MTX-DIN Family
www.mtxm2m.com
2015/10 v2.3
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52
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MTX M2M® by MATRIX ELECTRONICA S.L.U
4.2.7.2
3G models
The ADC/DAC of the MTX-DIN-3G consists of two independent, unbalanced, multiplexed analog
inputs as well as one unbalanced analog output, which can be used for measuring external DC
voltages in the range of 0mV…+2988.3mV, or generate them in the same range. Both ADC and DAC
have a resolution of 8 bits, which means that the voltage resolution in the given range is
11.71875mV.
The MTX-DIN-3G has an internal ADC/DAC chip (PCF8591) connected to its I2C bus, at a 7-bit address
0x48 hexadecimal. Maximum bus frequency allowed is 100kHz.
The A/D converter uses the successive approximation conversion technique. The D/A converter and a
high-gain comparator are used temporarily during the A/D conversion cycle.
The D/A converter consists of a resistor divider chain connected to the external reference voltage
with 256 taps and selection switches. The tap-decoder switches one of these taps to the DAC output
line. The analog output voltage is buffered by an auto-zeroed unity gain amplifier.
In order to release the DAC for an A/D conversion cycle the unity gain amplifier is equipped with a
track and hold circuit. This circuit holds the output voltage while executing the A/D conversion.
You can configure and use the ADC/DAC converters by issuing I2C related AT commands or via the
I2C Java class.
The maximum A/D and D/A conversion rate is given by the actual speed of the I2C bus.
You can find Java code examples in the
Downloads
section at
D/A conversion
In order to write a new sample to the DAC buffer, you have to send three bytes over the I2C bus
before sending the I2C STOP condition
1.
Address byte (write mode): 0x90 hexadecimal
2.
Control byte: 0x40 hexadecimal
3.
Data byte: the 8-bit sample value in straight binary format
You can calculate the sample value you have to write to the D/A converter using the following
expression:
𝐷 =
256
3
𝑉
𝐷𝐴𝐶𝑂𝑈𝑇
where
D
is the sample value in decimal format (0 ≤
D
≤ 255) and
V
DACOUT
is the voltage value you want
to generate (0 ≤
V
DACOUT
≤ 2988.3mV)
