QTech Data Systems DNP3 RTU - Manual (Page 20)

DATRAN XL4 RTU – Owners Manual – v1.1 June 2012
www.qtech.co.nz Page 20
Circuit details of Differential Analog Input Channel.
It can be seen from the above diagram that the analog preconditioning networks are identical to those in the single
ended input circuit. Therefore the various analog input mode configurations and jumper settings are the same as
for the single ended analog input circuit.
The op-amp is configured as a unity gain differential amplifier. Jumper J21 selects the input mode as follows:
When J21 is open the circuit is configured for differential input mode. That is the negative analog input line
is above ground therefore allowing for common mode input voltages up to 10 volts.
When jumper J21 is closed the circuit is configured for single ended input mode.
Note. Each of the inputs to the differential analog inputs are clamped with BAV99 high speed switching diodes to
both ground and +15 volts to keep the input voltage to op-amp in the range 0 to +15 volts. Differential and
common mode analog input voltages outside 0 to +15 volts will cause the clamping diodes to conduct and
draw current from the analog input source. The BAV99 diodes are rated at 200mA forward current.
Therefore care must be taken to ensure that over-voltage/under-voltage analog input currents do not
exceed 200mA.