AD7795 Analog Devices, AD7795 Datasheet - Page 32

• Manufacturer Part Number: AD7795
• Description: 6-Channel, Low Noise, Low Power, 16-Bit Sigma Delta ADC with On-Chip In-Amp and Reference
• Manufacturer: Analog Devices

Specifications of AD7795

• Resolution (bits): 16bit
• # Chan: 6
• Sample Rate: n/a
• Interface: Ser,SPI
• Analog Input Type: Diff-Uni
• Ain Range: (2Vref/PGA Gain) p-p
• Adc Architecture: Sigma-Delta
• Pkg Type: SOP

AD7794/AD7795
DATA OUTPUT CODING
When the ADC is configured for unipolar operation, the output
code is natural (straight) binary with a zero differential input
voltage resulting in a code of 00...00, a miscalled voltage
resulting in a code of 100...000, and a full-scale input voltage
resulting in a code of 111...111. The output code for any analog
input voltage can be represented as
When the ADC is configured for bipolar operation, the output
code is offset binary with a negative full-scale voltage resulting
in a code of 000...000, a zero differential input voltage resulting
in a code of 100...000, and a positive full-scale input voltage
resulting in a code of 111...111. The output code for any analog
input voltage can be represented as
where:
AIN is the analog input voltage.
GAIN is the in-amp setting (1 to 128).
N = 24.
BURNOUT CURRENTS
The AD7794/AD7795 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 resulting 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 that the
reference may be absent and the NOXREF bit is set, 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.
Code = (2
Code = 2
N – 1
N
× AIN × GAIN)/V
× [(AIN × GAIN/V
REF
REF
) + 1]
DD
to AIN(+), and one
Rev. D | Page 32 of 36
EXCITATION CURRENTS
The AD7794/AD7795 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
AV
device. These current sources are controlled via bits in the IO
register. The configuration bits enable the current sources and
direct the current sources to IOUT1 or IOUT2, along with
selecting 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 AD7794/AD7795. It
biases the negative terminal of the selected input channel to
AV
AIN3(−). It is useful in thermocouple applications, as the
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. If
there is no headroom, signals close to GND or AV
convert accurately.
The bias voltage generator is controlled using the VBIAS1 and
VBIAS0 bits in conjunction with the BOOST bit in the
configuration register. The power-up time of the bias voltage
generator is dependent on the load capacitance. To accommodate
higher load capacitances, each AD7794/AD7795 has a BOOST
bit. When this bit is set to 1, the current consumed by the bias
voltage generator is increased so that power-up time is reduced
considerably. Figure 11 shows the power-up times when
BOOST equals 0 and BOOST equals 1 for different load
capacitances. The current consumption of the AD7794/AD7795
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 AD7794/AD7795 have embedded 1.17 V references. These
references can be used to supply the ADC or external references
can be applied. The embedded references are low noise, low
drift references with 4 ppm/°C drift typically. For external
references, the ADC has a fully differential input capability for
the channel. In addition, the user has the option of selecting one
of two external reference options (REFIN1 or REFIN2). The
reference source for the AD7794/AD7795 is selected using the
REFSEL1 and REFSEL0 bits in the configuration register. When
the internal reference is selected, it is internally connected 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 AD7794/AD7795 are functional with reference
voltages from 0.1 V to AV
excitation (voltage or current) for the transducer on the analog
DD
DD
/2. This function is available on inputs AIN1(−) to
are directed to either the IOUT1 or IOUT2 pin of the
DD
. The reference input is unbuffered; therefore,
DD
. In applications where the
DD
do not