AD7792BRUZ Analog Devices Inc, AD7792BRUZ Datasheet - Page 25

• Manufacturer Part Number: AD7792BRUZ
• Description: IC ADC 16BIT SIG-DEL 3CH 16TSSOP
• Manufacturer: Analog Devices Inc

Specifications of AD7792BRUZ

• Data Interface: DSP, MICROWIRE™, QSPI™, Serial, SPI™
• Number Of Bits: 16
• Sampling Rate (per Second): 500
• Number Of Converters: 1
• Power Dissipation (max): 2.5mW
• Voltage Supply Source: Analog and Digital
• Operating Temperature: -40°C ~ 105°C
• Mounting Type: Surface Mount
• Package / Case: 16-TSSOP (0.173", 4.40mm Width)
• Resolution (bits): 16bit
• Sampling Rate: 470SPS
• Input Channel Type: Differential
• Supply Voltage Range - Analog: 2.7V To 5.25V
• Supply Voltage Range - Digital: 2.7V To 5.25V
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• For Use With: EVAL-AD7792EBZ - BOARD EVALUATION FOR AD7792
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant, Lead free / RoHS Compliant

BURNOUT CURRENTS
The AD7792/AD7793 contain two 100 nA constant current
generators, one sourcing current from AV
sinking current from AIN(–) to GND. The currents are
switched to the selected analog input pair. Both currents are
either on or off, depending on the burnout current enable (BO)
bit in the configuration register. These currents can be used to
verify that an external transducer is still operational before
attempting to take measurements on that channel. Once the
burnout currents are turned on, they flow in the external
transducer circuit, and a measurement of the input voltage on
the analog input channel can be taken. If the resultant voltage
measured is full scale, the user needs to verify why this is the
case. A full-scale reading could mean that the front-end sensor
is open circuit. It could also mean that the front-end sensor is
overloaded and is justified in outputting full scale, or the
reference may be absent, thus clamping the data to all 1s.
When reading all 1s from the output, the user needs to check
these three cases before making a judgment. If the voltage
measured is 0 V, it may indicate that the transducer has short
circuited. For normal operation, these burnout currents are
turned off by writing a 0 to the BO bit in the configuration
register. The current sources work over the normal absolute
input voltage range specifications with buffers on.
EXCITATION CURRENTS
The AD7792/AD7793 also contain two matched, software
configurable, constant current sources that can be programmed
to equal 10 μA, 210 μA, or 1 mA. Both source currents from the
AV
device. These current sources are controlled via bits in the IO
register. The configuration bits enable the current sources,
direct the current sources to IOUT1 or IOUT2, and select the
value of the current. These current sources can be used to excite
external resistive bridge or RTD sensors.
BIAS VOLTAGE GENERATOR
A bias voltage generator is included on the AD7792/AD7793.
This biases the negative terminal of the selected input channel
to AV
voltage generated by the thermocouple must be biased about
some dc voltage if the gain is greater than 2. This is necessary
because the instrumentation amplifier requires headroom to
ensure that signals close to GND or AV
accurately.
The bias voltage generator is controlled using the VBIAS1 and
VBIAS0 bits in conjunction with the boost bit in the configura-
tion register. The power-up time of the bias voltage generator is
dependent on the load capacitance. To accommodate higher
load capacitances, the AD7792/AD7793 have a boost bit. When
this bit is set to 1, the current consumed by the bias voltage
generator increases, so that the power-up time is considerably
reduced. Figure 10 shows the power-up time when boost equals
0 and 1 for different load capacitances.
DD
DD
are directed to either the IOUT1 or IOUT2 pin of the
/2. It is useful in thermocouple applications, because the
DD
DD
are converted
to AIN(+) and one
Rev. B | Page 25 of 32
The current consumption of the AD7792/AD7793 increases by
40 μA when the bias voltage generator is enabled, and boost
equals 0. With the boost function enabled, the current
consumption increases by 250 μA.
REFERENCE
The AD7792/AD7793 have an embedded 1.17 V reference that
can be used to supply the ADC, or an external reference can be
applied. The embedded reference is a low noise, low drift
reference, the drift being 4 ppm/°C typically. For external
references, the ADC has a fully differential input capability for
the channel. The reference source for the AD7792/AD7793 is
selected using the REFSEL bit in the configuration register.
When the internal reference is selected, it is internally con-
nected to the modulator. It is not available on the REFIN pins.
The common-mode range for these differential inputs is from
GND to AV
excessive R-C source impedances introduce gain errors. The
reference voltage REFIN (REFIN(+) − REFIN(−)) is 2.5 V
nominal, but the AD7792/AD7793 are functional with reference
voltages from 0.1 V to AV
tation (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 AD7792/AD7793
are used in a nonratiometric application, a low noise reference
should be used.
Recommended 2.5 V reference voltage sources for the AD7792/
AD7793 include the ADR381 and ADR391, which are low noise,
low power references. Also note that the reference inputs
provide a high impedance, dynamic load. Because the input
impedance of each reference input is dynamic, resistor/capacitor
combinations on these inputs can cause dc gain errors, depending
on the output impedance of the source that is driving the
reference inputs.
Reference voltage sources like those recommended above (such
as ADR391) typically have low output impedances and are,
therefore, tolerant to having 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.
RESET
The circuitry and serial interface of the AD7792/AD7793 can
be reset by writing 32 consecutive 1s to the device. This resets
the logic, the digital filter, and the analog modulator while all
on-chip registers are reset to their default values. A reset is
automatically performed on power-up. When a reset is initiated,
the user must allow a period of 500 μs before accessing any of
the on-chip registers. A reset is useful if the serial interface
becomes asynchronous due to noise on the SCLK line.
DD
. The reference input is unbuffered; therefore,
DD
. In applications where the exci-
AD7792/AD7793