The configuration of the switches on S1:A determines if either the on-board emulator or an external emulator is
connected to the MCU, and if the SCI (UART) pins on the MCU are connected to the COM port on the USB-C
connector (see
).
5.2 Clocking Methodology
This controlCARD is required to support a broad range of TI's baseboards. Several designs rely on GPIO18
and GPIO19 for SPIA, while others require these GPIO to be utilized as a precision clock input source. To
accommodate both of these systems, a switch (S1) has been added to the design. This methodology should not
be used in a final system as it increases EMI emissions and creates robustness susceptibilities. It is up to the
system designer to choose the best way to implement the clocking circuity for a given system.
5.3 Evaluation of the Analog-to-Digital Converters (ADCs)
When using the F280039C on-chip ADCs, there are some useful guidelines to follow to realize the performance
numbers listed in the device-specific data sheet. This is especially true for the AC parameters such as: SNR,
THD, and SINAD. Furthermore, it can also be shown that there is a direct correlation between the SNR of the
ADC result and the spread of ADC codes seen for a DC input; as such these tips will improve the range and
standard deviation of a DC input as well. Finally, while topics addressed will be with respect to the controlCARD,
they are applicable to other implementations using the F28003x MCUs as well.
On-board resistors and capacitors:
By default, all inline resistors to the ADC pins are a simple 0-Ω shunt and
all capacitors to the ground plane are not populated. While this circuit can be used to supply the ADC inputs with
a voltage, likely both the resistor (R) and capacitor (C) will need to be populated based on the voltage source's
characteristics. Referring to the ADC Input Model, the ADC input has its own RC network made up of the internal
sample-and-hold capacitor, switch resistance, and parasitic capacitance. By changing the inline resistance and
parallel capacitor, we can optimize the input circuit to assist with settling time and/or filtering the input signal.
Finally, it is recommended in general to use Negative-Positive 0 PPM/°C (NP0/C0G) capacitors as these have
better stability over temperature and across input frequencies than other types of capacitors.
Voltage source and drive circuitry:
While the on-chip ADCs are 12-bit architecture (4096 distinct output codes
when converting an analog signal to the digital domain); the translation will only be as precise as the input
provided to the ADC. The typical rule of thumb when defining the source resolution to realize the full specification
of an ADC is to have a 1-bit better source than the converter. In this case that would mean that ideally the analog
input should be accurate to 13-bits.
Typically, voltage supplies or regulators are not designed to be precise, but rather accommodate a wide range of
current loads within a certain tolerance, and for this reason, are not ideal to show the performance of a higher-bit
ADC, like the one on the F280039C. This also does not take into account that many times, the supply in question
is providing the main voltage to power the MCU itself, which also introduces noise and other artifacts into the
signal.
In addition to the quality of the input signal, there is also the aspect of the load presented to the ADC when it
samples the input. Ideally, an input to an ADC would have zero impedance so as not to impact the internal R/C
network when the sampling event takes place. In many applications, however, the voltages that are sampled
by the ADC are derived from a series of resistor networks, often large in value to decrease the active current
consumption of the system. A solution to isolate the source impedance from the ADC sampling network is to
place an operational amplifier in the signal path. Not only does this isolate the impedance of the signal from the
ADC, it also shields the source itself from any effects the sampling network may have on the system.
Recommended source for evaluation:
The
Precision Signal Injector (PSI) EVM
validate the ADC performance on the F280039C ControlCARD. This EVM supports both single-ended as well
as differential-ended outputs using a
as the signal source, then passed through a
with post amplifier filtering. The EVM is powered and controlled through a standard USB connection
from a host PC and includes a GUI to control its output. The outputs are routed through single- or dual-SMA
type connectors; it is highly recommended to place an additional female SMA connector (
controlCARD docking station to receive the signal via SMA for best noise immunity. For the local RC network,
30-Ω resistors and 300-pF capacitors were used. Using this setup, the ADC parameters were observed to be
consistent with the numbers in the device-specific data sheet.
Special Notes
6
TMS320F280039C controlCARD Information Guide
SPRUIZ4B – MAY 2021 – REVISED JUNE 2022
Copyright © 2022 Texas Instruments Incorporated
