AD7780 Datasheet, PDF (14/16 Pages) Analog Devices – 24-Bit, Pin-Programmable, Ultralow Power Sigma-Delta ADC

AD7780
APPLICATIONS INFORMATION
The AD7780 provides a low cost, high resolution analog-to-digital
function. Because the analog-to-digital function is provided by a
Σ-Δ architecture, the parts are more immune to noisy environ-
ments, making them ideal for use in sensor measurement and
industrial and process control applications.
WEIGH SCALES
Figure 23 shows the AD7780 being used in a weigh scale
application. The load cell is arranged in a bridge network and
gives a differential output voltage between its OUT+ and OUT−
terminals. Assuming a 5 V excitation voltage, the full-scale
output range from the transducer is 10 mV when the sensitivity
is 2 mV/V. The excitation voltage for the bridge can be used to
directly provide the reference for the ADC because the refer-
ence input range includes the supply voltage.
A second advantage of using the AD7780 in transducer-based
applications is that the bridge power-down switch (BPDSW)
can be fully utilized in low power applications. The bridge power-
down switch is connected in series with the low side of the bridge.
In normal operation, the switch is closed and measurements
can be taken. In applications where power is of concern, the
AD7780 can be placed in power-down mode, significantly
reducing the power consumed in the application. In addition,
the bridge power-down switch is opened while in power-down
mode, thus avoiding unnecessary power consumption by the
front-end transducer. When the part is taken out of power-down
mode and the bridge power-down switch is closed, the user should
ensure that the front-end circuitry is fully settled before attempting
a read from the AD7780.
The load cell has an offset or tare associated with it. This tare is
the main component of the system offset (load cell + ADC) and is
similar in magnitude to the full-scale signal from the load cell.
For this reason, calibrating the offset and gain of the AD7780 alone
is not sufficient for optimum accuracy; a system calibration that
calibrates the offset and gain of the ADC, plus the load cell, is
required. A microprocessor can be used to perform the calibra-
tions. The offset (the conversion result from the AD7780 when
no load is applied to the load cell) and the full-scale error (the
conversion result from the ADC when the maximum load is
applied to the load cell) must be determined. Subsequent conver-
sions from the AD7780 are then corrected, using the offset and
gain coefficients that were calculated from these calibrations.
PERFORMANCE IN A WEIGH SCALE SYSTEM
If the load cell has a sensitivity of 2 mV/V and a 5 V excitation
voltage is used, the full-scale signal from the load cell is 10 mV.
When the AD7780 operates with a 10 Hz output data rate and
the gain is set to 128, the device has a p-p resolution of 18.2 bits
when the reference is equal to 5 V. Postprocessing the data from
the AD7780 using a microprocessor increases the p-p resolution.
For example, an average by 4 in the microprocessor increases
the accuracy by 2 bits. The noise-free counts is equal to the
following:
Noise-Free Counts = (2^{Effective Bits})(FSLC/FSADC)
where:
Effective Bits = 18.2 bits + 2 bits (due to post-processing in the
microprocessor).
FSLC is the full-scale signal from the load cell (10 mV).
FSADC is the full-scale input range when gain = 128 and VREF = 5 V
(78 mV).
The noise-free counts are equal to the following:
(2^{18.2 + 2})(10 mV/78 mV) = 154,422
This example shows that with a 5 V supply, 154,422 noise-free
counts can be achieved with the AD7780.
EMI RECOMMENDATIONS
For simplicity, the EMI filters are not included in Figure 23.
However, an R-C antialiasing filter should be included on each
analog input. This filter is needed because the on-chip digital
filter does not provide any rejection around the master clock or
multiples of the master clock. Suitable values are a 1 kΩ resistor
in series with each analog input, a 0.1 μF capacitor from AIN(+)
to AIN(−), and 0.01 μF capacitors from AIN(+)/AIN(−) to GND.
VDD
IN+
OUT–
IN–
REFIN(+)
OUT+ ^AIN(+)
AIN(–)
GND
AV_DD
G=1
OR 128
24-BIT Σ-Δ
ADC
REFIN(–)
BPDSW
INTERNAL
CLOCK
AD7780
Figure 23. Weigh Scales Using the AD7780
DOUT/RDY
SCLK
DV_DD
FILTER
PDRST
GAIN
Rev. A | Page 14 of 16