ADA4522-4ARZ-R7 datasheet(25/32 Pages) AD

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Data Sheet

ADA4522-1/ADA4522-2/ADA4522-4

Rev. E | Page 25 of 32

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.

100

10k

100k

10M

100M

FREQUENCY (Hz)

AV = 1

AV = 1 (POST FILTER AT 80kHz)

AV = 1 (POST FILTER AT 8kHz)

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

determined 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 capaci-

tance 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.

0.1

100

10k

100k

FREQUENCY (Hz)

VSY = ±2.5V

VSY = ±15V

VSY = ±27.5V

RS = 100kΩ

AV = 1

Figure 78. Current Noise Density at Gain = 1

EMI REJECTION RATIO

Circuit performance is often adversely affected by high fre-

quency EMI. When the signal strength is low and transmission

lines are long, an op amp must accurately amplify the input

signals. However, all op amp pins—the noninverting input,

inverting input, positive supply, negative supply, and output

pins—are susceptible to EMI signals. These high frequency

signals are coupled into an op amp by various means, such as

conduction, near field radiation, or far field radiation. For example,

wires and printed circuit board (PCB) traces can act as antennas

and pick up high frequency EMI signals.

Amplifiers do not amplify EMI or RF signals due to their rela-

tively low bandwidth. However, due to the nonlinearities of the

input devices, op amps can rectify these out of band signals. When

these high frequency signals are rectified, they appear as a dc

offset at the output.

The ADA4522-1/ADA4522-2/ADA4522-4 have integrated EMI

filters at their input stage. To describe the ability of the

ADA4522-1/ADA4522-2/ADA4522-4 to perform as intended

in the presence of electromagnetic energy, the electromagnetic

interference rejection ratio (EMIRR) of the noninverting pin is

specified in Table 2, Table 3, and Table 4 of the Specifications

section. A mathematical method of measuring EMIRR is

defined as follows:

EMIRR = 20log(VIN_PEAK/ΔVOS)

100

10M

100M

10G

FREQUENCY (Hz)

55V

30V

VIN = 100mV p-p

Figure 79. EMIRR vs. Frequency

CAPACITIVE LOAD STABILITY

The ADA4522-1/ADA4522-2/ADA4522-4 can safely drive capaci-

tive loads of up to 250 pF in any configuration. As with most

amplifiers, driving larger capacitive loads than specified may cause

excessive overshoot and ringing, or even oscillation. A heavy

capacitive load reduces the phase margin and causes the amplifier

frequency response to peak. Peaking corresponds to overshooting

or ringing in the time domain. Therefore, it is recommended that

external compensation be used if the ADA4522-1/ADA4522-2/

ADA4522-4 must drive a load exceeding 250 pF. This compensa-

tion is particularly important in the unity-gain configuration,

which is the worst case for stability.

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