Circuit Note

CN-0216

Rev. 0 | Page 3 of 5

100

10

1

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:

705

,

688

V

μ

1

.

1

6

.

6

V

5

=

×

where the factor of 6.6 converts the rms voltage into a peak-to-

peak voltage.

The corresponding noise-free code resolution is, therefore:

(

)

(

)

( )

bits

5

.

19

2

log

705

,

688

log

705

,

688

log

10

10

2

=

=

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

μV

596

.

0

2

V

10

LSB

1

=

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

557

,

39

V

μ

8

.

94

V

3.75

=

Therefore, the corresponding noise-free code resolution of the

total system is:

(

)

(

)

( )

bits

3

.

15

2

log

557

,

39

log

557

,

39

log

10

10

2

=

=

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.

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

g

05

.

0

557

,

39

kg

2

=

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.