Dual DAC - MAX549A
Dual 8 bits DAC - SPI - MAX549
Fig. 1 Dual 8bits SPI DAC
Description:
This board is a dual 8bits DAC with an SPI interface. It is based around 2 Maxim chip: a dual 8 bits digital-to-analog converter MAX549A, and a voltage reference MAX61xx.
It connects to a host with an SPI bus, up to 10 MHz.
The voltage reference can be choosen between MAX6125, 6141 or 6145 for a voltage reference (and maximum output voltage) of 2.5, 4.096 or 4.5 V respectively.
The board can be powered from 2.5 to 5V. But the voltage reference dictates the lower power supply. The power supply needs to be at least 200mv higher than the voltage reference.
Schematic:
The schematic is as simple as it can get. U2 provides voltage reference to the DAC chip U1.
U2 can be selected from the MAX 61xx chips listed above. The thing to remember is that the power supply needs to be at least 200mV higher than the desired reference voltage.
Specifications:
Input voltage 2.5 to 5V
InterfaceSPI 10MHz
CS selectable by jumper
Output voltage0 to Vref
8 bits resolution
Voltage referencedefined by chip MAX61xx
Initially, a MAX6141 was selected for it, in an SO08 package, simply because I had one in my drawer. The board is powered with a 5V processor board. It provides an output from 0 to 4.096 V
The CS signal is pulled high by default with R1. U1 is active with CS low, so it keeps U1 inactive by default. JP1 allows to select which CS line from the SPI interface is used.
The SPI interface is common across the SPI boards, and provides the following signals: SCK, MISO, MOSI, 3 chip select signals and power supply.
The outputs are on JP3 and JP4, also common across analog boards.
Fig. 2 Dual 8 bits SPI DAC schematic
Construction:
The PCB fits on a small single sided board. The PCB is designed to fit on one of the typical size for the board designed here. All components are on the top side except for U2, the voltage reference, which is on the bottom side (copper side).
Fig. 3a Dual 8 bits DAC SPI layout
Start with the lowest components, and continue with the higher ones on the top side. I put component in sockets when possible, so I usually test the board without them first, making sure the voltage is as expected... Only then are they put into their socket. Keep the MAX549 out for now.
The connectors JP2, JP3 and JP4 are optional. You can either solder cables directly on the board, or use connector to connect various boards together.
Once all the components are soldered on the top side, solder U2 on the copper side.
Fig. 3b Bottom side with U2
Before fitting the DAC in its socket, you can test the board as described below.
Pads 1 to 4 can be drilled to 3mm to fix the board, or put spacers.
| Dual 8bits SPI DAC Component list |
|---|
| C1: 100 nF C2: 2.2 nF R1: 100 K U1: MAX549A U2: MAX6125, 6141 or 6145 JP5: jumper 3x2 JP2: Connector AMP MT 8 pins male (optional) JP3, JP4: Connector AMP MT 3 pins male (optional) |
How to use / Testing:
To check the board, connect the board to an SPI interface. Leave jumper off JP5 for now. Power up the board.
Check you have the Vcc supply on pin 8 of U1 socket and the reference volateg on pin 7.
With U1 in its socket, the output voltage will be 0V after power up.
To check the DAC is working, you need a processor board running the SPI DAC tutorial. Just make sure you set the switch on JP1 to the proper CS line.
Files and links:
Eagle PCB and schematics files for Dual 8 bits SPI DAC.PDFs of PCB, layout and schematics of Dual 8 bits SPI DAC.
PIC Tutorials for Dual 8 bits SPI DAC.
Maxim MAX549A.
Maxim MAX6125, MAX6141, MAX6145.