MT-X1S
Manual
disconnected using the solder jumpers. The IC is connected to a 3pin screw terminal with 3.5mm pin
spacing. The pin closest to header F is ground. The center pin is RX with RS232 installed, or
Inverting () with RS485. The pin next to the MicroSD slot is TX with RS232 or NonInverting (+) with
RS485. Note that the A/Y, B/Z naming is not used due to differing definitions among different
manufacturers. When RS485 is installed, there is a 120ohm termination resistor installed between the
inverting and noninverting pins, which can be disconnected using the solder jumper. Additionally, two
680 ohm resistors are installed that can be used to bias the inverting and noninverting pins to
negative and positive voltages respectively. The IC does not require this biasing so the bias resistors
are disconnected by default. When connecting wires to the screw terminal, take care not to short a
wire to an LED as they are close in proximity. The location of the LEDs was chosen to minimize
differences between the MTX1 and the MTX1S. When using RS232, be aware that the auto
powerdown feature is enabled. This causes the TX driver to power down when the RX line is
disconnected (no valid RS232 level present). One consequence of this is that a loopback test
requires a pull resistor on RX to enable the TX driver, which will then keep RX at a valid level
thereafter. For more information on either IC, consult the appropriate datasheet.
Audio Amplifier
The audio amplifier is the LM4889MM/NOPB from National Semiconductor. It is a single
channel, class AB, 400mW @ 3.3V amplifier with depop and thermal protection. It is connected to the
XMEGA DAC A0 on pin A2, which can be disconnected using a solder jumper. The shutdown pin is
routed to pin Q3 and has a 47 Kohm pull resistor to keep the IC in shutdown when Q3 is not driven.
The differential gain is set to 2, so the internal 1V reference or the external 1.25V reference can be
used. An 8ohm or 16ohm speaker can be connected to the output which is routed to a two pin header.
A 4ohm load can also be connected, but the amplifier may enter thermal shutdown if using a higher
voltage reference and the signal magnitude remains large for a long enough period of time. For more
information on this IC, please consult the datasheet.
1.25V Precision Reference
The 1.25V precision reference is the ISL60002DIH312ZTK from Intersil. It is a lowpower FGA
reference with a low 20ppm/C temperature coefficient. The initial accuracy is +/5mV. Because each
MTX1S board is intended to be calibrated individually, the initial accuracy was deemed less important
than the temperature coefficient. The reference is connected to both reference inputs, pins A0 and B0.
It can be disconnected using the solder jumpers. The reference voltage of 1.25V was chosen as a
workaround to the ADC errata of the ATxmega128a1. It is intended to be used with the ADC in
differential mode and with signed conversions. The voltage to be measured is connected to the
positive input, and the reference to the negative. This results in conversions in the ~0 to 2.5V range.
See the source code for example setup and usage. For more information on this IC, please consult
the datasheet.
Temperature Sensor
The temperature sensor is the MCP9701ATE/TT from Microchip. It is connected to pin A1,
and can be disconnected by using a solder jumper. It can sense from 10C to 125C, though the high
March 2, 2017
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