ADA4661-2ARMZ-R7 datasheet(28/32 Pages) AD

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ADA4661-2

Data Sheet

Rev. 0 | Page 28 of 32

NOISE CONSIDERATIONS WITH HIGH IMPEDANCE

SOURCES

Current noise from input terminals can become a dominant

contributor to the total circuit noise when an amplifier is driven

with a high impedance source. Unlike bipolar amplifiers,

CMOS amplifiers like the ADA4661-2 do not have an intrinsic

shot noise source at the input terminals. The small amount of

shot noise present is produced by the reverse saturation current

in the ESD protection diodes. This current noise is typically on

the order of 1 fA/√Hz to 10 fA/√Hz. Therefore, to measure

current noise in this range, a large source impedance of greater

than 10 GΩ is required.

For the ADA4661-2, the more relevant discussion centers

around an effect referred to as blowback noise. The blowback

effect comes from noise in the tail current source of the

amplifier, which is capacitively coupled to the amplifier inputs

through the gate-to-source capacitance (CGS) of the input

transistors. This blowback noise is multiplied by the source

impedance and appears as voltage noise at the input terminal. A

10× increase in the source impedance results in a 10× increase

in the voltage noise due to blowback.

The blowback noise spectrum has a high-pass response at low

frequencies due to CGS coupling. At high frequencies, the

spectrum tends to roll off with two poles: an internal pole due

to parasitic capacitances of the tail current source and an

external pole due to parasitic capacitances on the PCB.

Figure 89 shows the voltage noise density of the ADA4661-2

with source impedances of 1 MΩ and 10 MΩ. At low frequencies

(<1 Hz to 10 Hz), the amplifier 1/f voltage noise dominates the

spectrum. At moderate frequencies, the spectrum flattens due

to the thermal noise of the source resistors. As the frequency

increases, blowback noise dominates and causes the voltage

noise spectrum to increase. The noise spectrum continues to

increase until it reaches either the internal or external pole

frequency. After these poles, the spectrum starts to decrease.

Figure 89. Voltage Noise Density vs. Frequency (with Input Series Resistor RS)

Figure 90. Current Noise Density vs. Frequency

Figure 90 shows the current noise density of the ADA4661-2

with source impedances of 1 MΩ and 10 MΩ. This current

noise is extracted only from the voltage noise density curves in

the frequency band where blowback noise is the dominant

contributor. At low frequencies, the noise measurement is

dominated by resistor thermal noise and amplifier 1/f noise. At

high frequencies, parasitic capacitances dominate the source

impedance. The uncertainty of this scale factor prevents an

accurate current noise measurement for the entire frequency

range.

Blowback noise is present in all amplifiers. The magnitude of

the effect depends on the size of the input transistors and the

construction of the biasing circuitry. CMOS amplifiers typically

have more blowback noise than JFET amplifiers due to noisier

MOS transistor biasing. On the other hand, bipolar amplifiers

typically do not exhibit blowback noise because the large base

current shot noise masks any blowback noise present.

0.1

0.01

0.1

100

10k

100k

FREQUENCY (Hz)

RS = 10MΩ

RS = 1MΩ

0.01

0.1

0.01

0.1

100

10k

100k

FREQUENCY (Hz)

RS = 1MΩ

RS = 10MΩ

NOISE MEASUREMENT

LIMITATION

NOISE BANDWIDTH

LIMITATION

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