Agilent Technologies 4294A Specifications download pdf (Page 23)

2-4-2. DC bias

In addition to the AC test signal, a DC voltage can be output through the Hc terminal and applied to

the DUT. A simplified output circuit, with a DC bias source, is shown in Figure 2-7. Many of the

conventional impedance measurement instruments have a voltage bias function, which assumes that

almost no bias current flows (the DUT has a high resistance). If the DUT’s DC resistance is low, a

bias current flows through the DUT and into the range resistor Rr, thereby raising the DC potential

of the virtual ground point. Also the bias voltage is dropped at source resistor Rs. As a result, the

specified bias voltage is not applied to the DUT and, in some cases, it may cause measurement error.

This must be taken into consideration when a low resistivity semiconductor device is measured.

The Agilent 4294A (and some other impedance analyzers) has advanced DC bias function that can

be set to either voltage source mode or current source mode. Because the bias output is automati-

cally regulated according to the monitored bias voltage and current, the actual bias voltage or cur-

rent applied across the DUT is always maintained at the setting value regardless of the DUT’s DC

resistance. The bias voltage or current can be regulated when the output is within the specified

compliance range.

Inductors are conductive at DC. Often a DC current dependency of inductance needs to be mea-

sured. Generally the internal bias output current is not enough to bias the inductor at the required

current levels. To apply a high DC bias current to the DUT, an external current bias unit or adapter

can be used with specific instruments. The 42841A and its bias accessories are available for high

current bias measurements using the E4980A, 4284A and 4285A precision LCR meters.

Figure 2-7. DC bias applied to DUT referenced to virtual ground

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Agilent Technologies 4294A Specifications Page 23