Analog Devices EVAL-ADAQ7768-1 User Manual page 17

User Guide
CONFIGURING THE BOARD AND THE ADAQ7768-1
Figure 22. FFT Plot for a Typical AC Measurement, with AFE_GAIN = 1.3 V/V (Gain 2 Mode), Differential Input of 2.98 Vp (-0.5 dBFS) 1 kHz, ODR = 256 kSPS, 32768
DC Measurement
A sinc3 filter with ODR = 50 SPS is an optimum point to measure a
pure DC signal. The sinc notch appears at 50 Hz and can be used
to reject the 50 Hz line frequency. Follow these steps to set up the
ADAQ7768-1 for a 50 SPS sinc3 filter:
1. Short the inputs (IN+ and IN–) to ground (GND) to measure the
RMS noise.
2. In the Chip View of the ACE software, click Filter Config-
uration. Use the dropdown to select Sinc3 (programmable
decimation rate), as shown in
3. Configure the Ssinc3 decimation rate by typing '13FF' in the
text box near the lower right corner, as shown in
is also reflected in the registers SINC3_DEC_RATE_LSB and
SINC3_DEC_ RATE_MSB located in the memory map of the
ACE software (Figure 25). Then, click Apply Changes for this
to take effect.
4. Before clicking on Run Once, make sure to adjust the number
of samples to collect accordingly. Collecting the default 8192
analog.com
Figure 23.
Figure 24. This
Samples
samples at 50 SPS takes 8192 × 1/50/60 = 2.73 minutes!
Start with 1024 samples, which should take about 20 seconds.
Generate a new capture using 1024 samples, as shown in
Figure 26.
5. The same condition (sinc3 50 SPS) is used to measure low
frequency noise, as specified in the data sheet. In the histogram
tab, noise is displayed as transition noise in LSB. To compare it
with the data sheet's referred to input (RTI) low frequency noise
in µV RMS, apply the formula:
RTI Noise
= Transition Noise × 1 LSB
= Transition Noise ×
= Transition Noise ×
6. For example, using Gain6 mode (AFE_GAIN = 20.8 V/V) with
measured transition noise = 12.077 LSB = 0.284 µV RMS RTI.
EVAL-ADAQ7768-1
4 . 096
24 − 1
2 × AFE_GAIN
488 nV
AFE_GAIN
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