NanoBoard 3000 - DAC Interface

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Modified by Admin on Nov 6, 2013

Digital-to-analog conversion on the NanoBoard 3000 is provided by a 4-channel, 8-bit DAC device – a DAC084S085 from National Semiconductor. The device delivers rail-to-rail output voltage swing (0V up to a supplied reference voltage) with a maximum zero code error of 15mV and maximum full scale error of -0.75%FSR.

The device is powered from a regulated 3.3V power supply, obtained by passing the motherboard's 5V supply through a low dropout (LDO) regulator (a TC1017R, from Microchip).

The device interfaces to the User FPGA over an SPI bus, operating at clock rates up to 40MHz.

The four converted analog output signals ('DAC0' - 'DAC3') are delivered to a 6-way screw terminal block. This terminal block also provides 5V power and analog ground signals.

DAC interface.

The four digital input channels of the DAC084S085 are multiplexed – selectable by application software via the SPI connection. An individual channel can be written, or all channels simultaneously. The mode of operation, selection of specific channel and actual 8-bit data is written to the device's internal 16-bit input shift register. Table 1 illustrates this register.

Table 1. Bit content of the input shift register.

OP1

OP0

For the 8-bit data, D7 is the MSB. 00000000 corresponds to an output voltage of 0V. 11111111 corresponds to an output voltage of VRef - 1 LSB.

Tables 2 and 3 showing how the channel selection and operational mode bits are used.

Table 2. Selection of input channel using address bits A1:A0.

A1	A0	Channel
0	0	`'DAC0'`
0	1	`'DAC1'`
1	0	`'DAC2'`
1	1	`'DAC3'`

Table 3. Selection of operational mode using bits OP1:OP0.

OP1	OP0	Operational Mode
0	0	Write data to the DAC register for the selected channel specified by bits A1:A0, but do not update outputs
0	1	Write data to the DAC register for the selected channel specified by bits A1:A0, and update outputs
1	0	Write data to the DAC register for all channels, and update outputs
1	1	Power-down all outputs

At powerup, the output voltage for each channel is held at 0V until a valid write sequence is performed.

Digital-to-analog conversion in the DAC084S085 device is achieved using 8-bit resistor string DACs. The input supply reference to the device, VRef, used for each channel's DAC, is tied to the same regulated 3.3V supply – allowing the full dynamic range of the device to be utilized.

The four analog signal lines, as well as the 3.3V supply line, are protected against transient voltages (caused for example by ESD) through use of a TVS diode array device – an SMF05C, from Semtech.

Location on Board

The DAC084S085 device (designated U28), the screw terminal block (designated J15) and the SMF05C device (designated U29) are all located on the component side of the board, to the left of the 120R termination enable jumper for the RS-485 interface (J26).

The TC1017R device (designated U58) is located on the solder side of the board, to the bottom left of the HIN232A high-speed RS-232 Transceiver device (U59).

LDO regulator (U58) used for the DAC interface.

Schematic Reference

The DAC circuitry can be found on the following sheets of the motherboard schematics:

Sheet 39 (DAC_DAC084S085_SPI.SchDoc, entitled DAC084S085 8-Bit DAC IC)

Sheet 40 (CON_DACx4_KMRJIO3_5MM_6WAY.SchDoc, entitled Screw Header For DAC)

Sheet 9 (PSU_TC1017R_3V3.SchDoc, entitled PSU TC1017R 3V3).

Design Interface Component

Table 4 summarizes the available design interface component that can be placed from the FPGA NB3000 Port-Plugin.IntLib, to access and use the on-board DAC device over the SPI bus.

Table 4. DAC port-plugin component.

Component Symbol	Component Name	Description
	`DAC`	Place this component to interface to the DAC084S085 device.

Further Device Information

For more information on the DAC084S085 device, refer to the datasheet (DAC084S085.pdf) available at www.national.com.

For more information on the SMF05C device, refer to the datasheet (SMF05C.pdf) available at www.semtech.com.

For more information on the TC1017R device, refer to the datasheet (21813d.pdf) available at www.microchip.com.