Xylem H-4280 - Manual (Page 6)

1-2 Introduction
H-4280
Unipolar 0 to 5.0 Volts (resolution is 1.2mV)
Bipolar: ±5.0 Volts (resolution is 2.4mV)
Single Ended: Measurement is referenced to ground
Differential: Measures the difference between two adjacent input channels (0-1, 2-3, 4-5, 6-7)
The H-4280 always makes 8-measurements, even if one or more of the channels is configured for a
differential measurement. For example; if Channel 0 is configured for a differential measurement, the
Channel 0 measurement is determined by the input voltage on both Channel 0 & 1. Subsequently a
measurement is made for Channel 1, with the results determined separately by the settings of Channel 1.
Normally if Channel 0 is configured for a differential measurement, the results of the adjacent channel
measurement (Channel 1) is ignored by the data logger (and vice versa).
1.3 Single-Ended Analog Inputs
A single-ended analog measurement is one which is referenced to ground. The H-4280 can make up to eight
single-ended measurements. Both unipolar and bipolar measurements can be selected. The 12-position
input connector has 3-terminals for making ground connections. Unused inputs may be left floating,
however the measurement data from unused inputs channels is undefined and will drift.
1.4 Differential Analog Inputs
A differential analog measurement is one which measures the difference between two input terminals. The
two terminals have positive and negative input response respectively. The H-4280 can make up to 4
differential measurements. Both unipolar and bipolar measurements can be selected. If the (+) input will
always more positive than the (-) input, set the channel to “unipolar”. The unipolar setting has the best
resolution. If the (+) can be less positive than the (-) input, set the channel to “bipolar”. The bipolar setting
allows the measurement to go negative but has the shortcoming of 1-bit less resolution.
1.5 Excitation Output
The H-4280 has a precision 5.00V excitation output terminal for powering potentiometers, thermistors and
other sensors. The +5Vref terminal is internally connected to the A/D converter. Measurements made with
resistance sensors powered from the +5Vref terminal will be “ratio metric” in that errors in the absolute
value of the reference cancel out. The +5Vref output is normally turned off to save power while the
module is in its sleep mode and is switched on before a measurement is made. The “warmup time” is a
programmable delay (1 to 999 seconds) which elapses before the first measurement is made. If desired, the
excitation output can be programmed to remain active while the module is asleep. The +5Vref terminal is
for powering potentiometers, thermistors and other low current senors. Do not draw more than 50mA from
the +5Vref terminal.
1.6 Sensors With Current Output
The H-4280 measures input voltage. Sensors which output current can be connected to the H-4280 with
the aid of a shunt resistor. For example, a 4-20mA sensor can be interfaced by connecting a precision 250
ohm shunt resistor between the input and a ground terminal. When the sensor is transmitting its 20mA
maximum output the H-4280 will input 5.0Volts: ( 5.0V = .020mA x 250 Ohms).