4-20 mA source circuit

0-1V to 4-20 mA Converter
Ensure +5/-5 dual supply for chip TL062 IC3. Gnd is common
ps ground, let grounds radiate from ground plane in one
side of PCB. R3-R8 is an attenuator that may need to be
designed or modified.
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1-5 V to 4-20 mA Circuit
The input voltage to this circuit is assumed to be coming from
some type of physical transducer/amplifier arrangement,
calibrated to produce 1 volt at 0 percent of physical measurement,
and 5 volts at 100 percent of physical measurement. The
standard analog current signal range is 4 mA to 20 mA,
signifying 0% to 100% of measurement range, respectively. At 5
volts input, the 250 Ω (precision) resistor will have 5 volts applied
across it, resulting in 20 mA of current in the large loop circuit
(with Rload). It does not matter what resistance value Rload is, or
how much wire resistance is present in that large loop, so long as
the op-amp has a high enough power supply voltage to output the
voltage necessary to get 20 mA flowing through Rload. The 250 Ω
resistor establishes the relationship between input voltage and
output current, in this case creating the equivalence of 1-5 V in /
4-20 mA out. If we were converting the 1-5 volt input signal to a
10-50 mA output signal (an older, obsolete instrumentation
standard for industry), we'd use a 100 Ω precision resistor instead.

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Howland Current Source for Grounded Load

Тhis genius idea is implemented in the famous Howland current
source. In this clever circuit, the excitation voltage V and the
resistor R form a basic current source. It produces a current
I = V/R - VL/R, which passes from the left hand side through the
load RL.

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Constant-current circuits
Constant-current circuits are usually implemented with an op
amp and a discrete external transistor.

Circuit E is a current source, which requires close matching of
the R2-R3 and R4-R5 resistor pairs to ensure insensitivity to
changes in the supply voltage.
Circuit (E) Iout = Vin/Rload
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Computer Controlled 100ma Current Source
Often in industrial control systems a constant current source
is needed, which is controlled by a computer and referenced to
circuit ground. The circuit below converts a zero to 5v
signal from a computer’s analog output into a current, with
a full scale of 100ma. The circuit shown requires a 9v DC
supply but any voltage from 9v to 12v will work.

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Constant current source circuit
We can use a voltage reference turn into a Constant current
source circuit and use pnp-transistors for current-boosting

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Voltage-to-Current Converter
Voltage-to-Current Converter or Current Source provided more
High-quality Signals Transmission in Long Lines instead of
Tension Transmission.more

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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