Re: 4-20mA Application: need some advice, please

On 06 Apr 2007 15:23:51 GMT, the renowned andrea@xxxxxxxxxxxxxxxx
wrote:

1. You don't need an analog mux at all, half a dozen 1N4148s will do
at least as well.

2. A single chip will work to switch power to the transmitters, eg. a
Micrel 2981

3. Pay attention to the specs as Grant Edwards suggests, some
transmitters may be spec'd with 30 minute warm-up.

4. A constant-current circuit would be better than a series resistor
or polyswitch (eg. a 25mA 2-wire constant current circuit using
an LM317 and a resistor. It will need a few volts to work, so it
affects the compliance. You will need to check the specs to
determine the minimum supply voltage for your application (
the compliance range of the transmitter, the drop on your sense
resistor, the drop in the driver circuit, the drop in the current
regulator or whatever, and a bit of margin, at say 25mA).

5. It's best to add some series impedance from your sense resistor,
clamp that to some fixed voltage (eg. Vdd) and perhaps divide
that down a bit and buffer it to go into your ADC. You can
Low-pass filter it a bit too. Take care that too much current
doesn't flow into Vdd such that it causes Vdd to increase
(regulators generally don't sink current, only source).


Best regards,
Spehro Pefhany
--
"it's the network..." "The Journey is the reward"
speff@xxxxxxxxxxxx Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog Info for designers: http://www.speff.com
.

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