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MC33077

Low Noise Dual Operational Amplifier

ON Semiconductor 

MC33077

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 kW 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 mF

10 W

100 kW

−

D.U.T.

+

2.0 kW

4.7 mF

Voltage Gain = 50,000

24.3 kW

+

1/2

MC33077

−

100 kW

0.1 mF

4.3 kW 22 mF

Scope × 1

Rin = 1.0 MW

2.2 mF

110 kW

Note: All capacitors are non−polarized.

Figure 37. Voltage Noise Test Circuit

(0.1 Hz to 10 Hzp−p)

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