Analog-To-Digital And Digital-To-Analog Converters - MTX -3G-JAVA series User Manual

4.4.4 Analog‐to‐Digital and Digital‐to‐Analog converters
The ADC/DAC of the MTX‐3G‐JAVA 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‐3G‐JAVA 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 switched 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 www.mtxm2m.com

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:
where D is the sample value in decimal format (0 ≤ D ≤ 255) and V
to generate (0 ≤ V
DACOUT
www.mtx‐terminals.com
256
≤ 2988.3mV)
www.mtxm2m.com
MTX M2M® by MATRIX ELECTRONICA S.L.U
MTX‐3G‐JAVA Family
3
DACOUT
2015/10 v1.6
is the voltage value you want
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