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handbook of modern sensors

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168 5 Interface Electronic Circuits<br />

(A)<br />

(B)<br />

Fig. 5.15. Current generators with ground-referenced loads: (A) Howland current pump; (B)<br />

current pump with two OPAMs.<br />

An efficient current pump with four matched resistors and two operational amplifiers<br />

is shown in Fig. 5.15B. Its output current is defined by the equation<br />

i out = (V 1 − V 2 )<br />

R s<br />

. (5.20)<br />

The advantage <strong>of</strong> this circuit is that resistors R may be selected with a relatively high<br />

value and housed in the same thermally homogeneous packaging for better thermal<br />

tracking.<br />

For the generation <strong>of</strong> low-level constant currents, a monolithic voltage reference<br />

shown in Fig. 5.16 may be found quite useful. The circuit contains a 2.5 V reference<br />

which is powered by the output current from the voltage follower U 1 . The voltage

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