Three analogue outputs are provided, labelled ‘A’, ‘B’ and ‘C’, which output longitudinal acceleration, lateral acceleration and combined speed respectively. The signal range of the analogue output is 0 to 5V. The acceleration is output with 0g at 2.5V, and a configurable scale. The speed output gives zero speed at 0.5V, and a user-configurable maximum speed at 4.5V.
Noise measurement tests have been performed by connecting the SPEEDBOX20 to a National Instruments (NI) 6510 USB data logger. This data logger has 16 bit analogue inputs and a maximum sample rate of 250kHz, this unit is representative of the equipment that the SPEEDBOX20 would be attached to in typical applications.
Noise measurements on the analogue outputs are performed by tilting the SPEEDBOX20 without a GPS antenna attached until the output from the unit clamps at the maximum output level (4.5V), the speed output from the analogue channel is logged at three different sample rates – 1kHz, 10kHz and 100kHz. The measured outputs are illustrated beneath for 1kHz, 10kHz and 100kHz sampling rates, respectively.
SPEEDBOX analogue output noise for 1kHz, 10kHz and 100kHz sampling
Below is shown a histogram of the noise for each sampling rate used. The vast majority of the noise lies within 1mV of the mean output.
SPEEDBOX analogue output noise distribution
The analogue outputs of the SPEEDBOX20 are factory calibrated using a traceable source and a recalibration service is available. Calibration certificates are provided with each unit.
Typical accuracy of the analogue outputs is within 10mV over the full working temperature range of the unit.