Start
Tsamp1
A ADINx
Sample time in normal operation mode
Sample time doubled in self-test mode
Conversion of last value sampled
AD_Core _In
AD
REFHI
AD
REFLO
A ADINx
time
Ext. Input
discharge of ext. cap
charging of ext. cap
Tsamp2
ADC Special Modes
714
SPNU503C – March 2018
Copyright © 2018, Texas Instruments Incorporated
Analog To Digital Converter (ADC) Module
Conversions in self-test mode are started just as they are in the normal operating mode (see
). The conversion starts according to the configuration set in the three mode control
registers (ADEVMODECR, ADG1MODECR, ADG2MODECR) and the sampling time control registers
(ADEVSAMP, ADG1SAMP, ADG2SAMP). The acquisition time for each conversion in self-test mode is
extended to twice the normal configured acquisition time. The selected reference voltage and the input
voltage from the ADINx input channel are both connected to the ADC internal sampling capacitor
throughout this extended acquisition period.
shows the self-test mode timing when the
ADREFLO is chosen as the reference voltage for the self-test mode conversion. It also assumes an
external capacitor connected to the ADC input channel.
Figure 19-14. Timing for Self-Test Mode
19.8.2.1 Use of Self-Test Mode to Determine Open/Short on ADC Input Channels
The following sequence needs to be used to deduce the ADC pin status:
•
Convert the channel with self test enabled and with the reference voltage as Vreflo. Store the
conversion result, say Vd.
•
Convert the channel with self test enabled and with the reference voltage as Vrefhi. Store the
conversion result, say Vu.
•
Convert the channel with self test disabled. Store the conversion result, say Vn.
The results can be interpreted using
.
Table 19-3. Determination of ADC Input Channel Condition
Normal Conversion
Result, Vn
Self-test Conversion
Result, Vu
Self-test Conversion
Result, Vd
Pin Condition
Vn
Vn < Vu < AD
REFHI
AD
REFLO
< Vd < Vn
Good
AD
REFHI
AD
REFHI
approx. AD
REFHI
Shorted to AD
REFHI
AD
REFLO
approx. AD
REFLO
AD
REFLO
Shorted to AD
REFLO
Unknown
AD
REFHI
AD
REFLO
Open