc-

tance, low value capacitor (typically a 0.01 µF ceramic)

to create a good high frequency ground. If long supply

leads are unavoidable, a small resistor (z10

Ω) in series

with the bypass capacitor may be needed and using

shielded wire for the supply leads is also recommended.

COMPENSATION

The LM359 is internally compensated for stability with closed

loop inverting gains of 10 or more. For an inverting gain of

less than 10 and all non-inverting amplifiers (the amplifier al-

ways has 100% negative current feedback regardless of the

DS007788-13

ISET(IN) = ISET(OUT) = ISET

FIGURE 8. Single Resistor Programming of I

SET

DS007788-14

FIGURE 9. Current Source Programming of I

SET

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