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ADA4638-1

Data Sheet

Rev. 0 | Page 18 of 24

TIME (2µs/DIV)

VSY = ±15V

VOUT

RL =10kΩ

VIN

Figure 58. No Phase Reversal

NOISE CONSIDERATIONS

1/f Noise

1/f noise, also known as pink noise or flicker noise, is inherent

in semiconductor devices and increases as frequency decreases.

At low frequency, 1/f noise is a major noise contributor and

causes a significant output voltage offset when amplified by the

noise gain of the circuit. However, the ADA4638-1 eliminates

the 1/f noise internally, thus making it an excellent choice for dc

or low frequency high precision applications. The 0.1 Hz to

10 Hz voltage noise is only 1.2 μV p-p at ±15 V of supply voltage.

The low frequency 1/f noise appears as a slow varying offset to

the ADA4638-1 and is greatly reduced by the combination of

auto-zeroing and chopping technique. This allows the ADA4638-1

to have a much lower noise at dc and low frequency in comparison

to standard low noise amplifiers that are susceptible to 1/f noise.

Figure 47 and Figure 50 show the voltage noise density of the

ADA4638-1 with no 1/f noise.

COMPARATOR OPERATION

Op amps are designed to operate in a closed-loop configuration

with feedback from its output to its inverting input. Figure 59

shows the ADA4638-1 configured as a voltage follower with an

input voltage, which is kept at midpoint of the power supplies.

A1 and A2 indicate the placement of ammeters to measure supply

currents. ISY+ refers to the current flowing into the positive

supply pin of the op amp, and ISY− refers to the current flowing

out of the negative supply pin of the op amp. From Figure 60, as

expected in normal operating condition, the current flowing

into the op amp is equivalent to the current flowing out of the op

amp, where ISY+ = ISY−.

ADA4638-1

100kΩ

ISY+

+VSY

VOUT

–VSY

ISY–

Figure 59. Voltage Follower

–1.0

–0.8

–0.6

–0.4

–0.2

0.2

0.4

0.6

0.8

1.0

VSY (V)

ISY+

ISY–

Figure 60. Supply Current vs. Supply Voltage (Voltage Follower)

In contrast to op amps, comparators are designed to work in an

open-loop configuration and to drive logic circuits. Although

op amps are different from comparators, occasionally an unused

section of a dual op amp is used as a comparator to save board

space and cost; however, this is not recommended.

Figure 61 and Figure 62 show the ADA4638-1 configured as

a comparator, with resistors RIN1 and RIN2 in series with the

input pins.

ADA4638-1

ITOTAL+

ISY+

RIN1

+VSY

VOUT

–VSY

IINPUT+

RIN2

IINPUT–

ISY–

ITOTAL–

Figure 61. Comparator A

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