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handbook of modern sensors

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5.3 Excitation Circuits 167<br />

(A)<br />

(B)<br />

Fig. 5.14. Bipolar current generators with floating loads: (A) a noninverting circuit; (B) a circuit<br />

with a virtual ground.<br />

other reference potential. For some applications, this is quite all right; however, many<br />

<strong>sensors</strong> need to be grounded or otherwise referenced. The circuit shown in Fig. 5.14B<br />

keeps one side <strong>of</strong> the load impedance near the ground potential, because a noninverting<br />

input <strong>of</strong> the OPAM is a virtual ground. Nevertheless, even in this circuit, the load<br />

is still fully isolated from the ground. One negative implication <strong>of</strong> this isolation may<br />

be an increased pickup <strong>of</strong> various kinds <strong>of</strong> transmitted noise.<br />

In cases where the sensor must be grounded, a current pump invented by Brad<br />

Howland at MIT may be used (Fig. 5.15). The pump operation is based on utilizing<br />

both negative and positive feedbacks around the operational amplifier. The load is<br />

connected to the positive loop [2]. Current through the load is defined by<br />

A trimming resistor, P , must be adjusted to assure that<br />

i out = R 2<br />

R 1<br />

(V 1 − V 2 )<br />

R 5<br />

. (5.18)<br />

R 3 = R 1<br />

R 4 + R 5<br />

R 2<br />

. (5.19)<br />

In that circuit, each resistor may have a relatively high value (100 k or higher),<br />

but the value for R 5 should be relatively small. This condition improves the efficiency<br />

<strong>of</strong> the Howland current pump, as smaller voltage is wasted across R 5 and smaller<br />

current is wasted through R 4 and R 3 . The circuit is stable for most <strong>of</strong> the resistive<br />

loads; however, to ensure stability, a few-pic<strong>of</strong>arad capacitor C may be added in a<br />

negative feedback or/and from the positive input <strong>of</strong> an operational amplifier to ground.<br />

When the load is inductive, an infinitely large compliance voltage would be required<br />

to deliver the set current when a fast transient control signal is applied. Therefore, the<br />

current pump will produce a limited rising slope <strong>of</strong> the output current. The flowing<br />

current will generate an inductive spike across the output terminal, which may be<br />

fatal to the operational amplifier. For the large inductive load, it is advisable to clamp<br />

the load with diodes to the power-supply buses.

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