Datasheet

Data Sheet ADA4522-1/ADA4522-2/ADA4522-4
Rev. F | Page 25 of 33
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
10
1
1k 10k 100k 1M 10M 100M
VOLTAGE NOISE DENSITY (nV/Hz)
FREQUENCY (Hz)
A
V
= 1
A
V
= 1 (POST FILTER AT 80kHz)
A
V
= 1 (POST FILTER AT 8kHz)
13168-073
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.
10
1
0.1
10 100 1k 10k 100k
CURRENT NOISE DENSITY (pA/Hz)
FREQUENCY (Hz)
V
SY
= ±2.5V
V
SY
= ±15V
V
SY
= ±27.5V
R
S
= 100k
A
V
= 1
13168-074
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(V
IN_PEAK
/V
OS
)
100
50
90
40
80
30
70
20
60
10
0
10M 100M 1G 10G
EMIRR (dB)
FREQUENCY (Hz)
55V
30V
5V
V
IN
= 100mV p-p
13168-075
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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