CN0216 Datasheet, PDF (3/5 Pages) Analog Devices – Devices Connected

Circuit Note
100
10
1
CN-0216
Therefore, the corresponding noise-free code resolution of the
total system is:
log₂ (39,557)
=
log₁₀ (39,557)
log₁₀ (2)
= 15.3
bits
Figure 4 shows a plot of the ADC codes for 500 samples
(52.6 sec for 9.5 Hz data rate). Note that the peak-to-peak
spread is about 160 codes.
0.1
1
10
100
OUTPUT DATA RATE (Hz)
1000
Figure 3. AD7791 RMS Noise for Different Output Data Rates and a 2.5 V
Reference (5 V p-p Input Range), Buffer On
The AD7791 rms noise of 1.1 µV for a 9.5 Hz output data rate
and a reference of 2.5 V yields the following number of noise-
free counts:
6.6
5V
× 1.1 μ
V
=
688,705
where the factor of 6.6 converts the rms voltage into a peak-to-
peak voltage.
The corresponding noise-free code resolution is, therefore:
log₂ (688,705)
=
log₁₀ (688,705)
log₁₀ (2)
= 19.5
bits
Note that this represents the performance of the AD7791
without the load cell or the input amplifier connected.
The ADA4528-1 has 5.9 nV/√Hz of voltage noise density and,
therefore, the input amplifiers and resistors will add noise to the
system. In addition, the load cell itself will add noise.
In the circuit of Figure 1, a 5 V reference is used, therefore, the
peak-to-peak input range is 10 V. The LSB is, therefore, equal to
1LSB
=
10 V
2²⁴
=
0.596
μV
The 10 mV p-p full-scale signal from the load cell produces a
3.75 V p-p signal into the ADC, which is approximately 38% of
the ADC range.
Seven sets of 500 samples each were taken with the load cell
connected (no load). The peak-to-peak code spread for each
sample set was calculated, and the seven values averaged to
yield a code spread of 159 counts. This corresponds to 159 ×
0.596 µV = 94.8 µV p-p noise based on a full-scale input to the
ADC of 3.75 V p-p.
Therefore, the number of noise-free counts is
3.75 V _{= 39,557}
94.8 μ V
Figure 4. Measured Output Code for 500 Samples Showing
the Effects of Noise
Figure 5 shows the same data presented in a histogram. Figure 4
and Figure 5 show the actual (raw) conversions read back from
the AD7791. In practice, a digital post filter is typically used in a
weigh scale system. The additional averaging that is performed
in the post filter will further improve the number of noise-free
counts at the expense of a reduced data rate.
The resolution of the system in grams can be calculated by
2 kg _{= 0.05 g}
39,557
As with any high accuracy circuit, proper layout, grounding,
and decoupling techniques must be employed. See Tutorial
MT-031, Grounding Data Converters and Solving the Mystery of
AGND and DGND and Tutorial MT-101, Decoupling Techniques
for more details. A complete design support package for this
circuit note can be found at www.analog.com/CN0216-
DesignSupport.
Rev. 0 | Page 3 of 5