A simplified functional block diagram of the ADC module is shown in Figure 8-8. The ADC module consists of a 12-bit ADC with a built-in sample-and-hold (S/H) circuit. Functions of the ADC module include:
- 12-bit ADC core with built-in S/H
- Analog input: 0.0 V to 3.0 V (Voltages above 3.0 V produce full-scale conversion results.)
- Fast conversion rate: Up to 80 ns at 25-MHz ADC clock, 12.5 MSPS
- 16 dedicated ADC channels. 8 channels multiplexed per Sample/Hold
- Autosequencing capability provides up to 16 "autoconversions" in a single session. Each conversion can be programmed to select any 1 of 16 input channels
- Sequencer can be operated as two independent 8-state sequencers or as one large 16-state sequencer (that is, two cascaded 8-state sequencers)
- Sixteen result registers (individually addressable) to store conversion values
- The digital value of the input analog voltage is derived by:
- Multiple triggers as sources for the start-of-conversion (SOC) sequence
- S/W - software immediate start
- ePWM start of conversion
- XINT2 ADC start of conversion
- Flexible interrupt control allows interrupt request on every end-of-sequence (EOS) or every other EOS.
- Sequencer can operate in "start/stop" mode, allowing multiple "time-sequenced triggers" to synchronize conversions.
- SOCA and SOCB triggers can operate independently in dual-sequencer mode.
- Sample-and-hold (S/H) acquisition time window has separate prescale control.
The ADC module in the 2833x/2823x devices has been enhanced to provide flexible interface to ePWM peripherals. The ADC interface is built around a fast, 12-bit ADC module with a fast conversion rate of up to 80 ns at 25-MHz ADC clock. The ADC module has 16 channels, configurable as two independent 8-channel modules. The two independent 8-channel modules can be cascaded to form a 16-channel module. Although there are multiple input channels and two sequencers, there is only one converter in the ADC module. Figure 8-8 shows the block diagram of the ADC module.
The two 8-channel modules have the capability to autosequence a series of conversions, each module has the choice of selecting any one of the respective eight channels available through an analog MUX. In the cascaded mode, the autosequencer functions as a single 16-channel sequencer. On each sequencer, once the conversion is complete, the selected channel value is stored in its respective RESULT register. Autosequencing allows the system to convert the same channel multiple times, allowing the user to perform oversampling algorithms. This gives increased resolution over traditional single-sampled conversion results.
To obtain the specified accuracy of the ADC, proper board layout is very critical. To the best extent possible, traces leading to the ADCIN pins should not run in close proximity to the digital signal paths. This is to minimize switching noise on the digital lines from getting coupled to the ADC inputs. Furthermore, proper isolation techniques must be used to isolate the ADC module power pins ( VDD1A18, VDD2A18 , VDDA2, VDDAIO) from the digital supply.Figure 8-9 shows the ADC pin connections for the devices.
Note: - The ADC registers are accessed at the SYSCLKOUT rate. The internal timing of the ADC module is controlled by the high-speed peripheral clock (HSPCLK).
- The behavior of the ADC module based on the state of the ADCENCLK and HALT signals is as follows:
- ADCENCLK: On reset, this signal will be low. While reset is active-low (
XRS) the clock to the register will still function. This is necessary to make sure all registers and modes go into their default reset state. The analog module, however, will be in a low-power inactive state. As soon as reset goes high, then the clock to the registers will be disabled. When the user sets the ADCENCLK signal high, then the clocks to the registers will be enabled and the analog module will be enabled. There will be a certain time delay (ms range) before the ADC is stable and can be used.
- HALT: This mode only affects the analog module. It does not affect the registers. In this mode, the ADC module goes into low-power mode. This mode also will stop the clock to the CPU, which will stop the HSPCLK; therefore, the ADC register logic will be turned off indirectly.
Figure 8-9 shows the ADC pin-biasing for internal reference and Figure 8-10 shows the ADC pin-biasing for external reference.
A. TAIYO YUDEN LMK212BJ225MG-T or equivalent
B. External decoupling capacitors are recommended on all power pins.
C. Analog inputs must be driven from an operational amplifier that does not degrade the ADC performance.
D. External voltage on ADCREFIN is enabled by changing bits 15:14 in the ADC Reference Select register depending on the voltage used on this pin. TI recommends TI part REF3020 or equivalent for 2.048-V generation. Overall gain accuracy will be determined by accuracy of this voltage source.
Figure 8-10 ADC Pin Connections With External ReferenceNote: The temperature rating of any recommended component must match the rating of the end product.