AD7799 Analog Devices, AD7799 Datasheet - Page 23

• Manufacturer Part Number: AD7799
• Description: 3-Channel, Low Noise, Low Power, 24-Bit, Sigma Delta ADC with On-Chip In-Amp
• Manufacturer: Analog Devices

Specifications of AD7799

• Resolution (bits): 24bit
• # Chan: 3
• 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

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 midscale 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.
N = 16 for the AD7798, and N = 24 for the AD7799.
BURNOUT CURRENTS
The AD7798/AD7799 each contain two 100 nA constant
current generators—one sourcing current from AV
AIN(+), and one 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 into 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 must determine why this is the
case. A full-scale reading could mean that the front-end sensor
is open circuit, that the front-end sensor is overloaded and is
justified in outputting full scale, or that the reference is absent
and, thus, clamping the data to all 1s.
When reading all 1s from the output, the user should check
these three cases before making a judgment. If the voltage
measured is 0 V, it might 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.
REFERENCE
The common-mode range for these differential inputs is from
GND to AV
excessive resistance/capacitance source impedances introduce
gain errors. The reference voltage REFIN (REFIN(+) − REFIN(−))
is 2.5 V nominal, but the AD7798/AD7799 are functional with
reference voltages from 0.1 V to AV
excitation (voltage or current) for the transducer on the analog
input also drives the reference voltage for the part, the effect of
Code = 2
Code = (2
DD
N – 1
. The reference input is unbuffered; therefore,
N
× AIN × GAIN)/V
× [(AIN × GAIN/V
REF
DD
REF
. In applications where the
) + 1]
DD
to
Rev. A | Page 23 of 28
the low frequency noise in the excitation source is removed
because the application is ratiometric. If the AD7798/AD7799
are used in a nonratiometric application, a low noise reference
should be used.
Recommended 2.5 V reference voltage sources for the AD7798/
AD7799 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 combina-
tions on these inputs can cause dc gain errors, depending on the
output impedance of the source driving the reference inputs.
Reference voltage sources such as those recommended above
(for example, 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.
REFERENCE DETECT
The AD7798/AD7799 include on-chip circuitry to detect if
there is a valid reference for conversions or calibrations. This
feature is enabled when the REF_DET bit in the configuration
register is set to 1. If the voltage between the REFIN(+) and
REFIN(–) pins goes below 0.3 V, or either the REFIN(+) or
REFIN(–) inputs are open circuit, the AD7798/AD7799 detect
that there is no longer a valid reference. In this case, the NOREF
bit of the status register is set to 1. If the AD7798/AD7799 are
performing normal conversions and the NOREF bit becomes
active, the conversion results revert to all 1s. Therefore, it is not
necessary to continuously monitor the status of the NOREF bit
when performing conversions. It is only necessary to verify its
status if the conversion result read from the ADC data register
is all 1s. If the AD7798/AD7799 are performing an offset of full-
scale calibration and the NOREF bit becomes active, the updating
of the respective calibration registers is inhibited to avoid loading
incorrect coefficients to these registers, and the ERR bit in the
status register is set. If the user is concerned about verifying that
a valid reference is in place every time a calibration is performed,
the status of the ERR bit should be checked at the end of the
calibration cycle.
RESET
The circuitry and serial interface of the AD7798/AD7799 can
be reset by writing 32 consecutive 1s to the device. This resets
the logic, the digital filter, and the analog modulator, and all
on-chip registers are reset to their default values. A reset is
automatically performed upon power-up. When a reset is
initiated, the user must allow a period of 500 μs before
accessing an on-chip register. A reset is useful if the serial
interface becomes asynchronous due to noise on the SCLK line.
AD7798/AD7799