AD7771_17 Datasheet, PDF (51/99 Pages) Analog Devices – 8-Channel, 24-Bit, Simultaneous Sampling ADC

AD7771
Σ-Δ ADC Errors
Reference Detect (SPI Control Mode)
In SPI control mode, the AD7771 includes on-chip circuitry to
detect if there is a valid reference for conversions or calibrations. If
the voltage between the selected REFx+ and REFx− pins goes
below 0.7 V, the AD7771 detects that it no longer has a valid
reference. CHx_ERR_REF_DET can be interrogated to identify
the affected channel, which clears the bits if the error is no
longer present. The voltage detector can be disabled by clearing
the REF_DET_TEST_EN bit.
Use the Σ-Δ ADC diagnostic or the SAR ADC to verify the error.
Overvoltage and Undervoltage Events
The AD7771 includes on-chip overvoltage/undervoltage
circuitry on each analog input pin. When the voltage on an
analog input pin goes above AVDD1x + 0.04 mV, the CHx_
ERR_AINx_OV bit is set. The error disappears if the input
voltage falls below AVDD1x − 40 mV.
If an undervoltage event occurs (AVSSx − 40 mV), the CHx_
ERR_AINx_UV bit is set. The error disappears if the input
voltage increases to AVSSx + 0.04 V.
The CHx_ERR_AINM_UV, CHx_ERR_AINM_OV, CHx_ERR_
AINP_UV, and CHx_ERR_AINP_OV bits can be read back to
verify the affected channel input, which clears the bits if the
error is no longer present. The overvoltage and undervoltage
detection can be disabled independently by clearing the AINM_
UV_TEST_EN, AINM_OV_TEST_EN, AINP_UV_TEST_EN,
or AINP_OV_TEST_EN bit.
The input voltage can be checked independently with the
SAR ADC.
Modulator Saturation
The AD7771 includes modulator saturation detection on each
of the Σ-Δ ADCs. If 20 consecutive codes for the modulator
are either all 1s or 0s, this condition is flagged as a modulator
saturation event. Reading CHx_ERR_MOD_SAT clears the bit
if the error corrects itself.
Modulator saturation detection can be disabled by clearing the
MOD_SAT_TEST_EN bit.
Note that the modulator input voltage is attenuated internally,
which means that a modulator output of all 1s or 0s represents a
modulator that is out of bounds and that a RESET pulse is required.
Filter Saturation
TheAD7771 includes digital filter saturation detection on each
Σ-Δ ADC channel. This detection indicates that the filter output is
out of bounds, which represents an output code approximately 20%
higher than positive or negative full scale. Reading the CHx_ERR_
FILTER_SAT bit clears the bit if the error corrects itself.
The detection can be disabled by clearing FILTER_SAT_TEST_
EN bit.
Data Sheet
Output Saturation
An output saturation event can occur when gain and offset
calibration causes the output from the digital filter to clip at
either positive or negative full scale. The output does not wrap.
Reading the CHx_ERR_OUTPUT_SAT bit clears the bit if the
error corrects itself.
The detection can be disabled by clearing OUTPUT_SAT_
TEST_EN bit.
SPI Transmission Errors (SPI Control Mode)
All SPI errors clear after reading GEN_ERR_REG_1, which
contains the SPI errors. These errors are not recovered automatically
and, consequently, the ALERT pin and the ALERT bit remain
set until the error register is read back.
CRC Checksum Error
If the CRC checksum is enabled by setting the SPI_CRC_
TEST_EN bit, an error bit, SPI_CRC_ERR, is raised if the CRC
message does not match the message computed by the AD7771
internal CRC block. If the CRC message does not match the
internally computed message, the register is not updated.
SCLK Counter
If the number of clocks generated by the controller is not a
multiple of 8 after CS is pulled high, an error bit, SPI_CLK_
COUNT_ERR is raised. The last command multiple of 8 is
executed; however, the SCLK counter can be disabled by setting
the SPI_CLK_COUNT_TEST_EN bit.
Invalid Read
When attempting to read back an invalid register address, the
SPI_INVALID_READ_ERR bit is set.
The invalid readback address detection can be disabled by
setting the SPI_INVALID_READ_TEST_EN bit.
Invalid Write
When attempting to write to an invalid register address, the
SPI_INVALID_WRITE_ERR bit is set.
The invalid write address detection can be disabled by setting
the SPI_INVALID_WRITE_TEST_EN bit.
MONITORING USING THE AD7771 SAR ADC
(SPI CONTROL MODE)
The AD7771 contains an on-chip SAR ADC for chip diagnostics,
system diagnostics, or measurement verification. The SAR ADC
has a 12-bit resolution. The AVDD4 and AVSS4 pins operate in
complete independence of the Σ-Δ ADC supplies and, therefore,
can be used for chip diagnostics in systems where functional
safety is important. The reference for the SAR conversion
process is taken from the SAR ADC supply voltage (AVDD4/
AVSS4) and, therefore, the SAR analog input range is from AVSS4
to AVDD4.
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