AD7194 Analog Devices, AD7194 Datasheet - Page 31

• Manufacturer Part Number: AD7194
• Description: 8-Channel, 4.8 kHz, Ultralow Noise, 24-Bit Sigma-Delta ADC with PGA
• Manufacturer: Analog Devices

Specifications of AD7194

• Resolution (bits): 24bit
• # Chan: 8
• 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: CSP

impedance. External decoupling on the REFINx pins is not
recommended in this type of circuit configuration. Conversely,
if large decoupling capacitors are used on the reference inputs,
there should be no resistors in series with the reference inputs.
Recommended 2.5 V reference voltage sources for the AD7194
include the
These references tolerate decoupling capacitors on REFINx(+)
without introducing gain errors in the system. Figure 23 shows the
recommended connections between the ADR421 and the AD7194.
REFERENCE DETECT
The AD7194 includes on-chip circuitry to detect whether the
part has a valid reference for conversions or calibrations. This
feature is enabled when the REFDET bit in the configuration
register is set to 1. If the voltage between the selected REFINx(+)
and REFINx(−) pins is less than 0.3 V, the AD7194 detects that
it no longer has a valid reference. In this case, the NOREF bit
of the status register is set to 1. When the voltage between the
selected REFINx(+) and REFINx(−) pins is greater than 0.6 V,
the AD7194 detects a valid reference; thus, the NOREF bit is set
to 0. The operation of the NOREF bit is undefined when the
voltage between the selected REFINx(+) and REFINx(−) pins
is between 0.3 V and 0.6 V.
If the AD7194 is performing normal conversions and the
NOREF bit becomes active, the conversion result is 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 AD7194 is performing either an offset or full-scale cali-
bration 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.
BIPOLAR/UNIPOLAR CONFIGURATION
The analog input to the AD7194 can accept either unipolar or
bipolar input voltage ranges. A bipolar input range does not
imply that the part can tolerate negative voltages with respect
to system AGND. In pseudo differential mode, signals are
referenced to AINCOM, whereas in differential mode, signals
are referenced to the negative input of the differential pair. For
example, if AINCOM is 2.5 V and the AD7194 AIN1 analog
0.1µF
AVDD
ADR421
Figure 23. ADR421 to AD7194 Connections
10µF
and ADR431, which are low noise references.
2
4
V
GND
IN
ADR421
TRIM
V
OUT
6
5
4.7µF
REFINx(+)
REFINx(–)
AD7194
Rev. 0 | Page 31 of 56
input is configured for unipolar mode with a gain of 2, the input
voltage range on the AIN1 pin is 2.5 V to 3.75 V when a 2.5 V
reference is used.
If AINCOM is 2.5 V and the AD7194 AIN1 analog input is
configured for bipolar mode with a gain of 2, the analog input
range on AIN1 is 1.25 V to 3.75 V. The bipolar/unipolar option
is chosen by programming the U/ B bit in the configuration
register.
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.
Gain is the PGA setting (1 to 128).
N = 24.
BURNOUT CURRENTS
The AD7194 contains two 500 nA constant current generators,
one sourcing current from AV
current from AIN(−) to AGND. The currents are switched to
the selected analog input pair. Both currents are either on or off,
depending on the burnout current enable (burn) bit in the
configuration register.
These currents can be used to verify that an external transducer
remains operational before attempting to take measurements on
that channel. After 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. It
takes some time for the burnout currents to detect an open
circuit condition because the currents must charge any external
capacitors.
There are several reasons that a fault condition is detected: the
front-end sensor may be either open circuit or overloaded, or
the reference may be absent and the NOREF bit in the status
register is set, thus clamping the data to all 1s. The user must
check these three cases before making a determination.
If the voltage measured is 0 V, it may indicate that the transducer
has short circuited. The current sources work over the normal
Code = (2N × AIN × Gain)/V
Code = 2N – 1 × [(AIN × Gain/V
DD
to AIN(+) and one sinking
REF
REF
) + 1]
AD7194