NanoBoard 3000 - ADC Interface

Parent article: NanoBoard 3000 - Motherboard Resources

Analog-to-digital conversion on the NanoBoard 3000 is provided by a 4-channel, 8-bit ADC device – an ADC084S021 from National Semiconductor. This device uses successive-approximation with an internal track-and-hold circuit.

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 and provides a sample rate range of 50ksps to 200ksps.

External analog signals destined for the User FPGA ('ADC0' - 'ADC3') are delivered to the motherboard through a 6-way screw terminal block. This terminal block also provides 5V power and analog ground signals.

Each signal is low-pass filtered prior to entering the ADC084S021 device. The cut-off frequency of each line filter (based on R = 27Ω and C = 33nF) is ~ 178kHz.

ADC interface.

The four analog input channels of the ADC084S021 are multiplexed – selectable by application software via the SPI connection. The analog input to be sampled and converted is determined by setting bits 4 (ADD1) and 3 (ADD0) of the device's internal 8-bit control register. The following table shows how these bits are used to select the input channel.

At powerup, channel 1 ('ADC0') will be used by default.

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 ADC084S021 device (designated U24), the screw terminal block (designated J13) and the SMF05C device (designated U25) are all located on the component side of the board, between the DAC interface and the Ethernet status LEDs.
• 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 ADC interface.

Schematic Reference

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

• Sheet 37 (ADC_ADC084S021_SPI.SchDoc, entitled ADC084S021 8-Bit ADC IC)
• Sheet 38 (CON_ADCx4_KMRJIO3_5MM_6WAY.SchDoc, entitled Screw Header For ADC)
• Sheet 9 (PSU_TC1017R_3V3.SchDoc, entitled PSU TC1017R 3V3).

Design Interface Component

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

Further Device Information

• For more information on the ADC084S021 device, refer to the datasheet (ADC084S021.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.