Applications Information; Weigh Scales - Analog Devices AD7195 Manual

APPLICATIONS INFORMATION

The AD7195 provides a low-cost, high resolution analog-to-
digital function. Because the analog-to-digital function is
provided by a Σ-Δ architecture, the part is more immune
to noisy environments, making it ideal for use in sensor
measurement and industrial and process control applications.

WEIGH SCALES

Figure 50 shows the AD7195 being used in a weigh scale applica-
tion which uses ac excitation. 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 reference input range includes the supply voltage.
With ac-excitation, the excitation voltage to the load cell is
changed on each phase. In Phase 1, the T2 and T4 transistors
are turned on using ACX1 and ACX1 while the T1 and T3
transistors are turned off. The bridge is forward biased. During
Phase 2, Transistor T1 and Transistor T3 are turned on using
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+5V
T2
T1
IN+
OUT+
OUT–
IN–
T3 T4
AV
DD
1MΩ
1MΩ
ACX2 and ACX2 . In this phase, the excitation voltage to the
bridge is reversed while the analog input signal and the
reference voltage are also reversed. The AD7195 averages the
conversions from the two phases so that any offsets and thermal
affects are cancelled.
AC excitation is enabled by setting Bit ACX in the configuration
register to 1. When the ACX bit is set to 0, the bridge is dc
excited. When the AD7195 is in power-down mode, the bridge
is disconnected from the excitation voltage, which minimizes
power consumption of the system. Following a reset, the ac
excitation pins are undefined for a few milliseconds. Thus, pull-
up/pull-down resistors should be used on the pins to prevent
the excitation voltage being shorted to AGND.
For simplicity, external filters are not included in Figure 50.
However, an R-C antialias filter must be included on each
analog input. This is required because the on-chip digital filter
does not provide any rejection around the modulator sampling
frequency or multiples of this frequency.
AV AGND DV DGND
DD DD
REFIN(+)
AIN1
AV
DD
AIN2
AIN3
AIN4
MUX
AINCOM
PGA
REFIN(–)
AGND
TEMP
SENSOR
BPDSW
EXCITATION
AD7195
ACX1 ACX1
Figure 50. Typical Application (Weigh Scale)
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REFERENCE
DETECT
SERIAL
INTERFACE
Σ-Δ
AND
ADC
CONTROL
LOGIC
AC
CLOCK
CIRCUITRY
CLOCK
ACX2 ACX2
MCLK1 MCLK2
AD7195
DOUT/RDY
DIN
SCLK
CS
SYNC