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Monday, March 23, 2009

4-20mA to Voltage Circuit

Isolate 4-20mA to Voltage Circuit
Picture shows an isolator receiver that translates a 4-20 mA
process current signal into a 0 V to 10 V output. A 1 V to 5 V
signal appears at the isolator’s output, and a –1 V reference
applied to output LO provides the necessary level shift
(in multichannel applications, the reference can be shared by
all channels). This technique is often useful for getting offset
with a follower-type output buffer.



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Circuit for receive signal 4-20mA
Differential amp are the simplest circuit for receive signal 4-20mA
where a signal source flow to resistor 100 ohm will create voltage has
both of its terminals biased at several volts above ground, you need to
amplify the difference between the terminals. What about noise that
adds an unwanted voltage equally to both terminals of a sensor?
The differential amp reject the noise and rescue the signal.




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Two ICs Convert 4-20mA Signal to 0-5V Output

The circuit in Figure 1 monitors loop current with a current-sense
amplifier (IC1), and employs a comparator/reference/op-amp
device (IC2) to generate a ground-referenced output that ranges
from 0V at 4mA to 5V at the full-scale loop current (20mA).
For the resistor values shown (R2-R6), IC1 produces an
output at pin 8 of approximately 1.25V at 4mA, and 6.25V at
20mA. In turn, the IC2 op amp (configured as a unity-gain
difference amplifier) generates an output range of 0.05V to
5.045V. The IC2 comparator can be used to monitor input
voltage or flag a pre-set loop current.




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Current Feedback Amplifier
Solution
Current Feedback Amplifiers (CFA) are especially suited to
implement this function, as shown in Figure 1. With an
effective internal buffer on the inverting node of the op amp,
the output impedance RO (internal to U1, not shown) and the
photo-diode’s output capacitance CIN (typically 10-200pF)
introduce a zero in the noise gain at approximately 1/2 π x
(RO x CIN). In comparison, the zero produced by a Voltage
Feedback op amp in a similar configuration [1/2π x
(RINRFRBIAS) x CIN] tends to be much lower in frequency
and more troublesome. This being the case, CIN has less of
an effect on reduction of the converter bandwidth, and
achieving stability is easier when using a CFA.



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Current-to-Voltage Amplifier
If you said that the easy application of a transimpedance amp
is too good to be true - you'd be right! Optimizing current-to-voltage
amplifiers can be one of the most challenging aspects of op amp
design. Why? The main culprit is the sensor's own capacitance CS.


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