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3.8. ADC and DAC interfaces

The interface module provides two A to D Converters (ADC) and a D to A Converter (DAC). The two ADCs each provide eight analog inputs with buffered 0-5V inputs, an internal multiplexer, and a 12-bit converter. The ADCs provide a 16-bit host interface with conversion data appearing on D[11:0] (and zeros on D[15:12]). The ADCs are clocked by a 4MHz crystal and are able to perform 200ksamples/s.

The DAC provides two 0-5V outputs with a 12-bit resolution.

The ADCs and DAC are powered from a 5V supply and share buffers to interface them to the 3.3V system bus provided by the logic module.

Figure 3.10 shows the architecture of the ADCs and DACs.

Figure 3.10. ADC and DAC interface architecture

ADC and DAC interface architecture

All of the interface signals are routed to the FPGA on the logic module. The ADCs and DAC are supported by an AHB interface that is instantiated in the logic module code example supplied with the IM-AD1.

Table 3.10 shows the assignment of the ADC and DAC interface signals to the logic module signals on the EXPIM connector.

ADC and DAC interface signals
SignalEXPIM connectorDescription
AD_D[15:0]IM_ABANK[47:32]ADC and DAC data bus
AD_T/RIM_ABANK48Buffer direction control
AD_nOEIM_ABANK49Buffer output enable
ADC1_nCONVIM_ABANK50ADC1 conversion start signal
ADC1_nCSIM_ABANK51ADC1 chip select
ADC1_nWRIM_ABANK52ADC1 write strobe
ADC1_nRDIM_ABANK53ADC1 read strobe
ADC2_nCONVIM_ABANK54ADC2 conversion start signal
ADC2_nCSIM_ABANK55ADC2 chip select
ADC2_nWRIM_ABANK56ADC2 write strobe
ADC2_nRDIM_ABANK57ADC2 read strobe
DAC_nLDACIM_BBANK50DAC load signal
DAC_A0IM_BBANK51DAC address bit
DAC_nCSIM_BBANK52DAC chip select
DAC_nWRIM_BBANK53DAC write strobe

The ADCs are clocked from a 4MHz oscillator. This also supplies the IM_CLK signal routed to the logic module FPGA. This is used in the example logic to clock the DC-DC converter PrimeCell peripheral and the stepper motor interfaces.

The analog inputs to the ADCs are buffered by LMV324 operational amplifiers (op-amps). The op-amps are configured to give unity gain but the inputs have a resistive divider that divides the input voltage by 2. A 0-5V input signal range at the buffer inputs provides a 0-2.5V full range at the ADC input. If different input ranges are required the divider resistor values can be changed.

The op-amp buffers cannot drive their outputs lower than 65mV. This means input signals less than 130mV will have incorrect ADC values.

The reference voltage from one of the ADCs is buffered and fed to the reference inputs of the other ADC and the DAC so that all devices share a common reference. The GAIN input to the DAC is tied HIGH to configure the output range of the DAC to be 0 to 2xVref.

Figure 3.11 shows the pinout of the ADC interface connector (J1).

Figure 3.11. ADC connector pinout

ADC connector pinout

Figure 3.12 shows the pinout of the DAC interface connector (J2).

Figure 3.12. DAC connector pinout

DAC connector pinout
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