AD7781CRUZ Analog Devices Inc, AD7781CRUZ Datasheet - Page 12

• Manufacturer Part Number: AD7781CRUZ
• Description: 1CHANNEL L/POWER 20-BIT SD GAIN 128 IC
• Manufacturer: Analog Devices Inc

Specifications of AD7781CRUZ

• Design Resources: Weigh Scale Design Using AD7781 with Internal PGA (CN0108)
• Number Of Bits: 20
• Data Interface: Serial, SPI™
• Number Of Converters: 1
• Voltage Supply Source: Single Supply
• Operating Temperature: -40°C ~ 105°C
• Mounting Type: Surface Mount
• Package / Case: 16-TSSOP (0.173", 4.40mm Width)
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

AD7781
POWER-DOWN/RESET (PDRST)
The PDRST pin functions as a power-down pin and a reset pin.
When PDRST is taken low, the AD7781 is powered down. The
entire ADC is powered down (including the on-chip clock), the
low-side power switch is opened, and the DOUT/ RDY pin is
tristated. The circuitry and serial interface are also reset, which
resets the logic, the digital filter, and the analog modulator.
PDRST must be held low for 100 ns minimum to initiate the
reset function (see
When PDRST is taken high, the AD7781 is taken out of power-
down mode. When the on-chip clock has powered up (1 ms,
typically), the modulator begins sampling the analog input.
The low-side power switch is closed, and the DOUT/ RDY pin
becomes active.
A reset is automatically performed on power-up.
ANALOG INPUT CHANNEL
The AD7781 has one differential analog input channel. The
input channel feeds into a high impedance input stage of the
amplifier. Therefore, the input can tolerate significant source
impedances and is tailored for direct connection to external
resistive-type sensors such as strain gages.
The absolute input voltage range is restricted to a range between
GND + 450 mV and AV
up the common-mode voltage to avoid exceeding these limits.
Otherwise, there is degradation in linearity and noise performance.
The low noise in-amp means that signals of small amplitude can
be amplified within the AD7781, which still maintains excellent
noise performance. The amplifier can be configured to have a gain
of 128 or 1, using the GAIN pin. The analog input range is equal
to ±V
must be ≥0.5 V.
BIPOLAR CONFIGURATION
The AD7781 accepts a bipolar input range. A bipolar input range
does not imply that the part can tolerate negative voltages with
respect to system GND. Signals on the AIN(+) input are refer-
enced to the voltage on the AIN(−) input. For example, if AIN(−)
is 2.5 V, the analog input range on the AIN(+) input is 2.46 V to
2.54 V for a gain of 128.
DATA OUTPUT CODING
The AD7781 uses offset binary coding. Thus, a negative full-
scale voltage results in a code of 000...000, a zero differential
input voltage results in a code of 100...000, and a positive full-
scale input voltage results in a code of 111...111.
REF
/gain. The common-mode voltage (AIN(+) + AIN(−))/2
Figure 4
DD
− 1.1 V. Care must be taken in setting
).
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The output code for any analog input voltage can be represented as
where:
AIN is the analog input voltage.
Gain is 1 or 128.
N = 20.
REFERENCE
The AD7781 has a fully differential input capability for the channel.
The common-mode range for these differential inputs is GND to
AV
source impedances introduce gain errors. The reference voltage of
REFIN (REFIN(+) − REFIN(−)) is AV
is functional with reference voltages of 0.5 V to AV
tions where the excitation (voltage or current) for the transducer
on the analog input also drives the reference voltage for the part,
the effect of the low frequency noise in the excitation source is
removed because the application is ratiometric. If the AD7781
is used in a nonratiometric application, a low noise reference
should be used.
Recommended 2.5 V reference voltage sources for the AD7781
include the
references. These references have low output impedances and
are, therefore, tolerant to decoupling capacitors on REFIN(+)
without introducing gain errors in the system. Deriving the
reference input voltage across an external resistor means that
the reference input sees a significant external source impedance.
External decoupling on the REFIN pins is not recommended in
this type of circuit configuration.
BRIDGE POWER-DOWN SWITCH
The bridge power-down switch (BPDSW) is useful in battery-
powered applications where the optimization of system power
consumption is essential. A 350 Ω load cell typically consumes
15 mA when excited with a 5 V power supply. To minimize
current consumption, the load cell is disconnected when it is
not being used. The bridge power-down switch can be included
in series with the load cell. When PDRST is high, the bridge power-
down switch is closed, and the load cell measures the strain. When
PDRST is low, the bridge power-down switch is opened so no
current flows through the load cell. Therefore, the current
consumption of the system is minimized. The bridge power-
down switch has an on resistance of 9 Ω maximum. The switch
is capable of withstanding 30 mA of continuous current.
DD
Code = 2
. The reference input is unbuffered; therefore, excessive R-C
ADR381
N − 1
× [(AIN × Gain/V
and ADR391, which are low noise, low power
REF
DD
) + 1]
nominal, but the AD7781
DD
. In applica-