Hi all
I am new to electronics and I am looking for a DAC that has 4-20ma
output.
I googled and one option I got was the AD421 but the problem is that
it uses a mosfet that I cannot find anywhere the DN25D FET transistor
from Supertex1.
You don't need a FET.
What solutions can you suggest to my problem? Are there any
alternatives both to the AD421 or the fet?
Are you trying to make a "two wire" system where the power to run it
comes from the 4mA?
The basic current sink looks like:
U1
Vcontrol ---------!+\ !!------- Sink
! >----!!- N MOSFET
--!-/ !!-
! !
-------------+-/\/\/---GND
U1 is an op-amp that can swing to ground if needed and has an input
range including ground. The MOSFET is just about any small power
MOSFET.
Parts are commonly added for scaling and stability.
In self powered 4-20mA systems the resistor can't be as shown. It has
to be in the power wiring of the whole system. The concept of the
circuit is the same but there isn't the thing called "GND"
U1
Vcontrol ---------!+\ !!--------------- +Line
! >----!!- N MOSFET
--!-/ !!-
! B !
-------------+-/\/\/-
!
Local power -------------+
!
/---/
^
!
Local GND --------------+-------------- -Line
There is a problem with this in that VControl and U1 must all be above
the local power. The second part is easy to solve by running U1 on
+Line and the point labeled "B".
Solving the first is easiest if U1 is a dual op-amp. The other
section can be used as part of the circuit that translates the control
signal up to the local levels. This can be just a current source
feeding an inverting amplifier.