ada4528-1 Analog Devices, Inc., ada4528-1 Datasheet - Page 16

• Manufacturer Part Number: ada4528-1
• Description: Precision, Ultralow Noise, Rrio, Zero-drift Op Amp Ada4528-1
• Manufacturer: Analog Devices, Inc.

ADA4528-1
The total equivalent rms noise over a specific bandwidth is
expressed as
where BW is the bandwidth in hertz.
This analysis is valid for broadband noise calculation. If the
bandwidth of concern includes the chopping frequency, more
complicated calculations must be made to include the effect of
the noise spike at the chopping frequency (see Figure 59).
With a low source resistance of R
the amplifier dominates. As source resistance increases, the
thermal noise of R
increases, where R
main contributor of the total input noise. A good selection table
for low noise op amps can be found in the AN-940 Application
Note, Low Noise Amplifier Selection Guide for Optimal Noise
Performance.
Voltage Noise Density with Different Gain
Configurations
Figure 57 shows the voltage noise density vs. closed-loop gain of
a zero-drift amplifier from Competitor A. The voltage noise density
of the amplifier increases from 11 nV/√Hz to 21 nV/√Hz as closed-
loop gain decreases from 1000 to 1. Figure 58 shows the voltage
noise density vs. frequency of the ADA4528-1 for three different
gain configurations. The ADA4528-1 offers lower input voltage
noise density of 6 nV/√Hz to 7 nV/√Hz regardless of gain
configurations.
Figure 57. Competitor A: Voltage Noise Density vs. Closed-Loop Gain
e
N,RMS
24
20
16
12
8
4
0
1
= e
N
total √BW
S
S
dominates. As the source resistance further
> 100 kΩ, the current noise becomes the
CLOSED-LOOP GAIN (V/V)
10
S
< 1 kΩ, the voltage noise of
100
V
f = 100Hz
COMPETITOR A
SY
= 5V
1000
Rev. 0 | Page 16 of 20
Residual Ripple
Although the ACFB suppresses the chopping related ripples,
there exists higher noise spectrum at the chopping frequency
and its harmonics due to the remaining ripples. Figure 59 shows
the voltage noise density of the ADA4528-1 configured in unity
gain. A noise spike of 50 nV/√Hz can be seen at the chopping
frequency of 200 kHz. This noise spike is significant when the
op amp has a closed-loop frequency that is higher than the
chopping frequency. To further suppress the noise to a desired
level, it is recommended to have a post filter at the output of the
amplifier.
100
100
10
10
1
1
1
1
V
A
V
V
V
SY
V
CM
SY
CM
Figure 58. Voltage Noise Density vs. Frequency
= 1
A
A
A
= 5V
= V
= 5V
= V
10
V
V
V
= 1
= 10
= 100
SY
SY
Figure 59. Voltage Noise Density
/2
/2
10
100
FREQUENCY (Hz)
FREQUENCY (Hz)
1k
100
10k
100k
1k
1M
10M
10k