4mA-20mA to 0mA-20mA converter’s current mirror drives grounded load
The ubiquity of the 4 to 20mA current loop in analog process monitoring and control creates possibilities for peculiar designs of circuits for unusual accessory functions. Figure 1 shows an example. It does precision conversion of 4—20mA to 0—20mA. That’s useful for accommodating analog inputs that wouldn’t like a 4mA zero offset.
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Figure 1 This current conversion circuit’s function is define by the following equation: Iout = (IinR1 – 1.24v)/R2 = 1.25(Iin – 4mA).
The core of the circuit is the Vin = IR1 = 1.24v to 6.20v developed by the 4mA – 20mA input working into R1 and sensed by the Vref input of Z1. The principle in play is discussed here.
A potentially annoying shortcoming of the Figure 1 design, however, is its current sink output that’s referred not to ground but to the V+ source node, which needs to be at least 8v. Figure 2 offers an accurate and straightforward fix: an active current mirror as described here. The input max overhead voltage is 8v.
Figure 2 This circuit adds an active current mirror to its predecessor to drive a grounded load.
Stephen Woodward‘s relationship with EDN’s DI column goes back quite a long way. Over 200 submissions have been accepted since his first contribution back in 1974. They have included best Design Idea of the year in 1974 and 2001.
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