HMR2300

10

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OUTPUT SAMPLE RATES

The sample rate can be varied from 10 samples per second (sps) to 154 sps using the *ddR=nnn command. Each sample

contains an X, Y, and Z reading and can be outputted in either 16-bit signed binary or binary coded decimal (BCD) ASCII.

The ASCII format shows the standard numeric characters displayed on the host computer display. Some sample rates

may have restrictions on the format and baud rate used, due to transmission time constraints.

There are 7 Bytes transmitted for every reading binary format and 28 Bytes per reading in ASCII format. Transmission

times for 9600 baud are about 1 msec/Byte and for 19,200 baud are about 0.5msec/Byte. The combinations of format and

and baud rate selections are shown in the above Table. The default setting of ASCII format and 9600 baud will only

transmit correctly up to 30 sps. Note the HMR2300 will output a higher data settings, but the readings may be incorrect

and will be at alower output rate than selected.

For higher sample rates (>60 sps), it is advised that host computer settings for the terminal preferences be set so a line

feed <lf> is not appended to the sent commands. This slows down the reception of data, and it will not be able to keep up

with the incoming data stream.

INPUT SIGNAL ATTENUATION

Magnetic signals being measured will be attenuated based on the sample rate selected. The bandwidth, defined by the

3dB point, is shown in the above Table for each sample rate. The default rate of 20 sps has a bandwidth of 17Hz. The

digital filter inside the HMR2300 is the combination of a comb filter and a low pass filter. This provides a linear phase

response with a transfer function that has zeros in it.

When the 10 or 20 sps rate is used, the zeros are at the line frequencies of 50 and 60 Hz. These zeros provide better than

125 dB rejection. All multiples of the zeros extend throughout the transfer function. For example, the 10 and 20 sps rate

has zeros at 50, 60, 100, 120, 150, 180, ... Hz. The multiples of the zeros apply to all the sample rates against the stated

notch frequencies in the above Table.

COMMAND INPUT RATE

The HMR2300 limits how fast the command bytes can be recieved based on the sample rate selected. The above Table

shows the minimum time between command bytes for the HMR2300 to correctly read them. This is usually not a problem

when the user is typing the commands from the host computer. The problem could arise from an application program

outputting command bytes too quickly.

CIRCUIT DESCRIPTION

The HMR2200 Smart Digital Magnetometer contains all the basic sensors and electronics to provide digital indication of

magnetic field strength and direction. The HMR2300 has all three axis of magnetic sensors on the far end of the printed

circuit board, away from the J1 and J2 connector interfaces. The HMR2300 uses the circuit board mounting holes or the

enclosure surfaces as the reference mechanical directions. The complete HMR2300 PCB assembly consists of a mother

board, daughter board, and the 9-pin D-connector (J1).

The HMR2300 circuit starts with Honeywell HMC2003 3-Axis Magnetic Sensor Hybrid to provide X, Y, and Z axis

magnetic sensing of the earth’s field. The HMC2003 contains the AMR sensing bridge elements, a constant current

source bridge supply, three precision instrumentation amplifiers, and factory hand-selected trim resistors optimized for

performance for magnetic field gain and offset. The HMC2003 is a daughter board that plugs into the HMC2300

motherboard, and the hybrid analog voltages from each axis is into analog multiplexors and then into three 16-bit Analog

to Digital Converters (ADCs) for digitization. No calibration is necessary as the HMC2003 hybrid contains all the

compensation for the sensors, and the set/reset routine handles the temperature drift corrections. A microcontroller

integrated circuit receives the digitized magnetic field values (readings) by periodically querying the ADCs and performs

any offset corrections. This microcontroller also performs the external serial data interface and other housekeeping

functions. An onboard EEPROM integrated circuit is employed to retain necessary setup variables for best performance.

The power supply for the HMR2300 circuit is regulated +5 volt design (LM2931M) with series polarity power inputs diodes

in case of accidental polarity reversal. A charge pump circuit is used to boost the regulated voltage for the set/reset pulse

function going to the set/reset straps onboard the HMC2003. Transient protection absorbers are placed on the TD, RD,

and V+ connections to J1.