Column: Circuit drill


Voltage-to-current converter performance at different supply voltages


T


By Sulaiman Algharbi Alsayed, Managing Director, Smart PCB Solutions RLoad


he widely-applied voltage- to-current conversion circuit is regularly found in industrial facilities. Its function is to receive inputs from industrial instruments


and sensors and generate a correlated output. Te signal can travel long distances by first converting instrument output voltages to current, without suffering major distortion, which is essential in industrial facilities, where operating parameters are measured and transmitted to a central signal processing hub.


Simple or complex A voltage-to-current converter circuit can be simple or complex, but they all perform the same function, with variations in operationg range, accuracy, output signal stability, and more. Here we will examine one stability


aspect of such a circuit: we will establish the impact DC power supply changes have on the stability of this converter. Tis is important to know because a designer of any network must secure the control system against any problems caused by power supply fluctuations, which can affect operations.


The experiment We will use a simple and well-known voltage-to-current converter circuit (Figure 1) to establish the impact of any DC power supply changes on its output. Vsource Vin


is the output current sent from the


voltage-to-current converter circuit to the process interface hub. When fed with stable power, Vsource


,


ideally the circuit’s output current across RLoad


measurement range is at its best between Vmin


input voltage signal, Vin and Vmax


input voltage (Vin , where Vmin


should correlate with the . Tis useful


is the minimum ) that can be measured by


the circuit to produce a correlated output current. Similarly, Vmax


is the maximum input


voltage that the circuit can handle. Hence, the useful Vin


is between Vmin If Vin is below Vmin or above Vmax


and Vmax , the


.


circuit’s output won’t be useful, because the generated output current doesn’t correlate with the input voltage; hence, we can call Vmin


to Vmax the “useful input range”. In this experiment, we monitor the


useful input voltage span and the output current generated by the circuit at various supply voltages, Vsource


. To do so:


• Te supply voltage and input voltage are tested between 1Vdc and 9Vdc.


• Te temperature of the circuit is kept constant, to eliminate any temperature- related impacts on circuit performance.


Impacts Te minimum and maximum measurable input voltages are measured at every change in supply voltage (Vsource


). Ten,


the measurable input voltage span (Vmax – Vmin


) is calculated from the collected


represents the DC power supply and is the measured signal voltage received from the instrument. Te current through


values. Te circuit performance can be divided into three zones by plotting the data; see


08 November 2022 www.electronicsworld.co.uk


Zone 1: Tis zone covers the supply voltage below 3Vdc. In it, the circuit shows that output current does not correlate with the input voltage. Also, the span of the useful and measurable input voltage is saturated at 1Vdc only. Tus, this zone is not useful. In addition, the supply voltage must be above 3Vdc all the times.


Zone 2: Tis zone covers supply voltage range between 3Vdc and 8Vdc. Here, the circuit shows stable performance, with output current in good correlation with input voltage. Tis is the circuit’s zone of operation.


Zone 3: In this zone the supply voltage is above 8Vdc. Figure 2 shows that when the supply voltage is above 8Vdc, output current is saturated at less than 0.6mA, destroying any output current correlation with the applied input voltage. Terefore, the circuit must not operate within this zone, either.


Circuit misbehaviour As a circuit designer, you must pay good attention to the stability of any voltage- to-current converter circuit with regards to its supply voltage. Ignoring this important concern can result in control system misbehaviour.


Figure 2. Te graph has two Y axes, one for input voltages (leſt) and one for output current (right).


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