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Fabricante

MC330077D

Dual, Low Noise Operational Amplifier

Motorola => Freescale 

MC33077

occur, the amplifier’s phase will degrade severely causing the

amplifier to become unstable. Effective source resistances,

acting in conjunction with the input capacitance of the

amplifier, should be kept to a minimum to avoid creating such

a pole at the input (see Figure 31). There is minimal effect on

stability where the created input pole is much greater than the

closed loop corner frequency. Where amplifier stability is

affected as a result of a negative feedback resistor in

conjunction with the amplifier’s input capacitance, creating a

pole near the closed loop corner frequency, lead capacitor

compensation techniques (lead capacitor in parallel with the

feedback resistor) can be employed to improve stability. The

feedback resistor and lead capacitor RC time constant

should be larger than that of the uncompensated input pole

frequency. Having a high resistance connected to the

noninverting input of the amplifier can create a like instability

problem. Compensation for this condition can be

accomplished by adding a lead capacitor in parallel with the

noninverting input resistor of such a value as to make the RC

time constant larger than the RC time constant of the

uncompensated input resistor acting in conjunction with the

amplifiers input capacitance.

For optimum frequency performance and stability, careful

component placement and printed circuit board layout should

be exercised. For example, long unshielded input or output

leads may result in unwanted input output coupling. In order

to reduce the input capacitance, the body of resistors

connected to the input pins should be physically close to the

input pins. This not only minimizes the input pole creation for

optimum frequency response, but also minimizes extraneous

signal “pickup” at this node. Power supplies should be

decoupled with adequate capacitance as close as possible to

the device supply pin.

In addition to amplifier stability considerations, input

source resistance values should be low to take full advantage

of the low noise characteristics of the amplifier. Thermal

noise (Johnson Noise) of a resistor is generated by

thermally–charged carriers randomly moving within the

resistor creating a voltage. The rms thermal noise voltage in

a resistor can be calculated from:

Enr = / 4k TR × BW

where:

k = Boltzmann’s Constant (1.38 × 10–23 joules/k)

T = Kelvin temperature

R = Resistance in ohms

BW = Upper and lower frequency limit in Hertz.

By way of reference, a 1.0 kΩ resistor at 25°C will produce

a 4.0 nV/√Hz of rms noise voltage. If this resistor is

connected to the input of the amplifier, the noise voltage will

be gained–up in accordance to the amplifier’s gain

configuration. For this reason, the selection of input source

resistance for low noise circuit applications warrants serious

consideration. The total noise of the amplifier, as referred to

its inputs, is typically only 4.4 nV/√Hz at 1.0 kHz.

The output of any one amplifier is current limited and thus

protected from a direct short to ground, However, under such

conditions, it is important not to allow the amplifier to exceed

the maximum junction temperature rating. Typically for ±15 V

supplies, any one output can be shorted continuously to

ground without exceeding the temperature rating.

0.1 µF

Figure 36. Voltage Noise Test Circuit

(0.1 Hz to 10 Hzp–p)

10 Ω

100 kΩ

–

2.0 kΩ

D.U.T.

+

4.7 µF

Voltage Gain = 50,000

24.3 kΩ

+

1/2

MC33077

–

100 kΩ

0.1 µF

4.3 kΩ

2.2 µF

22 µF

Scope

×1

Rin = 1.0 MΩ

110 kΩ

Note: All capacitors are non–polarized.

MOTOROLA ANALOG IC DEVICE DATA

11

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