Cirrus Logic AN330 REV2 - Manual (Page 4)

4 AN330REV2
AN330
The noise contributed by the ADC is distributed across the spectrum up to one-half the converter output word rate.
The sample rate of the CS5513 is nominally 107 Sps. This is illustrated in the following figure:
Figure 4.
As shown earlier, the op amp noise (after being amplified) is 2642 nV /
√
Hz, and the estimate for the ADC spot noise
is 1640 /
√
Hz. These summed together would be (2642
2
+ 1640
2
) = 3110 nV /
√
Hz, but this is only valid for frequen-
cies below the noise bandwidth of the analog low-pass filter. Above the noise bandwidth of the filter, only the spot
noise of the ADC will be present.
Figure 5.
The amplifier noise and the ADC noise below 7.5 Hz will add in rms fashion to produce
(2642
2
+ 1640
2
) = 3110 nV /
√
Hz. Across 7.5 Hz the total integrated noise will be
√
7.5 times
3110 nV /
√
Hz = 8.52
μVrms. The total noise from the ADC from 7.5 Hz to 53.5 Hz will be √
(53.5 – 7.5) times
1640 nV /
√
Hz = 11.1
μV rms. Adding these two noise numbers in rms fashion will result in an estimate for the total
noise exhibited by the ADC. This will be ( 8.52
2
+ 11.1
2
) = 14 μV rms. The peak-to-peak noise would be the rms
value, 14 μV, multiplied by 6.6 (±3.3 standard deviations) or 92.4 μVpeak to peak.
Recall that the 5 mV signal from the load cell became 2.04 volts into the ADC. Using the noise estimate one can
estimate the noise-free counts from the converter as 2.04 V / 92.4
μV = 22,078 noise-free counts out of the ADC.
This is the performance on the output words of the CS5513 with no post filtering performed by the microcontroller.
This circuit with the CS3001 and the CS5513 was constructed and tested in the lab. With the input held stable, (some
comments on testing will be stated later) 256 conversion words from the CS5513 were collected and the standard
0 7.5 53.5
Frequency (Hz)
M
a
gn
it
ud
e
A D C Spot N oise
A D C Spot N oise = 1640 nV / √HZ
0
0 7.5 53.5
Frequency (Hz)
M
a
gn
it
ud
e
A D C Spot N oise
Op A mp Spot N oise
Op A m p Spot N oise = 2642 nV / √HZ
A D C Spot N oise = 1640 nV / √HZ
R MS Sum of Op A mp & A D C N oise = 3110 nV / √HZ
0