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Signal conditioning


A large current source with high accuracy and fast settling


Nick Jiang, product applications engineer in the Analog Devices Linear and Precision Technology Group, analyses the enhanced Howland current source (EHCS) circuit’s limitations and shows how to improve it


V


oltage controlled current sources (VCCSs) are widely used in many areas, like medical machine and industrial automation. The dc precision, ac performance, and drive capability of a VCCS


are highly important in these applications. This article analyses the EHCS circuit’s limitations and shows how to improve it with a composite amplifier topology to implement a ±500 mA current source with high precision and fast settling.


ENHANCED HowLAND CurrENT SourCE


If R1/R2 = R3/R4 = k, the equation is changed to


Equation 3. The output current is independent of the load and only controlled by the input voltage. It is an ideal VCCS.


PErformANCE ANALySiS Equation 3 is based on the ideal system. Figure 3 shows the dc error


analysis model of the EHCS. VOS and IB+/IB– are the input offset voltage and bias current of the main amplifier. VOSbuf and IBbuf are the input offset voltage and bias current of the buffer. The total output error can be calculated by Equation 4.


(3)


Figure 1. Howland current source circuit


Figure 1 shows the traditional Howland current source (HCS) circuit, while Equation 1 shows how the output current can be calculated. The output current will be constant if R2 is large enough.


(4) (1)


Figure 2. Enhanced Howland current source circuit


inserting a buffer into the feedback route to form an enhanced Howland current source will eliminate this, as shown in Figure 2. All the current flows


While a large R2 will reduce the speed and precision of the circuit,


through R0 is through into RL. The output current is calculated with Equation 2.


Figure 3. Offset voltage calculation


R3/R4 = k, R1//R2= R3//R4. The output offset current depends on the amplifiers’ offset and bias current, as shown in Equation 5.


Ignore the mismatch from the gain resistors, and consider R1/R2 =


(5) (2) 56


Taking the mismatch of R1/R2 and R3/R4 into consideration, RL will influence the output offset current. The worst relative error is shown in


August 2019 Instrumentation Monthly


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