ADA4522-4ARZ-R7 datasheet(25/33 Pages) AD

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Data Sheet

ADA4522-1/ADA4522-2/ADA4522-4

THEORY OF OPERATION

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where BW is the bandwidth in hertz.

This analysis is valid for broadband noise calculation up to a

decade before the switching frequency. If the bandwidth of concern

includes the switching frequency, more complicated calculations

must be made to include the effect of the increase in noise at the

switching frequency.

With a low source resistance of RS < 1 kΩ, the voltage noise of

the amplifier dominates. As the source resistance increases, the

thermal noise of RS dominates. As the source resistance further

increases, where RS > 50 kΩ, the current noise becomes the main

contributor of the total input noise.

Residual Ripple

As shown in Figure 60, Figure 61, and Figure 62, the ADA4522-1/

ADA4522-2/ADA4522-4 have a flat noise spectrum density at lower

frequencies and exhibits spectrum density bumps and peaks at

higher frequencies.

The largest noise bump is centered at 6 MHz; this bump is due

to the decrease in the input gain at higher frequencies. This de-

crease is a typical phenomenon and can also be seen in other

amplifiers. In addition to the noise bump, a sharp peak due to the

chopping networks is seen at 4.8 MHz. However, this magnitude

is significantly reduced by the offset and ripple correction loop. Its

magnitude may be different with different amplifier units or with

different circuitries around the amplifier. This peak can potentially

be hidden by the noise bump and, therefore, may not be detected.

The offset and ripple correction loop, designed to reduce the

4.8 MHz switching artifact, also creates a noise bump centered

at 800 kHz and a noise peak on top of this noise bump. Although

the magnitude of the bump is mostly constant, the magnitude of

the 800 kHz peak is different from unit to unit. Some units may

not exhibit the 800 kHz noise peak; however, for other units, peaks

occur at multiple integrals of 800 kHz, such as 1.6 MHz or 2.4 MHz.

These noise peaks, albeit small in magnitude, can be significant

when the amplifier has a closed-loop frequency that is higher than

the chopping frequency. To suppress the noise spike to a desired

level, either configure the amplifier in a high gain configuration or

apply a post filter at the output of the amplifier.

Figure 76 shows the voltage noise density of the ADA4522-1/

ADA4522-2/ADA4522-4 in various gain configurations. Note that

the higher the gain, the lower the available bandwidth is. The earlier

bandwidth roll-off effectively filters out the higher noise spectrum.

Figure 76. Voltage Noise Density with Various Gains

Figure 77 shows the voltage noise density of the ADA4522-1/

ADA4522-2/ADA4522-4 without and with post filters at different

frequencies. The post filter serves to roll off the bandwidth before

the switching frequency. In this example, the noise peak at 800 kHz

is about 38 nV/√Hz. With a post filter at 80 kHz, the noise peak is

reduced to 4.1 nV/√Hz. With a post filter at 8 kHz, the noise peak is

lower than the noise floor and cannot be detected.

Figure 77. Voltage Noise Density with Post Filters

Current Noise Density

Figure 78 shows the current noise density of the ADA4522-1/

ADA4522-2/ADA4522-4 at unity gain. At 1 kHz, the current noise

density is about 1.3 pA/√Hz. The current noise density is deter-

mined by measuring the voltage noise due to current noise flowing

through a resistor. Due to the low current noise density of the

amplifier, the voltage noise is usually measured with a high value

resistor; in this case, a 100 kΩ source resistor is used. However,

the source resistor interacts with the input capacitance of the

amplifier and board, causing the bandwidth to roll off. Note that

Figure 78 shows the current noise density rolling off much earlier

than the unity-gain bandwidth; this roll-off is expected.

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