Cirrus Logic AN330 REV2 - Manual (Page 9)

AN330REV2 9
AN330
If 32 output words were averaged, the effective output would yield results at 326 Sps / 32 = 10.18 Sps with an ef-
fective noise bandwidth of about 5 Hz.
Across 5 Hz, the integrated noise would be
√
5 • 8.6 nV / Hz = 19.5 nV rms. The peak-to-peak noise of the amplifier
would be 6.6 times 19.2 or 126.7 nVpp. The noise-free counts would be 5 mV / 126.7 nVpp = 39,463. This is across
±3.3 standard deviations. For ±2 standard deviations, the converter would yield 65,104 noise-free counts over 95%
of the time. This would yield 16 bits noise-free on a 5 mV load cell signal.
The following figure illustrates how filter components for RFI can be easily added.
The 200 ohm resistor is chosen to minimize noise. Two capacitors are chosen, the 0.02 microfarad to provide a low-
er-frequency corner to the filter, and the 300 picofarad to provide much better high-frequency filtering. The 300 pi-
cofarad capacitor should be chosen from a capacitor technology that provides the highest self-resonant frequency.
Figure 8.
This application note has illustrated several circuits that are suitable for use in battery-powered weigh scales. The
application note has illustrated how noise performance can be estimated by analysis and verified by measurement.
The devices mentioned are all configured to operate with 5V supply. The devices use the following typical currents:
• CS3001 – 2.1 mA
• CS3011 – 0.9 mA
• CS3002 – 3.6 mA
• CS3012 – 1.7 mA
• CS5512 – 0.36 mA
The CS3002 and CS5512 together use about 3.96 mA typical, which results in less than 20 mW of power consump-
tion.
+5 V
1 mV/V
-
+
350
140 k
0.22 F
CS3002
-
+
-
+
140 k
0.22 F
392
200
300 pF
200
0.02uF
300 pF 0.02uF