Description: This is an isolated signal analog output card used industry wide. The analyzer’s potentiometric output (DC volts) signal drives the signal for the 4-20 mADC card (usually 0VDC input = 4 mADC output & 5 VDC input = 20 mADC output). The term “isolated” refers to the fact that the output signal is not referenced to earth ground and is therefore much less susceptible to interference from other voltage sources. Milliamp signals are very useful when running long signal loops (on the order of tens of feet to thousands of feet long; note that total loop resistance has to be less than 850 ohms).
The board is built and tested as 620433. Rosemount packages this board in a kit (p/n: 621023) that contains the 620433 plus an instruction sheet & hold-down screw.
This is an excerpt from the Rosemount 755A instruction manual: An isolated current output is obtainable with the optional Current Output Board, either included with the Model 755A or added at a later date in the field. This option provides a current output of either 0 to 20mA or 4 to 20mA for a maximum of 850 ohms.
Where used: 400A,755,755A,755R,870
Typical failure modes:
- total failure: most likely due to the fullwave bridge rectifier or output opamp.
- non-linearity: most likely due to the charge pump circuit.
- inability to calibrate: most likely due to the charge pump circuit.
- noisy output: output opamp is failing –or– input signal is noisy –or– board mounted isolated power supply(ies) are failing –or– loop resistance is very high –or– it could be that your mADC reader (DCS, DAS, etc) is misbehaving.
Static sensitive: This board is not very susceptible to ESD but handle it with some ESD protocols.
Repairable: Yes! Our flat rate fee is $350 and assumes that we won’t have to replace the $250 Burr Brown isolation amplifier (we haven’t had to replace one yet!). Call use for a RMA number … 877-616-0600.
** (Pricing is valid through October 1, 2012) **
Replaced by: P/N: 25C0007R0 … RIGAS has released its compatible version of this board. Some nice enhancements were included:  hook-type test points,  better labeling,  heat sinks,  zero & span potentiometers can dual configuration (i.e., adjustable from the top or from the front).