Xylem SDI-12 Data Logger iRIS 220, iRIS 320, iRIS 350, iRIS 150, iRIS 300 - Manual (Page 16)

iRIS Datalogger User Guide V1.50
10 iQuest (NZ) Ltd - PO Box 15169, Hamilton, New Zealand Tel: +64 7 857-0810 Fax: +64 7 857-0811 Email: [email protected]
3.5.4 Analog I/O
Analog Inputs
On the iRIS 350, the four analog inputs are uni-polar 0-5Vdc with 16-bit resolution. Each input presents a
load impedance of 97K to the input signal. The iRIS 220 and iRIS 320 have 12-bit resolution inputs with an
impedance of 98 K.
For the iRIS 350, scaling factors should be chosen to convert from a raw value of 0.0000 – 5.0000, which
reflects the input signal range of 0-5V. When current sources such as 0-20mA or 4-20mA are connected, an
internal sink resistor (100) is enabled by an internal user-settable link. In this mode the actual voltage range
is 0-2V and the scaling factor should take this into account.
As the analog inputs have an input impedance of 97K, the actual sink resistor impedance
will be slightly lower than the value fitted. When, for example, the current mode link is fitted, a
sink resistor of 100 ohms is installed. The actual impedance will theoretically be 99.71;
therefore the voltage measured by the iRIS will also be slightly lower than expected. See
Section 8 for details on the analog input scaling method to use to optimise the calibration.
Figure 3 - Simplified iRIS 350 Analog Input Circuit
Figure 4 - iRIS 350 Analog Input / Output Links
Analog Output
The iRIS 350 has a single variable analog output. This may be configured to deliver either a voltage output
ranging between 0-5V or a current output ranging from 0-20mA or 4-20mA. The output mode is link
selectable.
In the current software this output is fixed to 5V output making it compatible with the fixed 5V
excitation output on the iRIS 220/320.
See Section 2.4 for a list of the I/O differences between the models.
1nF
22K
75K
AGND
AINx
To ADC
AIN4 100 Ω current
sink enable link (J1)
AIN1 100 Ω current
sink enable link (J4)
AIN3 100 Ω current
sink enable link (J2)
AIN2 100 Ω current
sink enable link (J3)
AOUT mode link. (J5)
4-20mA
0-5V (default)